Postural Instability in Subjects With Usher Syndrome

Characterization of Vestibular Phenotypes in Patients with Genetic Hearing Loss

Background: The vestibular phenotypes of patients with genetic hearing loss are poorly understood. Methods: we performed genetic testing including exome sequencing and vestibular function tests to investigate vestibular phenotypes and functions in patients with genetic hearing loss. Results: Among 627 patients, 143 (22.8%) had vestibular symptoms. Genetic variations were confirmed in 45 (31.5%) of the 143 patients. Nineteen deafness genes were linked with vestibular symptoms; the most frequent genes in autosomal dominant and recessive individuals were COCH and SLC26A4, respectively. Vestibular symptoms were mostly of the vertigo type, recurrent, and persisted for hours in the genetically confirmed and unconfirmed groups. Decreased vestibular function in the caloric test, video head impulse test, cervical vestibular-evoked myogenic potential, and ocular vestibular-evoked myogenic potential was observed in 42.0%, 16.3%, 57.8%, and 85.0% of the patients, respectively. The caloric test revealed a significantly higher incidence of abnormal results in autosomal recessive individuals than in autosomal dominant individuals (p = 0.011). The genes, including SLC26A4, COCH, KCNQ4, MYH9, NLRP3, EYA4, MYO7A, MYO15A, and MYH9, were heterogeneously associated with abnormalities in the vestibular function test. Conclusions: In conclusion, diverse vestibular symptoms are commonly concomitant with genetic hearing loss and are easily overlooked.

Balance control impairments in Fabry disease

Background

Fabry disease (FD) is a rare inherited lysosomal storage disorder caused by the deficiency of the enzyme alpha-galactosidase A. This deficiency leads to an accumulation of glycosphingolipids leading to progressive and multisystemic disease, including renal, cardiac, and neurological damages. FD may also have neuro-otological and visual impairments, which can generate postural control alterations, inner ear, and vision being involved in this function. This study aimed to evaluate the impact of FD on postural control.

Methods

In total, fourteen adult patients (8 men/6 women, mean age = 37.6 ± 11.4 years) and two children (mean age = 11 years) with FD and 19 healthy adults (12 men/7 women, mean age = 36.5 ± 16.9 years) and two healthy children (mean age = 10.5 years) took part in this study. Postural control was evaluated by a sensory organization test combining three visual situations (eyes open, eyes closed, and sway referenced visual surround motion) with two platform situations (stable platform and sway referenced platform motion), aiming to calculate a composite equilibrium score (CES), a high score being representative of good postural control. Somatosensory (R^SOM), visual (R^VIS), and vestibular (R^VEST) contributions to postural control were calculated, a low score reflecting a poor use of the indicated sensory input.

Results

The CES was lower in adult patients with FD compared with the healthy subjects (p < 0.001). R^VIS (p = 0.001) and R^VEST (p = 0.003) were lower in patients with FD compared with the control group, whereas no difference in R^SOM was observed.

Conclusion

Inner ear and visual pathologies associated with the central nervous system impairments are factors of postural control impairments. Physical activities, which can also be rehabilitative, by maintaining or increasing the weight of proprioception, may help diminish dependency on altered sensorial inputs.

Genetics, pathogenesis and therapeutic developments for Usher syndrome type 2

Usher syndrome (USH) is a rare, autosomal recessively inherited disorder resulting in a combination of sensorineural hearing loss and a progressive loss of vision resulting from retinitis pigmentosa (RP), occasionally accompanied by an altered vestibular function. More and more evidence is building up indicating that also sleep deprivation, olfactory dysfunction, deficits in tactile perception and reduced sperm motility are part of the disease etiology. USH can be clinically classified into three different types, of which Usher syndrome type 2 (USH2) is the most prevalent. In this review, we, therefore, assess the genetic and clinical aspects, available models and therapeutic developments for USH2. Mutations in USH2A, ADGRV1 and WHRN have been described to be responsible for USH2, with USH2A being the most frequently mutated USH-associated gene, explaining 50% of all cases. The proteins encoded by the USH2 genes together function in a dynamic protein complex that, among others, is found at the photoreceptor periciliary membrane and at the base of the hair bundles of inner ear hair cells. To unravel the pathogenic mechanisms underlying USH2, patient-derived cellular models and animal models including mouse, zebrafish and drosophila, have been generated that all in part mimic the USH phenotype. Multiple cellular and genetic therapeutic approaches are currently under development for USH2, mainly focused on preserving or partially restoring the visual function of which one is already in the clinical phase. These developments are opening a new gate towards a possible treatment for USH2 patients.

