Samuel Benkimoun
email: s.benkimoun@gmail.com

Célestine Atyame

Marion Haramboure

Pascal Degenne

Hélène Thébault

Jean-Sébastien Dehecq

Annelise Tran

Dynamic mapping of dengue basic reproduction number

Keywords:

We developed a method to produce time-varying maps for dengue transmission risk, using the Ross-Macdonald framework and differential equations to estimate spatially the basic reproduction number (R 0 ) of a vector-borne disease. The components of the R 0 formula were derived partly from a mosquito population dynamics model integrating meteorological and environmental variables, and partly from temperature-dependent functions of vector competence and the extrinsic incubation period. The method was applied on Reunion Island, a tropical island located in the Indian Ocean, where the mosquito Aedes (Stegomyia) albopictus has been responsible for large and numerous outbreaks of dengue. As a validation, predicted maps and dynamic outputs were compared with the distribution of confirmed dengue cases registered during the year 2018 in Reunion Island. The results highlight strong agreements between the observed epidemiological patterns and predicted R 0 distribution and temporal dynamics. This finding demonstrates the relevance and efficiency of the spatialised basic reproduction number (R 0 ) to develop an operational dynamic mapping tool for dengue surveillance and control. The resulting method could be of great use in a health policy-making context, providing a time and space awareness to the dengue risk perception.

Introduction

Vector-borne diseases (VBD) are caused by pathogensparasites, bacteria, or viruses, whose transmission between vertebrate hosts requires a hematophagous arthropod. According to the World Health Organization, they account for more than 17% of infectious diseases, causing 700,000 deaths annually [START_REF] Who | Vector-borne diseases -Key facts[END_REF]. Among them, mosquito-borne diseases such as malaria and dengue place a high burden on the impacted countries and their health systems, mainly in tropical and subtropical areas. The high diversity of species involved in the transmission of mosquito-borne pathogens increases the occurrence of these diseases in both rural and environments [START_REF] Cox | Habitat segregation of dengue vectors along an urban environmental gradient[END_REF][START_REF] Harrington | Dispersal of the dengue vector Aedes aegypti within and between rural communities[END_REF][START_REF] Vazquez-Prokopec | Quantifying the spatial dimension of dengue virus epidemic spread within a tropical urban environment[END_REF]. In the absence of effective vaccines and specific treatments, vector control is the only measure to interrupt the transmission of vector-borne pathogens. Therefore, the capacity to predict periods and locations at risk of disease transmission would help improve vector control measures, with better-measured impacts. Moreover, mapping the epidemic risks is an essential step to further orientate health policies for sensitization campaigns of human populations [START_REF] Ouedraogo | Evaluation of effectiveness of a communitybased intervention for control of dengue virus vector, Ouagadougou, Burkina Faso[END_REF].

Modelling of vector-borne diseases, including process-based and empirical approaches, has a long and rich history [START_REF] Jr | A systematic review of mathematical models of mosquito-borne pathogen transmission: 1970-2010[END_REF][START_REF] Smith | Recasting the theory of mosquito-borne pathogen transmission dynamics and control[END_REF][START_REF] Sadeghieh | A scoping review of importation and predictive models related to vector-borne diseases, pathogens, reservoirs, or vectors (1999-2016)[END_REF][START_REF] Rogers | Models for vectors and vector-borne diseases[END_REF][START_REF] Aliyu | A fractional model of vertical transmission and cure of vector-borne diseases pertaining to the Atangana-Baleanu fractional derivatives[END_REF]. Most mathematical models use the theoretical framework developed for malaria by Ross and Macdonald [START_REF] Macdonald | The epidemiology and control of malaria[END_REF][START_REF] Smith | Ross, Macdonald, and a theory for the dynamics and control of mosquito-transmitted pathogens[END_REF]. Indeed, such a framework allowed a simple formulation of the basic reproduction number (R 0 ) of a vectorborne disease. This reproduction number can be understood in the following way: it is the expected number of hosts that would become infected from the introduction of a single infected host in a fully susceptible population, as a function of entomological and epidemiological parameters (vector biting rate, mortality rate, competence and extrinsic incubation period, vector-to-host ratio, transmission probability, duration of infectiousness in hosts) [START_REF] Macdonald | The epidemiology and control of malaria[END_REF][START_REF] Liu | Reviewing estimates of the basic reproduction number for dengue, Zika and chikungunya across global climate zones[END_REF]. R 0 has become a key notion in mathematical epidemiology as R 0 = 1 is considered as the epidemic threshold that predicts whether a disease will spread at the onset of the epidemic [START_REF] Heffernan | Perspectives on the basic reproductive ratio[END_REF].

As arthropods' life cycle is very much driven by climatic and environmental determinants, this latter also strongly impacts the transmission of vector-borne diseases, which may vary in space and time. Thus, methods have been developed to map R 0 for vector-borne diseases from environmental and meteorological variables as predictors [START_REF] Hartemink | Mapping the basic reproduction number (R(0)) for vector-borne diseases: a case study on bluetongue virus[END_REF]. For example, seasonal R 0 maps accounting for meteorological variables were produced at a national level for bluetongue [START_REF] Hartemink | Mapping the basic reproduction number (R(0)) for vector-borne diseases: a case study on bluetongue virus[END_REF], and maps at continental scale were used to better understand and predict the impact of climate change on the risk of bluetongue in Europe [START_REF] Guis | Modelling the effects of past and future climate on the risk of bluetongue emergence in Europe[END_REF]. Recently, R 0 maps of malaria were produced at a regional scale in Greece, integrating meteorological variables as well as entomological, environmental, and social data [START_REF] Pergantas | A spatial predictive model for malaria resurgence in central Greece integrating entomological, environmental and social data[END_REF].

Yet, as far as we know this R 0 mapping approach has not been used so far for arthropod-borne viruses (arboviruses) transmitted by Aedes mosquitoes. In particular, it was not applied to dengue fever, the most prevalent arboviral infection, endemic in more than 100 countries, including Reunion Island, and responsible for 390 million infections estimated annually [START_REF] Who | Dengue and severe dengue -Key facts[END_REF]. A wide variety of predictors and modelling approaches have been used either to create dengue risk maps [START_REF] Louis | Modeling tools for dengue risk mapping -a systematic review[END_REF] or estimate R 0 values [START_REF] Liu | Reviewing estimates of the basic reproduction number for dengue, Zika and chikungunya across global climate zones[END_REF][START_REF] Andraud | Dynamic epidemiological models for dengue transmission: a systematic review of structural approaches[END_REF][START_REF] Johansson | Models of the impact of dengue vaccines: a review of current research and potential approaches[END_REF][START_REF] Favier | Early determination of the reproductive number for vector-borne diseases: the case of dengue in Brazil[END_REF] but to our knowledge, both approaches have never been combined. A recent study provided a framework for mapping dengue spatially time-varying reproductive numbers from reported cases locations [START_REF] Ng | Spatially adjusted time-varying reproductive numbers: understanding the geographical expansion of urban dengue outbreaks[END_REF], but such approach does not account for meteorological and environmental drivers (i.e. vegetation, urbanization type, etc.) that could impact spatial and temporal vector abundances, thus resulting in heterogeneities in virus transmission.

In this study, we aimed to produce time-varying dengue R 0 maps in Reunion Island, a tropical island located in the Indian Ocean, where the mosquito Aedes (Stegomyia) albopictus is responsible for a current large outbreak of dengue [START_REF] Vincent | From the threat to the large outbreak: dengue on Reunion Island, 2015 to 2018[END_REF]. As highly heterogeneous vector densities have been reported in the island [START_REF] Boyer | Spatial and temporal heterogeneities of Aedes albopictus density in La Reunion Island: rise and weakness of entomological indices[END_REF], a rainfall-and temperature-driven model of Ae. albopictus populations ('ALBORUN') was recently developed to predict vector abundance at a local scale, accounting for variations in the availability of breeding sites [START_REF] Tran | Complementarity of empirical and process-based approaches to modelling mosquito population dynamics with Aedes albopictus as an example-Application to the development of an operational mapping tool of vector populations[END_REF]. We used the 'ALBORUN' model and the Ross-Macdonald framework to derive dynamic dengue R 0 maps from meteorological and environmental variables, and assessed the relevance of such an approach by comparing the predicted maps and dynamics with the number of confirmed dengue cases registered during the year 2018.

Material and methods

Study area. Reunion Island (2500 km 2 and 865,826 inhabitants in 2018) is a French overseas territory located in the south-western Indian Ocean (Fig. 1). Its subtropical climate with mild austral winters (May-October) and warm austral summers (November-April) is suitable for the development of Aedes albopictus mosquitoes throughout the year [START_REF] Boyer | Spatial and temporal heterogeneities of Aedes albopictus density in La Reunion Island: rise and weakness of entomological indices[END_REF][START_REF] Tran | Complementarity of empirical and process-based approaches to modelling mosquito population dynamics with Aedes albopictus as an example-Application to the development of an operational mapping tool of vector populations[END_REF] local variations of climate are important due to a highly uneven relief and different wind expositions.

Arboviruses are a recurring public health issue on Reunion Island, with a massive dengue epidemic in 1977 that affected up to 35% of the population [START_REF] Paupy | Population structure of Aedes albopictus from La Reunion Island (Indian Ocean) with respect to susceptibility to a dengue virus[END_REF]. In 2005, a chikungunya epidemic again reached 35% of the island residents [START_REF] Renault | A major epidemic of chikungunya virus infection on Reunion Island, France, 2005-2006[END_REF]. After several years of moderate virus circulation, a large dengue outbreak started in 2018, with 28,141 cases reported between January 2018 and April 2020 [START_REF]Surveillance de la dengue à la Réunion[END_REF].

Human population data. 2016 census data were obtained at IRIS ("Ilots regroupés pour l'information statistique") level (https://www. insee.fr/fr/statistiques/4228434). Those sub-urban units correspond to the most detailed level of population census available in Reunion (n = 344) (Fig. 1). The population was then estimated at the scale of the operational zones mapping used by the local vector control services (n = 1203). To scale down from the IRIS level of the French census, built-up Fig. 1. Location of the study area, Reunion Island. spatial data were downloaded from the French National Geographic Institute (https://data.geocatalogue.fr/id/dataset/a51dafeb-bd2a-4b8 a-929b-ca539a44217e) and the population was reallocated at the prorata of the houses within the operational zones, as compared to the total number of houses in the intersected IRIS zones. Geographic Information System spatial analysis functions were used (QGIS 3.4). Only operational zones located in residential areas with an estimated population greater than 10 (i.e., excluding administrative, business, or sparse habitat) were selected for R 0 computation (n = 1189).

Epidemiological data. The weekly confirmed dengue cases recorded in 2018 were aggregated at the IRIS level. The incidence rate (number of confirmed cases in 2018 per 10,000 inhabitants) per IRIS was calculated using 2016 census data.

Meteorological data. We obtained daily mean temperature and rainfall recorded from 2017 to 2018 at 32 weather stations (Fig. 1) from the national meteorological service 'Meteo France' (https://publitheque .meteo.fr). Indeed, the acquisition of meteorological data covering the year before the period of interest (2018) was necessary for the initialization of the mosquito population dynamics model [START_REF] Tran | Complementarity of empirical and process-based approaches to modelling mosquito population dynamics with Aedes albopictus as an example-Application to the development of an operational mapping tool of vector populations[END_REF].

'ALBORUN' population dynamics model. This process-based model of Ae. albopictus population dynamics on Reunion Island is based on a system of ordinary differential equations (ODE) [START_REF] Amann | Ordinary differential equations: an introduction to nonlinear analysis[END_REF][START_REF] Chicone | Ordinary differential equations with applications[END_REF] that represents all steps of the mosquito life cycle, considering aquatic juvenile and aerial female adult stages (Fig. 2) [START_REF] Tran | Complementarity of empirical and process-based approaches to modelling mosquito population dynamics with Aedes albopictus as an example-Application to the development of an operational mapping tool of vector populations[END_REF] (https://doi. org/10.18167/DVN1/XF2I3L). Transition functions from one stage to the next and mortality rates are driven by daily rainfall and temperature. The model inputs are i) the operational zones used by the vector control service, characterized by their standard fixed and variable environment carrying capacities, two values describing the availability and characteristics of the mosquito breeding sites in the zone (based on field observations, see [START_REF] Tran | Complementarity of empirical and process-based approaches to modelling mosquito population dynamics with Aedes albopictus as an example-Application to the development of an operational mapping tool of vector populations[END_REF] for details), ii) the location of weather stations, and iii) the corresponding daily rainfall and temperature. As output, 'ALBORUN' predicts the abundances of Ae. albopictus mosquitoes per stage at a weekly frequency, and for each operational zone for which the rainfall and temperature values from the closest weather station are simply attributed.

The following parameters, functions, and outputs are used for the R 0 model (see below): the daily transition rate from host-seeking to engorged adults (γ Ah ), the mortality rate of adult mosquitoes (m A ), the mortality rate related to seeking behaviour (μ r ), the number of hostseeking female mosquitoes (A h ) and the total number of Ae. albopictus female mosquitoes (A tot ). R 0 model. We used the classic computation of R 0 from the vectorial capacity (V), which estimates the daily number of new vector infectious bites that arises from one infected host introduced into an entirely susceptible host population [START_REF]Prognosis for Interruption of Malaria Transmission through Assessment of the Mosquito's Vectorial Capacity[END_REF][START_REF] Lambrechts | Impact of daily temperature fluctuations on dengue virus transmission by Aedes aegypti[END_REF]:

V = ma 2 p n -ln(p) (1)
where m is the vector density per host, a the daily biting rate, p the daily survival rate, n the duration of the extrinsic incubation period (EIP).

The basic reproduction number R 0 is obtained by multiplying V by the vector competence (b) and the duration of host viremia (1/r, where r is the host recovery rate):

R 0 = V r .b (2)
With the exception of r, all components of this R 0 formula (Eqs. ( 1) and ( 2)) were considered space-and time-dependent. Moreover, in absence of the genetic structure of Ae. albopictus populations in Reunion Island [START_REF] Latreille | High genetic diversity but no geographical structure of Aedes albopictus populations in Reunion Island[END_REF], meteorological and environmental factors were considered the main drivers of the vector competence and the vectorial capacity. Most components of the vectorial capacity (Eq. ( 1)), namely m, a, and p the daily survival rate, were derived from the 'ALBORUN' model for each operational zone and each time step (Table 1). The EIP (n) and the vector competence (b) were defined as functions of temperature varying over time. Polynomial regressions adjusted to experimental values [START_REF] Guo | Vector competence of Aedes albopictus and Aedes aegypti (Diptera: Culicidae) for DEN2-43 and New Guinea C virus strains of dengue 2 virus[END_REF] were used to express the relationship between the temperature and EIP and vector competence (Table 1), regarding the dengue virus serotype 2 (DENV-2), the serotype circulating in Reunion Island in 2018 [START_REF] Vincent | From the threat to the large outbreak: dengue on Reunion Island, 2015 to 2018[END_REF]. The impact of dengue infection on the mosquitoes' life cycle, very uncertain [START_REF] Lambrechts | Vector biology prospects in dengue research[END_REF], has been neglected, as well as an eventual variation between symptomatic and asymptomatic hosts regarding incubation period and viremia.

Simulations. The R 0 model was implemented in Ocelet language 1) and ( 2)). Simulations were run over two years (2017-2018) because the first year was not retained for output computation.

Evaluation of the R 0 model. The capacity of the R 0 model to predict the spatial distribution of dengue transmission risk was assessed by comparing the annual statistics (mean and maximum values) of simulated R 0 with the number of confirmed dengue cases reported, at the IRIS level. On the other hand, the capacity of the R 0 model to predict the seasonal variations of dengue transmission throughout the year was assessed by comparing for each week w, the weekly R 0 median value, with the ratio n w /n w-1 between the numbers of confirmed dengue cases reported during week w and the precedent week (w-1), using Pearson correlation coefficient.

Results

Dengue R 0 maps highlight spatial and seasonal variations of the risk of dengue transmission in Reunion Island. For each operational zone defined by the vector control services (mean surface of 33 ha), our model predicts dengue R 0 value at a weekly frequency. The resulting maps highlight high spatial and seasonal variations on Reunion Island (Fig. 3, Video file 1). Higher R 0 values are predicted during the austral summer (January-April), and in western and southern coastal areas (Fig. 3). When R 0 peaks, almost all regions of the islands are at risk (R 0 > 1), except the inner mountainous zones (e.g. April, Fig. 3). During the austral winter, R 0 drops below the threshold value of one in all regions (e.g. August, Fig. 3). At the beginning of the hot and wet season (November) R 0 values start increasing, mostly in the southern part of the island (Fig. 3). Spatio-temporal dynamics of the R 0 components, vectorial capacity, extrinsic incubation period, and vector competence, are provided in Supplementary Fig. S1-3.

The spatial distribution of predicted R 0 is significantly correlated with dengue incidence.

Significant correlations were found between annual maximum R 0 values computed at IRIS level (Fig. 4A) and dengue incidence (Fig. 4B) (Pearson r = 0.41, p < 10 -12 ). The correlation with the mean R 0 values was also significant (Pearson r = 0.30, p 〈10 -6 ). The map of the annual maximum of R 0 values computed at IRIS level highlights several areas with high predicted R 0 , mainly located on the western coast of the island (Fig. 4A). Seasonal dynamics of predicted dengue R 0 correlates with the temporal dynamics of dengue transmission. According to our model, the predicted median R 0 value on Reunion Island was over the threshold of one from January to May 2018 (Fig. 5A). It started decreasing at the beginning of April. During this period, the number of confirmed dengue cases on the island steadily increased. It is important to look at the moment where the R 0 crosses the 1 threshold, meaning a decrease in the transmission dynamic of the virus. In our model, it happens during the first week of May. Comparing this to the actual numbers of 2018 shows that a plateau is reached at this very same time. The number of recorded cases is stabilizing at its highest level for about a month (showing a comprehensible delay of a couple of weeks in the effect of this R 0 decrease, due to the incubation period of previously infected individuals for example). Then, the number of cases dropped at the beginning of the austral winter (end of June), when the median R 0 reaches its minimum value (R 0 = 0.03, week 25: June 17th). Very low R 0 values were predicted between July and November; R 0 started increasing again at the end of October (Fig. 5A). The variation of weekly R 0 median values could be compared with the ratio n w /n w-1 for the period with n greater than 10, i.e. between weeks 5 (January 28th) and 35 (August 28th) (Fig. 5B), depicting an even clearer adequation between the model outputs and the outbreak. The comparison between the two indices showed a strong and significant correlation (Pearson r = 0.77, p < 10 -6 ), and their respective drop below the value of 1 appears synchronized.