Changes in plantar load distribution in legally blind subjects

We investigated the impact of visual impairment on balance control. We measured the center of pressure (COP) between the two feet and plantar surface pressures on each foot in 18 normal-sighted participants and compared their data with measures from 18 legally blind participants, either acquired or congenital. Pressures were measured in open- and closed-eye conditions using a baropodometric resistive plate. In the eyes-open condition, there were no differences between the sighted and legally blind groups in COP displacement. However, participants with visual loss had significantly increased pressures in two metatarsal regions (M1 and M2 zones) of the plantar surface in both viewing conditions (p < 0.05). The differences in pressure measures between the normally sighted and legally blind groups could be attributed mainly to the subgroup of subjects with acquired impairment. Our findings suggest that subjects with visual impairment present increased metatarsal pressures (i.e. forefoot), not yet associated to anterior displacement of COP or impaired balance control.

Vestibular phenotype-genotype correlation in a cohort of 90 patients with Usher syndrome

Usher syndrome has been historically categorized into one of three classical types based on the patient phenotype. However, the vestibular phenotype does not infallibly predict which Usher genes are mutated. Conversely, the Usher syndrome genotype is not sufficient to reliably predict vestibular function. Here we present a characterization of the vestibular phenotype of 90 patients with clinical presentation of Usher syndrome (59 females), aged 10.9 to 75.5 years, with genetic variants in eight Usher syndromic genes and expand the description of atypical Usher syndrome. We identified unexpected horizontal semicircular canal reactivity in response to caloric and rotational stimuli in 12.5% (3 of 24) and 41.7% (10 of 24), respectively, of our USH1 cohort. These findings are not consistent with the classical phenotypic definition of vestibular areflexia in USH1. Similarly, 17% (6 of 35) of our cohort with USH2A mutations had saccular dysfunction as evidenced by absent cervical vestibular evoked myogenic potentials in contradiction to the classical assumption of normal vestibular function. The surprising lack of consistent genotypic to vestibular phenotypic findings as well as no clear vestibular phenotypic patterns among atypical USH cases, indicate that even rigorous vestibular phenotyping data will not reliably differentiate the three USH types.

Usher syndrome and Nebulin-associated myopathy in a single patient due to variants in MYO7A and NEB

In a patient with Usher syndrome and atypical muscle complaints, we have identified two separate variants in MYO7A andNEB genes by exome sequencing. The homozygous variants in these two recessive genes could explain the full phenotype of our patient.

Vestibular impact of Friedreich ataxia in early onset patients

Background

Friedreich ataxia (FRDA) is the most frequent form of inherited ataxias. Vestibular and auditory assessments are not commonly part of the check up for these patients despite hearing and balance complaints. Screening of vestibular and auditory function was performed in a large group of young patients with genetically confirmed FRDA.

Methods

Our study included 43 patients (7–24 years of age). A complete vestibular assessment was performed including the canals function evaluation at 3 head velocities (bithermal caloric test, earth vertical axis rotation (EVAR) and head impulse test (HIT)) and otolith function evaluation (cervical vestibular evoked myogenic potentials). Information regarding the hearing evaluation of the patients were also retrieved including impedance tympanometry, distortion product otoacoustic emissions (DPOAEs), air and bone conduction audiometry and auditory brainstem response (ABR).

Results

Vestibular responses were impaired for canal responses (only at high and middle head velocities) and vestibulospinal otolithic responses. Abnormal neural conduction in the central auditory pathways was frequently observed. Oculomotor abnormalities were frequent, mostly hypermetric saccades and gaze instability. Inhibition of the vestibulo-ocular reflex by fixation was normal.

Conclusions

We show that Friedreich ataxia, even at onset, frequently associate saccadic intrusions, abnormal ABRs and decreased vestibulo-ocular and vestibulospinal responses progressing over time. These sensory impairments combined with ataxia further impair patient’s autonomy. These vestibular, auditory and visual impairments could be used as markers of the severity and progression of the disease. Adding vestibular and auditory testing to Friedreich patient’s evaluation may help physicians improve patient’s management.