Discussion

In this study, we used the Ross and Macdonald framework [START_REF] Smith | Ross, Macdonald, and a theory for the dynamics and control of mosquito-transmitted pathogens[END_REF] to propose a real-time mapping method of the basic reproduction number of dengue fever from the weather (rainfall and temperature) and environmental variables (availability and characteristics of the Ae. albopictus breeding sites). Predicted R 0 values were compared with 2018 dengue outbreak data from Reunion Island at fine spatial and temporal scales [START_REF] Vincent | From the threat to the large outbreak: dengue on Reunion Island, 2015 to 2018[END_REF]. It is important to reassert that the model was at no point calibrated to specifically fit this data; instead, parameters and functions were defined based on the results of previous experimental and observational studies thus combining all current knowledge on the ecological mechanisms. Our results demonstrate the efficiency of such an approach to develop an operational dynamic mapping tool for dengue surveillance and control, potentially useful for decision-makers. The dengue R 0 module was implemented in the 'ALBORUN' tool, which is routinely used by the Health Regional Agency in Reunion Island [START_REF] Tran | Complementarity of empirical and process-based approaches to modelling mosquito population dynamics with Aedes albopictus as an example-Application to the development of an operational mapping tool of vector populations[END_REF].

Spatial patterns of dengue R 0 in Reunion Island. According to our results, R 0 values computed at the IRIS level ranged from 0 to 14.85 (mean: 5.12) (Fig. 4A), which is consistent with dengue R 0 estimations in other tropical countries [START_REF] Liu | Reviewing estimates of the basic reproduction number for dengue, Zika and chikungunya across global climate zones[END_REF]. Higher values of estimated dengue R 0 are predicted in the western and southern regions (Figs. 3 and4A): in these leeward coastal areas, temperatures are warmer than in other parts of the island, favouring dengue transmission with shortened extrinsic incubation period and increased vector capacity (Supplementary Figs. S1 andS2). Moreover, although the rainfall is lower in these areas, densities of Ae. albopictus populations are higher as compared to other areas, due to human-made oviposition sites such as flower plates, pots, or barrels that remain available over time in urban and peri-urban neighbourhoods [START_REF] Tran | Complementarity of empirical and process-based approaches to modelling mosquito population dynamics with Aedes albopictus as an example-Application to the development of an operational mapping tool of vector populations[END_REF][START_REF] Delatte | Geographic distribution and developmental sites of Aedes albopictus (Diptera: Culicidae) during a Chikungunya epidemic event[END_REF]. Interestingly, the differences in vector competence between Ae. albopictus populations from the eastern and western coasts (Supplementary Fig. S3) are less marked than those predicted with EIP (Supplementary Fig S2 ) and vectorial capacity (Supplementary Fig. S1), suggesting that in Reunion Island, the spatial heterogeneity of dengue predicted R 0 is mostly driven by vector ecology rather than molecular interactions between Ae. albopictus populations and dengue viruses. This result corroborates the conclusions of previous modelling and experimental studies on Ae. albopictus competence for dengue virus transmission, and on the importance of the presence of breeding sites to support the transmission [START_REF] Guo | Vector competence of Aedes albopictus and Aedes aegypti (Diptera: Culicidae) for DEN2-43 and New Guinea C virus strains of dengue 2 virus[END_REF][START_REF] Brady | Global temperature constraints on Aedes aegypti and Ae. albopictus persistence and competence for dengue virus transmission[END_REF][START_REF] Vazeille | Population genetic structure and competence as a vector for dengue type 2 virus of Aedes aegypti and Aedes albopictus from Madagascar[END_REF] and endemicity of dengue in tropical areas [START_REF] Barrera | Population dynamics of Aedes aegypti and dengue as influenced by weather and human behavior in San Juan, Puerto Rico[END_REF].

Predicted dengue R 0 values computed at IRIS level were significantly correlated with dengue incidence, suggesting that the resulting R 0 maps (Figs. 3 and4A) could be used to identify areas at risk for virus transmission. Our results thus stress that most of those areas are located on the western and southern coasts (Fig. 4A). Of note, areas with high R 0 predicted values do not fully overlap those with high observed dengue incidence, and overall, the R 0 index trends to overestimate the risk of dengue: it is the case for example in the southern and eastern coasts, where few dengue cases were reported in 2018 in areas with high predicted R 0 values (Fig. 4). Several non-exclusive assumptions may explain these discrepancies. Firstly, a large number of paucisymptomatic cases may remain undetected by the surveillance system [START_REF] Duong | Asymptomatic humans transmit dengue virus to mosquitoes Proceedings of the National Academy of Sciences of the United States of America[END_REF]. Secondly, by definition R 0 reveals the number of secondary cases arising from the introduction of a single case in a susceptible population, but it does not capture the risk of introduction, which is related to human mobility. Thus, a low number of cases may be reported in areas with high R 0 values but poorly connected to other regions of the island in terms of human population flows, for example, south-eastern areas (Supplementary Fig. S4). Thirdly, in R 0 computation the human population was assumed fully susceptible to dengue serotype 2, which may not be the case. An estimation of the real reproduction number R, taking into account the population immunity, would improve the model. Lastly, the vector control measures which are implemented when a dengue case is reported in a zone [START_REF] Vincent | From the threat to the large outbreak: dengue on Reunion Island, 2015 to 2018[END_REF] can substantially reduce the virus transmission and the number of secondary cases, if promptly detected. This effect could be considered by the model in future investigations.

Dynamics of dengue R 0 in Reunion Island. Our analysis highlighted R 0 variations over time in relation to temperature and rainfall: predicted R 0 values were over the threshold of one during the hot and rainy season, and below the threshold during the austral winter, mainly due to colder and potentially lethal temperatures for mosquito populations. R 0 starts increasing again in November, at the beginning of the austral summer (Fig. 5A). This result is in keeping with the seasonal pattern described in other tropical areas from observational and modelling studies [START_REF] Bartley | The seasonal pattern of dengue in endemic areas: mathematical models of mechanisms[END_REF][START_REF] Kim | Assessment of optimal strategies in a two-patch dengue transmission model with seasonality[END_REF][START_REF] Polwiang | The time series seasonal patterns of dengue fever and associated weather variables in Bangkok (2003-2017)[END_REF][START_REF] Teurlai | Can human movements explain heterogeneous propagation of dengue fever in Cambodia?[END_REF]. The predicted variations of R 0 are also consistent with the dynamics of dengue epidemics observed on Reunion Island (Fig. 5A): the number of cases increases steadily from January to May, when R 0 is greater than one, and strongly decreases at the end of June when R 0 reaches its minimum. The strong and significant correlation of R 0 with the ratio n w /n w-1 , an index depicting the variation of the number of cases on a weekly basis (Fig. 5B), suggests that predicted R 0 could be used to identify periods at risk for virus transmission.

Limitations and perspectives. The method proposed in this study has the advantage of simplicity and parsimony (Eqs (1) and ( 2)). The mechanist model behind those results has also been applied to other territories [START_REF] Iyaloo | ALBOMAURICE: A predictive model for mapping Aedes albopictus mosquito populations in Mauritius[END_REF] and gave similarly consistent outputs regarding the vector ecology, which tends to drive away the eventuality of an over-fit. The comparison with the dengue cases reported in Reunion Island in 2018 proves its efficiency as a dynamic mapping tool for dengue surveillance and control, which would be relevant to orientate further health policies. However, different weather-driven processes were not taken into account in our model, such as the impact of diurnal thermal range on vector competence [START_REF] Lambrechts | Impact of daily temperature fluctuations on dengue virus transmission by Aedes aegypti[END_REF] or a temperature-dependent hostvector transmission rate [START_REF] Mordecai | Detecting the impact of temperature on transmission of Zika, dengue, and chikungunya using mechanistic models[END_REF]. The inclusion of such processes could improve this first R 0 model. Moreover, as above-mentioned, by construction R 0 does not capture the risk of the introduction of an index case in an area. A model including both importation risk estimates, from human mobility (Supplementary Fig. S4), and onward transmission [START_REF] Lai | Seasonal and interannual risks of dengue introduction from South-East Asia into China, 2005-2015[END_REF] could be used to improve the predictions.

DENV2-serotype was the only serotype circulating on Reunion Island in 2018 [START_REF] Vincent | From the threat to the large outbreak: dengue on Reunion Island, 2015 to 2018[END_REF], thus in this study R 0 functions (Table 1) were defined for this particular serotype. To assess the risk of transmission of the three other dengue serotypes in Reunion Island, the functions would need to be modified as vector competence and the extrinsic incubation period can change according to the dengue virus serotype involved in the transmission [START_REF] Joanne | Vector competence of Malaysian Aedes albopictus with and without Wolbachia to four dengue virus serotypes[END_REF]. Indeed, DENV-1, DENV-3, and DENV-4 serotypes have been reported on the island in recent years and an autochthonous circulation of these three serotypes has been reported in 2020. Moreover, with time the immunity will increase and therefore the population could not be considered as fully susceptible: the cartography of the dengue reproduction rate (R), with a formula taking into account the immunity of the human population, would be more appropriate in the future. Another perspective would be to account for the vector control actions (fumigation of mosquito adults, cleaning the water-holding containers that serve as breeding sites for Ae. albopictus) in order to simulate in silico different scenarios of control strategies, and study their impact on the reduction of R 0 . Finally, the approach developed in this work could be applied to other pathogens transmitted by Ae. albopictus, such as the Chikungunya virus, which caused a very severe outbreak in Reunion Island in 2006, and the Zika virus, which recently spread worldwide and constitutes a threat for Reunion Island. Moreover, as the mosquito population dynamics model framework has already been used for other geographical contexts [START_REF] Tran | A rainfall-and temperature-driven abundance model for Aedes albopictus populations[END_REF] and/or mosquito species [START_REF] Cailly | A climate-driven abundance model to assess mosquito control strategies[END_REF][START_REF] Ezanno | A generic weather-driven model to predict mosquito population dynamics applied to species of Anopheles, Culex and Aedes genera of southern France[END_REF], our model also offers some broad transposition possibilities by adapting the parameters from the existing scientific literature.

Conclusion

In this paper, we presented an original method coupling a mechanistic mosquito population dynamics model with the Ross-Macdonald R 0 formula to evaluate risks with regard to mosquito-borne diseases. This model operates at a very fine spatial scale, adapted to vector surveillance and control interventions, and on a weekly frequency. This strong reactivity to spatial and temporal trends makes the model a reliable tool for health policymakers to implement effective public health policies against dengue. The proposed framework could be deployed in other and various geographical and epidemiological contexts.

Ethics statements

Dengue cases are reported to the regional health agency to conduct vector control measures. As data from an epidemiological record, institutional review and informed consent were not required. All data analyzed were anonymized.

Fig. 2 .

 2 Fig. 2. Diagram of the Aedes albopictus population dynamics model (ALBORUN). The model predictions focus on the sole female mosquito population, being the one that bites. In blue, the aquatic stages (E: eggs; L: larvae; P: pupae); in yellow the female adult stages (A em : emerging; A 1 : nulliparous, A 2 : parous, A tot : total, h: host-seeking; g: resting; o: ovipositing). Parameters (greek letters) are constant, functions (latin letters) are weather-driven functions, varying over time. For stage X ,γ X and f X are transition rates to the next stage; μ X and m X mortality rates; β X is the egg laying rate; σ is the sex-ratio at the emergence; μ r is an additional adult mortality rate related to seeking behavior; k X the environment carrying capacity. Stages, parameters and functions values used for R 0 computation are red highlighted. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

  ; H: human population; A tot : total number of Ae. albopictus female mosquitoes A h : number of Ae. albopictus host-seeking females; γ Ah : daily transition rate from host-seeking to engorged adults; m A : mortality rate of adult mosquitoes; μ r : mortality rate related to seeking behavior. A tot , A h , A 2h , A 2o and m A are estimated by 'ALBORUN' model (see[START_REF] Tran | Complementarity of empirical and process-based approaches to modelling mosquito population dynamics with Aedes albopictus as an example-Application to the development of an operational mapping tool of vector populations[END_REF] for details).(www.ocelet.org), by adapting the 'ALBORUN' mosquito population model (https://doi.org/10.18167/DVN1/XF2I3L), extended by a module of R 0 calculation. R 0 values are then computed for each operational zone from dynamically updated values of the different components of the vectorial capacity at each time step (Eqs. (

Fig. 3 .

 3 Fig. 3. Predicted dengue R 0 values, Reunion Island, 2018.

Fig. 4 .

 4 Fig. 4. Predicted R 0 and observed incidence for dengue, Reunion Island, 2018. A) Predicted annual maximum R 0 value computed at IRIS level. B) Dengue incidence on Reunion Island, 2018.

Fig. 5 .

 5 Fig. 5. A) Predicted median R 0 values dynamics (red line) and dengue confirmed cases (grey bars), Reunion Island, 2018. The red dotted line indicates the threshold of one. B) Predicted median R 0 values dynamics (red line) and ratio n w /n w-1 (dashed black line) between the numbers of confirmed dengue cases reported during week w and during the precedent week w-1, Reunion Island, 2018, weeks 5 (January 28th) to 35 (August 28th). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1

 1 Estimation of dengue R 0 parameters, Reunion Island.

	Notation	Definition*	Expression*	Reference
	m	Vector density per	Atot	[25,49]
		host	H
	a p	Daily biting rate Daily survival rate	A h Atot 1 -mA -.γ Ah , withγ Ah = 0.2 A 2h + A2o Atot .μ r , withμ r =	[25] [25]
			0.08
	n	EIP	0.11T

2 -7.13T + 121.17

Acknowledgments

This work was supported by the European Regional Development Fund (ERDF) INTERREG TROI project in the framework of the One Health Indian Ocean network (www.onehealth-oi.org). M. H. received a grant from European Research Council under the European Union's Horizon 2020 research and innovation program: ERC Consolidator grant n 682387 / REVOLINC project. APC was funded by ANISETTE, a project supported by the French Space Agency, CNES (DAR 4800001029). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The authors thank Catherine Cêtre-Sossah, CIRAD, for fruitful discussion on the results of this study.

Data Availability statement

ALBORUN code is available from CIRAD dataverse https://doi.org/1 0.18167/DVN1/XF2I3L

Authors' contributions

SB contributed to the acquisition and preparation, modelling, analysis and interpretation of data and helped draft the manuscript. CA contributed to analysis and interpretation of data and critically revised the manuscript. MH and PD contributed to modelling, analysis and interpretation of data and critically revised the manuscript. HT and JSD conceived of the study, contributed to the acquisition and interpretation of data, and revised the manuscript. AT conceived of the study, designed the study, contributed to modelling, analysis and interpretation of data and drafted the manuscript. All authors gave final approval for publication and agreed to be held accountable for the work performed therein.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.rinp.2021.104687.


Kenna D Peusner
email: kennad.peusnerpeusnerk@gwu.edu

Nina M Bell

June C Hirsch

Mathieu Beraneck

Anastas Popratiloff

Georges Habib

Rizk

Understanding the Pathophysiology of Congenital Vestibular Disorders: Current Challenges and Future Directions

Keywords: inner ear imaging, developmental balance disorders, vestibular system development, inner ear abnormalities, animal models

INTRODUCTION

The vestibular system processes signals generated by vestibular hair-cell mechanoreceptors of the inner ear in response to linear and angular acceleration to maintain posture, balance, and spatial orientation. The mature inner ear is a complex labyrinth including many elaborate sensory organs in a fixed configuration. However, inner ear malformation is not rare during development, despite the evolutionary significance of the labyrinths for survival. Over 40 genes affect the inner ear with many expressed during key developmental stages [START_REF] Wu | Molecular mechanisms of inner ear development[END_REF]. A major hallmark of many CVDs is the formation of a malformed inner ear into a saclike structure with the semicircular canals missing or truncated and the vestibular sensory organs relatively intact (Table 1). A sac-like inner ear is observed in CHARGE syndrome [START_REF] Haginomori | Temporal bone histopathology in CHARGE Association[END_REF][START_REF] Abadie | Vestibular anomalies in CHARGE syndrome: investigations on and consequences for postural development[END_REF][START_REF] Sando | Vestibular abnormalities in congenital disorders[END_REF], an acronym for a set of congenital malformations: Coloboma (eye defects), Heart defects, choanal Atresia (nasal passage defects), Retarded growth, Genital hypoplasia, and Ear abnormalities. In addition, Waardenburg [START_REF] Elmaleh-Bergès | Spectrum of temporal bone abnormalities in patients with Waardenburg syndrome and SOX10 mutations[END_REF], Noonan [START_REF] Naficy | Multiple temporal bone anomalies associated with Noonan syndrome[END_REF], Wildervanck [START_REF] Abu-Amero | Xq26.3 microdeletion in a male with Wildervanck syndrome[END_REF], Goldenhar [START_REF] Bisdas | Inner ear abnormalities in patients with Goldenhar syndrome[END_REF], and branchio-otorenal syndromes (BOR) [START_REF] Kemperman | Evidence of progression and fluctuation of hearing impairment in branchio-oto-renal syndrome[END_REF] form a sac-like inner ear during development. Epidemiological studies reveal that vestibular disorders affect about 3.3 million children in the US alone (5.3-10%) [START_REF] Li | Epidemiology of dizziness and balance problems in children in the United States: a population-based study[END_REF][START_REF] Rine | section on pediatric knowledge translation lecture: pediatric vestibular-related impairments[END_REF]. Due to the major role that the vestibular system plays in daily living, vestibular symptoms can impose life-altering disabilities that become a major healthcare burden. CVD children may experience disorientation, confusion, and fatigue while performing daily tasks, and severe challenges in eye-hand coordination, eye tracking, and reading that may lead to difficulties in physical and intellectual development [START_REF] Blake | Cranial nerve manifestations in CHARGE syndrome[END_REF]. Physical therapy is helpful to manage certain symptoms, but does not counter poor visual acuity or balance deficits experienced by patients with bilateral vestibular abnormalities [START_REF] Janky | Vestibular, visual acuity and balance outcomes in children with cochlear implants: a preliminary report[END_REF][START_REF] Guyot | Milestones in the development of a vestibular implant[END_REF]. Although clinical testing of CVD patients detects gross vestibular behavioral defects, testing is not sufficiently sensitive to identify the cellular sites of the pathology, within the cristae, maculae, or specific neuron subsets of the central vestibular network. To this end, animal models offer the opportunity to evaluate the origins of CVD dysfunction at the cellular level. The cellular level of analysis may offer new insight into the type and amount of vestibular adaptation necessary to restore function in CVD patients.

This review is focused on two main CVD phenotypes that result from gene mutation. The most common phenotype is the sac-like inner ear with the vestibular sensory organs relatively intact, whereas the less frequent phenotype forms more discrete abnormalities, such as abnormal hair-cell stereocilia. Why are there two main phenotypes? Since genes are known to be active at specific stages of inner ear development, the time of gene action determines the type of malformation. By reference to the schedule for inner ear morphogenesis in a species, the time of gene action can be predicted [chick [START_REF] Bissonnette | Standard atlas of the gross anatomy of the developing inner ear of the chicken[END_REF](30)[START_REF] Bok | Patterning and morphogenesis of the vertebrate inner ear[END_REF]], [mouse [START_REF] Morsli | Development of the mouse inner ear and origin of its sensory organs[END_REF]], and [human [START_REF] O'reilly | Development of the vestibular system and balance function: differential diagnosis in the pediatric population[END_REF][START_REF] Toyoda | Morphogenesis of the inner ear at different stages of normal human development[END_REF][START_REF] Chacko | Growth and cellular patterning during fetal human inner ear development studied by a correlative imaging approach[END_REF]]. For example, the chick otocyst forms a sac-like inner ear at embryonic day 4 (E4) [START_REF] Bissonnette | Standard atlas of the gross anatomy of the developing inner ear of the chicken[END_REF]. By E6, the superior and posterior canals emerge, with lateral canal outgrowth 6 h later. The vestibular sensory organs form before canal outgrowth, with the superior and posterior cristae developing at E3-3.5, macula sacculi and lateral cristae at E3.5-4, basilar papilla at E4, and macula utriculi at E4.5 (30). Thus, a sac-like, inner ear pathology results from gene mutation pin-pointed to E4-4.5 to produce a sac-like inner ear with missing or truncated canals and macula utriculi and cochlea development primarily affected. Regarding CVDs that form defects in the stereocilia linkage, the mutation must occur after vestibular hair cell formation that takes place in the latter half of chick embryonic development: E10 for type II hair cells and E15 for type I hair cells [START_REF] Masetto | Membrane properties of chick semicircular canal hair cells in situ during embryonic development[END_REF]. Altogether, it is intriguing that most CVDs result from mutation of genes acting relatively early in gestation.

VESTIBULAR SYSTEM PATHOLOGY IN CVD PATIENTS AND CLINICAL FINDINGS

Histological preparations of postmortem specimens have provided the most comprehensive descriptions of human innerear pathology in CVDs [e.g., [START_REF] Haginomori | Temporal bone histopathology in CHARGE Association[END_REF]]. When inner ear structures are imaged in the clinic, the configuration of the bony labyrinths are demonstrated by computed tomography (CT), while gross details of the membranous labyrinths, fluid-filled inner-ear spaces, and vestibular nerves are seen using magnetic resonance imaging (MRI). Dysfunction in the vestibular system of CVD children can be investigated in the clinic by testing directly the vestibular reflex pathways. For example, ocular and cervical vestibular evoked myogenic potentials reveal utricular or saccular dysfunction, while bithermic caloric tests, earth vertical axis rotation, and head impulse tests demonstrate semicircular canal pathology [START_REF] Janky | Vestibular, visual acuity and balance outcomes in children with cochlear implants: a preliminary report[END_REF].

CVDs Forming a Sac-Like Inner Ear

Charge Syndrome

The vast majority of CVD cases in the literature are based on evaluations of CHARGE syndrome patients, with most studies containing one or a few patients. Like most CVDs, CHARGE syndrome has a low incidence (1/10,000 births) [e.g., [START_REF] Basson | Functional insights into chromatic remodeling from studies on CHARGE syndrome[END_REF]]. CHARGE syndrome patients develop a sac-like inner ear with the semicircular canals missing or truncated, and few defects in the cristae or maculae [START_REF] Haginomori | Temporal bone histopathology in CHARGE Association[END_REF]. Vestibular sensory organs contacted by the anterior vestibular nerve tend to show more abnormalities, with the utricular maculae often decreased in overall extent [START_REF] Haginomori | Temporal bone histopathology in CHARGE Association[END_REF]. Also, the cochlea may be shortened or absent. In some CHARGE syndrome patients, the auditory or vestibular ganglion is small or absent [START_REF] Chen | Otopathology in CHARGE syndrome[END_REF][START_REF] Guyot | The temporal bone anomaly in CHARGE association[END_REF][START_REF] Wright | Auditory and temporal bone abnormalities in CHARGE association[END_REF]. From vestibuloocular reflex (VOR) testing, combined with CT and MRI imaging, CHARGE syndrome patients present relatively normal otolith function, but lack canal activity [START_REF] Abadie | Vestibular anomalies in CHARGE syndrome: investigations on and consequences for postural development[END_REF][START_REF] Wiener-Vacher | Vestibular function in children with the CHARGE association[END_REF][START_REF] Joshi | CT and MRI imaging of the inner ear and brain with children with congenital sensorineural hearing loss[END_REF]. Clinically, CHARGE syndrome patients present with mild to profound hearing loss and balance deficits [START_REF] Hartshorne | Quality of life in adolescents adults with CHARGE syndrome[END_REF].

Waardenburg Syndrome

With the incidence at 1/42,000, this syndrome is one of the most common causes of congenital, syndromic deafness [START_REF] Asher | Newsletter of the National Institute of deafness and other communication disorders hereditary hearing impairment resource registry[END_REF]. Although clinically and genetically heterogeneous, the disorder is characterized by sensorineural hearing loss and pigmentation deficits [START_REF] Elmaleh-Bergès | Spectrum of temporal bone abnormalities in patients with Waardenburg syndrome and SOX10 mutations[END_REF]: Depending on symptoms, four subtypes are identified, with vestibular deficits present in WS2 and WS4, and SOX10 mutation linked to these two subtypes [START_REF] Hao | Key genes and pathways associated with inner ear in SOX10 p.R109W mutation pigs[END_REF]. Temporal bone scans reveal deformed, highly variable semicircular canals, enlarged vestibule, and small, flattened cochlea [START_REF] Elmaleh-Bergès | Spectrum of temporal bone abnormalities in patients with Waardenburg syndrome and SOX10 mutations[END_REF]. All three semicircular canals are truncated or absent. In one study, three patients lacked all three semicircular canals, while two other patients showed only unilateral canal loss [START_REF] Elmaleh-Bergès | Spectrum of temporal bone abnormalities in patients with Waardenburg syndrome and SOX10 mutations[END_REF]. In another study, the semicircular canals were absent bilaterally [START_REF] Sznajer | A de novo SOX10 mutation causing severe type 4 Waardenburg syndrome without Hirschsprung disease[END_REF]. While vestibular dysfunction in Waardenburg syndrome is not wellstudied in the clinic, one study did show that patients missing all three semicircular canals presented with complete loss of vestibular function, while other patients with one hypoplastic canal showed some vestibular function, suggesting that truncated

A/P, anterior/posterior extent; CT, computed tomography; MRI, magnetic resonance imaging; ND, not done; VG, vestibular ganglion cells. The table was ordered by first listing human CVDs: CHARGE, Waardenburg, Noonan, Wildervanck, Goldenhar, Branchio-oto-renal followed by animal models: mouse, chick. The most common phenotype found is listed for each reference.

Frontiers in Neurology | www.frontiersin.org canals and a sac-like inner ear are capable of generating and transmitting vestibular signals [START_REF] Elmaleh-Bergès | Spectrum of temporal bone abnormalities in patients with Waardenburg syndrome and SOX10 mutations[END_REF]. In a case study of 22 Waardenburg patients, those patients with symptoms of vertigo, dizziness or imbalance, showed abnormal VOR and vestibulospinal tests [START_REF] Black | A vestibular phenotype for Waardenburg syndrome?[END_REF].

Noonan Syndrome

Noonan syndrome (NS) is a relatively common genetic disorder (incidence: 1/1,000 to 1/2,500) [START_REF] Mendez | Noonan syndrome: a review[END_REF], producing multiple abnormalities, including short stature, broad neck, and developmental delays. NS results from an autosomal dominant gene mutation that occurs in multiple loci. In a report from a single NS patient, CT imaging revealed absence of the semicircular canals and VOR testing showed no measurable vestibular function [START_REF] Naficy | Multiple temporal bone anomalies associated with Noonan syndrome[END_REF].

Wildervanck Syndrome

Wildervanck syndrome, also called cervico-oculo-acoustic syndrome, is a rare congenital disorder (incidence, 1/1,000,000; Orphanet), characterized by fusion of the cervical vertebrae, oculomotor dysfunction causing horizontal gaze paralysis, and congenital deafness. Wildervanck syndrome almost exclusively affects females, since the hemizygous male is lethal [START_REF] Rihani | Cervico-oculo-acoustic (Wildervanck) syndrome: clinic radiological findings[END_REF]. In a study of one Wildervanck syndrome patient, both inner ears lacked all three semicircular canals, with the vestibule missing on one side [START_REF] Abu-Amero | Xq26.3 microdeletion in a male with Wildervanck syndrome[END_REF]. Recurrent attacks of dizziness are experienced in some Wildervanck patients.

Goldenhar Syndrome

Goldenhar syndrome, also known as oculo-auriculo-vertebral syndrome, is a congenital disorder (incidence, 1/3,500 to 1/5,600) [START_REF] Hennersdorf | Temporal bone changes in patients with Goldenhar syndrome with special emphasis on inner ear abnormalities[END_REF] in which the patients present with diverse malformations, with the most characteristic features ocular anomalies, such as microphthalmia, external, middle, and inner ear malformation, and vertebral defects. Less attention has been paid to the inner ear malformations in this syndrome. One study of 21 patients showed that 33% of patients presented with inner ear malformations, including cochlear hypoplasia, underdeveloped or missing semicircular canals, enlarged vestibular aqueduct, and enlarged internal auditory meatus [START_REF] Hennersdorf | Temporal bone changes in patients with Goldenhar syndrome with special emphasis on inner ear abnormalities[END_REF]. Other studies report a variable number of semicircular canals in one or both ears [START_REF] Bisdas | Inner ear abnormalities in patients with Goldenhar syndrome[END_REF][START_REF] Lemmerling | Unilateral semicircular canal aplasia in Goldenhar's syndrome[END_REF]. Since conductive and sensorineural hearing deficits are prominent in Goldenhar patients, clinics have focused on managing their hearing loss rather than addressing the vestibular defects.

Branchio-Oto-Renal Syndrome BOR syndrome (incidence, 1/40,000; Orphanet) is an autosomal dominant congenital disorder, resulting from mutation of SIX1 or EYA1 genes that affect development of the ear, kidney, and neck [for review, see [START_REF] Lindau | Anatomical changes and audiological profile in branchio-oto-renal syndrome: a literature review[END_REF]]. There is considerable variability in the presence, type, and severity of the clinical abnormalities, but hearing loss is the most prevalent symptom [START_REF] Morisada | Branchio-oto-renal syndrome: comprehensive review based on nationwide surveillance in Japan[END_REF]. CT and MRI of the temporal bone reveal inconsistent inner ear abnormalities, but cochlea hypoplasia or dysplasia, vestibular aqueduct enlargement, and absence or hypoplasia of the semicircular canals are reported most often [START_REF] Ceruti | Temporal bone anomalies in the branchio-oto-renal syndrome: detailed computed tomographic and magnetic resonance imaging findings[END_REF][START_REF] Propst | Temporal bone findings on computed tomography imaging in branchio-oto-renal syndrome[END_REF].

Evidence of cochleovestibular nerve malformation is also found [START_REF] Kari | A de novo SIX1 variant in a patient with a rare nonsyndromic cochleovestibular nerve abnormality, cochlear hypoplasia and bilateral sensorineural hearing loss[END_REF], but no descriptions are provided on the condition of the vestibular sensory organs or vestibular symptoms in patients.

CVDs Forming Diverse Vestibular Inner Ear Defects

Enlarged Vestibular Aqueduct Syndrome

Enlarged vestibular aqueduct syndrome (EVA) is a common congenital inner ear disorder (incidence, 1/100-1,300) [START_REF] Kodama | Postnatal development of the vestibular aqueduct and endolymphatic sac[END_REF][START_REF] Pinto | Enlarged vestibular aqueduct syndrome: report of 3 cases and literature review[END_REF].

Diagnosis is made primarily from CT and MRI scans that show an enlarged vestibular aqueduct or enlarged endolymphatic duct and sac, with no abnormalities apparent in the semicircular canals or vestibular sensory organs [START_REF] Ralli | Unilateral enlarged vestibular aqueduct syndrome and bilateral endolymphatic hydrops[END_REF]. EVA primarily produces mild to profound sensorineural hearing loss accompanied by mild imbalance or episodic vertigo [START_REF] Ralli | Unilateral enlarged vestibular aqueduct syndrome and bilateral endolymphatic hydrops[END_REF].

In a study of 106 patients, 45% presented with vestibular symptoms [START_REF] Zalewski | Vestibular dysfunction in patients with enlarged vestibular aqueduct[END_REF].

Otogelin Disorder

Otogelin is an extracellular, N-glycosylated protein composing the fibrillar network linking the tectorial membrane to the outer hair cells and anchoring otoconia and cupula to the underlying neuroepithelium [START_REF] Simmler | Targeted disruption of Otog results in deafness and severe imbalance[END_REF]. Mutation of the gene encoding otogelin causes autosomal-recessive, non-syndromic, moderate hearing loss associated with severe imbalance, delayed motor development, and dizziness [START_REF] Schraders | Mutations of the gene encoding otogelin are a cause of autosomalrecessive nonsyndromic moderate hearing impairment[END_REF][START_REF] Yariz | Mutations in OTOGL, encoding the inner ear protein otogelin-like, cause moderate sensorineural hearing loss[END_REF]. CT scans do not reveal inner ear abnormalities.

Usher Syndrome

Usher syndrome is the leading cause of combined visual and hearing loss, with vestibular function abnormal in certain genotypes [START_REF] Mathur | Usher syndrome and non-syndromic deafness: functions of different Whirlin isoforms in the cochlea, vestibular organs, and retina[END_REF]. The syndrome is clinically and genetically diverse, with a low incidence of 1.6-4.4/10,000 [START_REF] Kimberling | Frequency of Usher syndrome in two pediatric populations: implications for genetic screening of deaf and hard of hearing children[END_REF]. Hearing and vestibular dysfunction result from defective tip linkages connecting the hair-cell stereocilia that prevents sensory transduction [START_REF] Michalski | Harmonin-b, an actin-binding scaffold protein, is involved in the adaptation of mechanoelectrical transduction by sensory hair cells[END_REF]. There are three types of Usher syndrome. Type 1 patients have profound hearing loss and severe balance deficits resulting in delays in sitting and walking. Type 2 patients show moderate-to-severe hearing loss that may be accompanied by balance deficits. Type 3 patients show progressive hearing loss, retinal degeneration, and unpredictable vestibular dysfunction.

Pejvakin Disorder

PJVK is a protein expressed in hair and supporting cells of the inner ear and spiral ganglion neurons that has been shown to be necessary to maintain stereocilia structure in outer hair cells [START_REF] Kazmierczak | Pejvakin, a candidate stereociliary rootlet protein, regulates hair cell function in a cell-autonomous manner[END_REF]. In mice, PJVK mutation is linked to an autosomal recessive, non-syndromic hearing loss and balance deficits [START_REF] Cheng | Generation and pathological characterization of a transgenic mouse model carrying a missense PJVK mutation[END_REF].

Variability in PJVK mutations underlies the diverse phenotypes in these patients. Clinical reports routinely describe the nature of the hearing loss, but do not report on balance defects in these patients.

CVD ANIMAL MODELS CVD Animal Models Forming a Sac-Like Inner Ear Phenotype

Charge Syndrome

Since most CHARGE syndrome patients show heterozygous mutation of the chromodomain helicase DNA binding protein 7 gene CHD7, animals with heterozygous Chd7 mutation are a popular research model [START_REF] Lalani | Charge syndrome. In: GeneReviews (Internet)[END_REF]. At present, there are two animal models commonly used to study CHARGE syndrome, Chd7 mutant mice and Chd7 mutant zebrafish. The Chd7 gene influences many downstream genes involved in inner ear development [START_REF] Vuorela | Molecular analysis of the CHD7 gene in CHARGE syndrome: identification of 22 novel mutations and evidence for a low contribution of large CHD7 deletions[END_REF] that may explain the diversity of inner ear phenotypes. At present, the downstream genetics of CHARGE syndrome are not sufficiently known to generate a mutant animal model with a consistent inner ear phenotype. Chd7 mutant mice show diverse semicircular-canal malformations [START_REF] Adams | Defects in vestibular sensory epithelia and innervation in mice with loss of Chd7 function: implications for human CHARGE syndrome[END_REF]. The superior canal is often normal, while the lateral canal is absent or has a reduced diameter in diverse configurations [START_REF] Adams | Defects in vestibular sensory epithelia and innervation in mice with loss of Chd7 function: implications for human CHARGE syndrome[END_REF]. The posterior canal often has a reduced diameter. The lateral ampullae may have a small width. The cristae and maculae may develop abnormal shapes. Beside the normal saddle-shape, the lateral cristae may have a smaller width and the posterior cristae may appear with round-patch-like or flattened epithelium that lacks calycine endings and vestibular afferents. Like CHARGE syndrome patients, vestibular ganglia are small in Chd7 mutant mice [START_REF] Hurd | The ATPdependent chromatin remodeling enzyme CHD7 regulates pro-neural gene expression and neurogenesis in the inner ear[END_REF]. Finally, Chd7 mutant mice may form a small ocular lens and display cerebellar hypoplasia [START_REF] Gage | Mouse models for the dissection of CHD7 functions in eye development and the molecular basis for ocular defects in CHARGE syndrome[END_REF][START_REF] Feng | Chd7 is indispensable for mammalian brain development through activation of a neuronal differentiation program[END_REF]. Since the visual system and cerebellum interact intimately with the vestibular system, their pathology makes it difficult if not impossible to sort out the origin of vestibular deficits in Chd7 mutant mice. Chd7 mutant mice perform rapid, bidirectional circling movements, indicating bilateral defects in the lateral canal [START_REF] Adams | Defects in vestibular sensory epithelia and innervation in mice with loss of Chd7 function: implications for human CHARGE syndrome[END_REF].

Chd7 mutant zebrafish primarily show defects in the otoliths, forming either asymmetric otoliths or one irregular otolith [START_REF] Patten | Role of Chd7 in zebrafish: a model for CHARGE syndrome[END_REF]. The general size and shape of the semicircular canals are also abnormal. Thus, Chd7 mutation in zebrafish does not mirror the inner ear phenotype found in CHD7 patients.

Waardenburg Syndrome

Various gene mutations can produce WS, including mutation of PAX3, MITF, EDNRB, EDN3, and SOX10 genes [for review, see [START_REF] Elmaleh-Bergès | Spectrum of temporal bone abnormalities in patients with Waardenburg syndrome and SOX10 mutations[END_REF][START_REF] Du | Degeneration of saccular hair cells caused by MITF gene mutation[END_REF]]. For example, the panda pig is used to study MITF-M expression, which is mutated in WS type 2. Unlike the human disorder, this model loses vestibular hair cells in the saccule, without defects in the utricle or cristae [START_REF] Du | Degeneration of saccular hair cells caused by MITF gene mutation[END_REF].

Noonan Syndrome

NS belongs to a group of clinically-related developmental disorders that result from mutations in the RAS signaling pathway [for review, see [START_REF] Kang | The impact of RASopathy-associated mutations on CNS development in mice and humans[END_REF]]. While there are studies using RAS mutant mice to study NS, vestibular inner ear abnormalities have not been evaluated [e.g., [START_REF] Kang | The impact of RASopathy-associated mutations on CNS development in mice and humans[END_REF][START_REF] Hernández-Porras | K-RasV14I recapitulates Noonan syndrome in mice[END_REF]].

Wildervanck Syndrome

No studies have been reported on an animal model to study this disorder.

Goldenhar Syndrome

The gene ZYG11B, thought to be defective in Goldenhar syndrome, is under study in zebrafish [START_REF] Tingaud-Sequeira | Functional genetic analyses of ZYG11B provide evidences for its involvement in OAVS[END_REF].

BOR Syndrome

Six1 heterozygous mutant mice have been used to study inner ear malformation in BOR syndrome [START_REF] Zheng | The role of Six1 in mammalian auditory system development[END_REF]. Unlike the human disorder, the mutant mouse model lacks all the sensory organs in the inner ear, along with a malformed saccule, no posterior ampulla, and truncated or missing posterior semicircular canal.

A Universal Model for the Sac-Like Inner Ear Phenotype

The chick has a long and distinguished history in biomedical research as a model to study inner ear and central nervous system development. To further advance understanding of the sac-like inner ear pathology in CVD patients, we designed and implemented a CVD animal model by surgically rotating the developing inner ear, or otocyst, in 2-day-old chick embryos (E2) [START_REF] Lilian | A new model for congenital vestibular disorders[END_REF][START_REF] Seal | Implementing the chick embryo model to study vestibular developmental disorders[END_REF]. The right otocyst can be readily accessed in E2 chick embryos in ovo for microsurgical manipulation (Figure 1A). Anterior-posterior axis Rotation of the Otocyst 180 • produces a sac-like inner ear on the right side, called the "ARO/s chick." This model is reproducible in 85% of cases, forming a saclike inner ear that mirrors the common inner ear pathology found in many CVD patients. Thus, the ARO/s chick develops without introducing genetic mutations that affect other targets beside the inner ear. Altogether, the ARO/s chick offers a highly tractable model to study the deterministic connections between the malformed inner ear and vestibular neural circuitry abnormalities/plasticity. Like CVD children, ARO/s hatchlings experience balance and walking problems (Figure 1B).

By E13, the chick semicircular canals, vestibular sensory organs, and certain vestibular nuclei neurons have differentiated so that they provide identifiable experimental targets [START_REF] Bissonnette | Standard atlas of the gross anatomy of the developing inner ear of the chicken[END_REF][START_REF] Shao | Maturation of firing pattern in chick vestibular nucleus neurons[END_REF][START_REF] Shao | Emergence of action potential generation and synaptic transmission in vestibular nucleus neurons[END_REF]. The membranous labyrinths can be imaged using opaque paint to view canal size and cochlea configuration compared to the normal counterparts [START_REF] Seal | Implementing the chick embryo model to study vestibular developmental disorders[END_REF]. ARO/s chicks form a sac-like inner ear with the semicircular canals missing or truncated [START_REF] Lilian | A new model for congenital vestibular disorders[END_REF]. Nisslstained, serial tissue sections show that the vestibular sensory organs associated with the anterior vestibular nerve are more often affected in ARO/s chicks [START_REF] Lilian | A new model for congenital vestibular disorders[END_REF], like CVD patients [START_REF] Haginomori | Temporal bone histopathology in CHARGE Association[END_REF]. In preliminary studies of the vestibular ganglion (VG), VG cell number is reduced to 62% on the rotated side compared to the non-rotated side, with vestibular ganglion cell number on the non-rotated side similar to controls [START_REF] Peusner | The chick embryo model for biomedical research on congenital vestibular disorders[END_REF]. The advantages of the ARO/s chick model include a reliable source of embryos with well-characterized phenotype, utility of the non-rotated inner ear as an internal control, and lack of pathology in related neural systems that could convolute the analysis and interpretation of the effects of the sac-like inner ear pathology on vestibular neural circuitry development. In addition, the tangential vestibular nucleus (TN) is a major vestibular nucleus whose principal cells FIGURE 1 | (A) E2 chick in ovo viewed through a shell window under a stereo dissecting microscope (Zeiss Discovery.V8) after otocyst rotation. Fast green dye was injected to improve otocyst visibility. After tearing open the chorion (Ch) and amnion with forceps, the otocyst was cut free from the surrounding epithelium with a tungsten needle (10 µm tip diameter; curved tip) and glided posteriorly along the epithelial surface before rotating it 180 • in the anterior-posterior and dorsal-ventral axes. The rotated otocyst was returned to the epithelial slot. Note that the endolymphatic duct (ED) normally situated on the dorsal surface of the otocyst is located on the ventral surface after rotation. The shell window was sealed with tape and the egg reincubated at high humidity (70%) without egg turning ( 75). E, eye; h, heart. (B) Five-day-old hatchling ARO/s chick. Note the widened base of the feet after performing the righting reflex, indicating stress placed on the balance system [START_REF] Lilian | A new model for congenital vestibular disorders[END_REF].

are vestibular reflex neurons. In E13 ARO/s chicks, the number of principal cells on the rotated side was reduced to 35% of the normal number in control chicks [START_REF] Lilian | A new model for congenital vestibular disorders[END_REF]. This indicates that the sac-like inner ear pathology produces major developmental abnormalities in the downstream vestibular reflex pathways. Finally, the vestibular behaviors observed in 5-day-old hatchling (H5) ARO/s chicks are consistent with a unilateral vestibular deficit [START_REF] Aldrich | Vestibular compensation after ganglionectomy: ultrastructural study of the tangential vestibular nucleus and behavioral study of the hatchling chick[END_REF]. ARO/s chicks with a right, sac-like inner ear tilt their head to the right, but do not display eye deviations or nystagmus at rest. However, after performing the righting reflex, ARO/s hatchlings stumble and close their eyes, in contrast to the normal hatchlings. After performing the righting reflex, ARO/s hatchlings stand with a widened base, suggesting balance deficits (Figure 1B) [START_REF] Lilian | A new model for congenital vestibular disorders[END_REF].

CVD Animal Models Forming Diverse Vestibular Inner Ear Abnormalities

EVA Syndrome

Mutation of SLC26A4 gene in mice results in profound hearing loss, vestibular abnormalities, and enlarged endolymphatic spaces throughout the inner ear [START_REF] Everett | Targeted disruption of mouse Pds provides insight about the innerear defects encountered in Pendred syndrome[END_REF]. In the utricle and saccule, the maculae are degenerated with the near complete absence of the otoconia. Unlike the human disorder, macular hair cells in the mouse model continue to degenerate as postnatal development progresses.

Otogelin

Rock solo AN66 is a zebrafish mutant of the glycoprotein otogelin [START_REF] Roberts | Delayed otolith development does not impair vestibular circuit formation in zebrafish[END_REF]. It is interesting that balance and postural deficits present in young mutant larvae later disappear with no permanent vestibular deficits, unlike the human disorder.

Usher Syndrome

Ush 1c mutant mice are defective for harmonin-b, a protein located in the tip links of hair-cell stereocilia [START_REF] Michalski | Harmonin-b, an actin-binding scaffold protein, is involved in the adaptation of mechanoelectrical transduction by sensory hair cells[END_REF]. Vestibular sensory evoked potentials are absent, indicating malfunction primarily in the utricular macula [START_REF] Vijayakumar | Rescue of peripheral vestibular function in Usher syndrome mice using a splice-switching antisense oligonucleotide[END_REF]. In another Usher 1 mutant mouse model, SANS, the defect occurs in a protein involved in stereocilia differentiation and VOR testing reveals that canal and otolith activity are absent [START_REF] Emptoz | Local gene therapy durably restores vestibular function in a mouse model of Usher syndrome type 1G[END_REF].

Pejvakin Syndrome

Several Pjvk-mutant mice models have been produced that show phenotypic variability [START_REF] Kazmierczak | Pejvakin, a candidate stereociliary rootlet protein, regulates hair cell function in a cell-autonomous manner[END_REF]. In studies of Pjvk-flox mutation, pejvakin was found to be localized to the base of the stereocilia rootlets in cochlear outer hair cells [START_REF] Cheng | Generation and pathological characterization of a transgenic mouse model carrying a missense PJVK mutation[END_REF]. Mice develop normal rootlets, but the stereociliary bundles and mechanotransduction are defective, resulting in deafness. In a study using Pjvk G292R mutant mice, progressive hearing loss occurs, with vestibular dysfunction indicated by head tilt, circling behavior, and abnormal swimming and balance [START_REF] Kazmierczak | Pejvakin, a candidate stereociliary rootlet protein, regulates hair cell function in a cell-autonomous manner[END_REF]. Fluorescence microscopy shows that the vestibular hair cells are intact, but the vestibular ganglion neurons are lost progressively.

DISCUSSION

Benefits and Limitations of CVD Animal Models

Animal models offer living, non-human subjects to understand human disease without compromising human life. Often, animal models are genetically engineered. Despite ortholog gene mutations, most animal models do not accurately replicate the human pathology because disease phenotypes result from complex interactions between genes and environment. Indeed, a phenotype does not result from disrupting one gene, but a cascade of genes whose dosage of alleles may vary [for review [START_REF] Haordouin | Mouse models for human disease[END_REF]]. Nonetheless, many advantageous drugs, treatments, and cures for human diseases have resulted from using animal models.

Animal models are most advantageous when they share the same cause and symptoms for a disease as the patients with the disease. Alternately, there are animal models that share the same symptoms, but not the same etiology, for example, the ARO/s chick. ARO/s chicks are formed by microsurgical manipulation of the otocyst rather than genetic mutation, but subsequent development of the animal model produces a similar sac-like inner ear phenotype with minimally-disrupted, vestibular sensory organs that parallels the human phenotype found in many CVDs. Finally, human genetic disorders show considerable variability in the presence, type, and severity of the abnormalities, even among family members with the same mutation, so that a specific mutation is not associated with a similar clinical presentation [e.g., [START_REF] Chang | Branchio-Oto-Renal syndrome: the mutation spectrum in EYA1 and its phenotypic consequences[END_REF]]. This holds true for animal models, as seen in the diversity of inner ear phenotypes found in Chd7 mutant mice used to study CHARGE syndrome.

Is There an Optimal Window for Therapeutic Intervention?

Genetic testing for potential inherited mutations can be performed early in fetal development so that therapeutic intervention can be initiated prenatally, possibly before vestibular dysfunction emerges [START_REF] Hastings | Fetal gene therapy and pharmacotherapy to treat congenital hearing loss and vestibular dysfunction[END_REF]. The time for treatment depends on the time when the pathology appears, which can be assayed in an animal model and then extrapolated to the equivalent human gestational age. Treatment delivered locally in the postnatal inner ear avoids potential toxic effects of systemic delivery [e.g., [START_REF] Emptoz | Local gene therapy durably restores vestibular function in a mouse model of Usher syndrome type 1G[END_REF]]. In addition, our chick model offers easy access to the embryonic inner ear in ovo without compromising continued development. The fluidfilled, developing inner ear also provides a suitable vehicle to inject drugs prenatally to determine whether balance can be rescued. After hatching, balance can be tested in the ARO/s hatchling [START_REF] Lilian | A new model for congenital vestibular disorders[END_REF].

What Is the Underlying Pathophysiology of CVDs?

Why is canal function lost on clinical VOR testing of CVD children? Canal sensation relies on endolymph fluid movement in the canal to convert angular head movements into displacement of the stereocilia on the apical surface of the vestibular hair cells. Endolymph displacement on head movements depends on key geometric features of the canals to maintain mechanical sensitivity, including canal radius and canal length [START_REF] Rabbitt | Semicircular canal biomechanics in health and disease[END_REF]. During Xenopus development, rotatory head movements generate signals in the crista only after the semicircular canals have acquired a minimal diameter, despite the presence of functional activity in other components of the vestibular reflex circuitry, including the hair cells, central vestibular pathways, and extraocular eye muscles [START_REF] Lambert | Semicircular canal size determines the developmental onset of angular vestibuloocular reflexes in larval Xenopus[END_REF]. Blocking individual semicircular canals, or "canal plugging, " is an approach that was used to determine the role of individual canals [START_REF] Ewald | Physiologische Untersuchungen Uber das Endorgan des Nervus octavus[END_REF]. At low rotational frequencies (<2 Hz), endolymph movement is reduced greatly in a plugged canal, with only minimal hair-cell stimulation in the blocked canal, but normal function preserved in the other canals and otoliths [START_REF] Carey | Semicircular canal function before and after surgery for superior canal dehiscence[END_REF][START_REF] Sadeghi | Effects of canal plugging on the vestibuloocular reflex and vestibular nerve discharge during passive and active head rotations[END_REF]. At present, canal plugging is a surgical approach performed on patients to alleviate vestibular symptoms resulting from canal dehiscence. To what extent does the sac-like inner ear, with or without truncated canals, affect vestibular signaling? Does synaptic plasticity in the peripheral or central vestibular neural circuitry impact vestibular signaling in CVD patients? These are some questions for animal models to address that may translate into clinical practice.

There have been no systematic studies of connectivity in the peripheral or central vestibular neural circuitry of CVD animal models. The ARO/s chick is now available to interrogate the anatomical and functional modifications, since the model is reproducible and recapitulates salient features found in CVD patients [START_REF] Lilian | A new model for congenital vestibular disorders[END_REF][START_REF] Seal | Implementing the chick embryo model to study vestibular developmental disorders[END_REF]. Developmental studies have been performed on normal and vestibular ganglionectomized chicks at critical pre-and post-natal ages [START_REF] Shao | Maturation of firing pattern in chick vestibular nucleus neurons[END_REF][START_REF] Shao | Emergence of action potential generation and synaptic transmission in vestibular nucleus neurons[END_REF][START_REF] Aldrich | Vestibular compensation after ganglionectomy: ultrastructural study of the tangential vestibular nucleus and behavioral study of the hatchling chick[END_REF][START_REF] Popratiloff | Otolith fibers and terminals in chick vestibular nuclei[END_REF][START_REF] Shao | Adaptation of chicken vestibular nucleus neuron to unilateral vestibular ganglionectomy[END_REF][START_REF] Shao | Plasticity of spontaneous excitatory and inhibitory synaptic activity in morphologically defined vestibular nuclei neurons during early vestibular compensation[END_REF][START_REF] Shao | Presynaptic GABAB receptors decrease neuro-transmitter release in vestibular nuclei neurons during vestibular compensation[END_REF]. TN principal cells offer a structurally-uniform subset of vestibular reflex projection neurons to determine whether the orderly ingrowth of the canal and otolith fibers occurs and whether passive and active membrane properties emerge within the normal time frame during ARO/s chick development. Is normal synaptic transmission conserved in principal cells of ARO/s chicks, or do major changes occur in the spontaneous excitatory and inhibitory synaptic events (93-95)? VOR testing using Earth vertical axis rotation for canal function and static tilting platform for otolith function has been performed on animal models and offers a powerful research tool to evaluate potential therapeutic intervention [START_REF] Beraneck | Ontogeny of mouse vestibulo-ocular reflex following genetic or environmental alteration of gravity sensing[END_REF][START_REF] Simon | Surgical techniques functional evaluation for vestibular lesions in the mouse: unilateral labyrinthectomy (UL) unilateral vestibular neurectomy (UVN)[END_REF]. Altogether, the outcome of animal model experiments could provide novel insights into the consequences of CVDs on the development, maintenance, and plasticity of the vestibular neural circuitry that will modify our thinking on how to treat the disorders. This includes insights into whether the neural circuitry may benefit from vestibular implant technology or specific pharmacological treatments [START_REF] Guyot | Milestones in the development of a vestibular implant[END_REF][START_REF] Strupp | Current treatment of vestibular, ocular motor disorders and nystagmus[END_REF][START_REF] Lewis | Vestibular implants studied in animal models: clinical and scientific implications[END_REF](100).

first draft of the review. All authors contributed to the article and approved the submitted version.

TABLE 1 |

 1 Sac-like phenotype in human congenital vestibular disorders and animal models.

	Syndrome	Approach	Inner ear	Canals	Cristae	Maculae	Calyces	VGs	Cochlea	Reference
			shape		Ampullae
	CHARGE	Human; n = 37; CT, MRI	Sac	Absent; superior	ND	ND	ND	ND	ND	(2)
				NM may be truncated
	CHARGE	Human; n = 1; CT,	Sac	Absent/	Ampullae	Atrophy,	ND	N ND	Aud ganglion	(3)
		histopathology		truncated	absent	utricle, saccule			reduced to 35%
	CHARGE	Human; n = 107; CT	Sac	Absent	ND	ND	ND	ND	ND	(4)
	CHARGE	Human; n = 42	Sac	Absent	ND	ND	ND	ND	Short	(5)
	CHARGE	Human; n = 18; CT	Sac	Absent/truncated	ND	ND	ND	ND	Short	(6)
	CHARGE	Human; n = 4; CT	Sac	Absent	ND	ND	ND	ND	ND	(7)
	CHARGE	Human; n = 13; CT	Sac	Absent	ND	ND	ND	ND	Short	(8)
	CHARGE	Human; n = 1;	Sac	Absent	Normal;	Utricle small;	ND	ND	Short;	(9) case 1
		histopathology			no ampullae	saccule normal			no cochlea nerve
									or ganglion
	CHARGE	Human; n = 1; CT	Sac	Absent	ND	ND	ND		Short	(10)
	CHARGE	Human; n = 12; CT	Sac	Absent	ND	ND	ND	ND	ND	(11)
	CHARGE	Human; n = 7;	Sac	Absent	ND	ND	ND	ND	Short	(12)
		CT; EVAR; OVAR
	CHARGE	Human, n = 1;	Sac	Absent	No cristae	Utricle absent	ND	Few	Short	(13)
		histopathology			No ampullae	saccule normal		VG
	CHARGE	Human; n = 1;	Sac	Absent	ND; no	utricle small saccule	ND	Few	Short	(14) Case 4
		histopathology			ampullae	reduced		VGs
	Waardenburg	Human; n = 151; CT; MRI	Sac	Absent or truncated	ND	ND	ND	ND	Length normal;	(15)
									width small and
									flat
	Noonan	Human; n = 1; CT	Sac	Absent	ND	ND	ND	ND	Short	(16)
	Wildervanck	Human; n = 1; MRI	Sac	Absent, truncated post	ND	ND	ND	ND	Short	(17)
				canal
	Goldenhar	Human; n = 5; CT	Sac	Absent/truncated	ND	ND	ND	ND	ND	(18)
	Goldenhar	Human; n = 1; CT and MRI Sac	Absent	ND	ND	ND	ND	Short	(19)
	Branchio-oto-	Human;	Sac	Absent	ND	ND	ND	ND	Short	(20)
	renal	n = 32; MRI
	Chd7 mutant	Mouse; fast green: n = 16;	Variable	Highly	Superior and	normal	present	ND	ND	(21)
		neuro-filament and f-actin;		variable/present/missing/	lateral normal;
		n = 31		truncated	posterior
					patch-like
	ARO/s	Chicken; paint-fill, n = 25;	Sac	Missing/truncated	Sup crista	utricle short A/P;	present	VGs reduced	Often missing
		Nissl sections, n = 8			short A/P	saccule normal		to 62%

Frontiers in Neurology | www.frontiersin.org

September 2021 | Volume 12 | Article 708395

FUNDING This work was supported in part by NIH grant R01 DC019369 (KP), The George Washington University School of Medicine and Health Sciences Research Funds (KP), Robert Vincent Research Fellowship of the Columbian College of Arts and Sciences of The George Washington University (NB), Centre National de la Recherche Scientifique (MB), Université de Paris (MB), IdEx Université de Paris ANR-18-IDEX-0001 (MB), and Intellectual and Developmental Disabilities Research Center [NICHD Grant U54-HD090257 (AP)].

AUTHOR CONTRIBUTIONS

KP is responsible for the conception of the review. KP, NB, JH, MB, and AP are responsible for the design of the review and contributed to manuscript revision, read, and approved the submitted version. KP and NB are responsible for writing the

Conflict of Interest:

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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