Auditory dysfunction in Friedreich ataxia: result of spiral ganglion degeneration

Oculomotor and Vestibular Deficits in Friedreich Ataxia - Systematic Review and Meta-Analysis of Quantitative Measurements

Disease-specific oculomotor assessments play a crucial role in the early diagnosis of hereditary cerebellar ataxias. Whereas several studies have reported on quantitative oculomotor and vestibular measurements in Friedreich's Ataxia (FRDA), the value of specific oculomotor paradigms remains unclear. We aimed to address this knowledge gap through a systematic literature review and providing disease-specific recommendations for a tailored set of eye-movement recordings in FRDA. MEDLINE and Embase were searched for studies reporting on quantitative oculomotor and/or vestibular measurements in FRDA-patients. Data on oculomotor and vestibular parameters were extracted and correlations with a range of clinical parameters were sought. Included studies (n = 17) reported on 185 patients. Abnormalities observed included the presence of saccadic intrusions (143/161) such as square-wave jerks (SWJ, 90/109) and ocular flutter (21/43), impaired eccentric gaze-holding (40/104), abnormal pursuit (81/93) and angular vestibulo-ocular reflex (aVOR) deficits (39/48). For visually-guided saccades (VGS), we frequently observed increases in saccade latency (27/38) and dysmetric saccades (71/93), whereas saccade velocity was more often preserved (37/43). Augmented anti-saccade (AS) latency, downbeat nystagmus and frequent macro-SWJ correlated with disease duration. Increased AS-latency and VGS-latency, frequent macro-SWJ, reduced aVOR-gain and augmented aVOR peak-latency correlated with disease severity. A broad range of oculomotor and vestibular deficits are documented in the literature. Impairments in pursuit, saccades and aVOR-responses are most commonly reported, and as such, should be prioritized as disease markers. Quantitative oculomotor testing in FRDA may facilitate early diagnosis and prove valuable in monitoring disease progression and treatment response.

Auditory neuropathy in mice and humans with Friedreich ataxia

OBJECTIVE

Recent studies have found that human Friedreich ataxia patients have dysfunction of transmission in the auditory neural pathways. Here, we characterize hearing deficits in a mouse model of Friedreich ataxia and compare these to a clinical population.

METHODS

Sixteen mice with a C57BL/6 background were evaluated. Eight were YG8Pook/J animals (Friedreich ataxia phenotype) and eight wild-type mice served as controls. Auditory function was assessed between ages 6 and 12 months using otoacoustic emissions and auditory steady-state responses. At study end, motor deficit was assessed using Rotorod testing and inner ear tissue was examined. Thirty-seven individuals with Friedreich ataxia underwent auditory steady-state evoked potential assessment and response amplitudes were compared with functional hearing ability (speech perception-in-noise) and disease status was measured by the Friedreich Ataxia Rating Scale.

RESULTS

The YG8Pook/J mice showed anatomic and functional abnormality. While otoacoustic emission responses from the cochlear hair cells were mildly affected, auditory steady-state responses showed exaggerated amplitude reductions as the animals aged with Friedreich ataxia mice showing a 50-60% decrease compared to controls who showed only a 20-25% reduction (F(2,94)  = 17.90, p < 0.00). Furthermore, the YG8Pook/J mice had fewer surviving spiral ganglion neurons, indicating greater degeneration of the auditory nerve. Neuronal density was 20-25% lower depending on cochlear region (F(1, 30)  = 45.02, p < 0.001). In human participants, auditory steady-state response amplitudes were correlated with both Consonant-Nucleus-Consonant word scores and Friedreich Ataxia Rating Scale score.

INTERPRETATION

This study found degenerative changes in auditory structure and function in YG8Pook/J mice, indicating that auditory measures in these animals may provide a model for testing Friedreich ataxia treatments. In addition, auditory steady-state response findings in a clinical population suggested that these scalp-recorded potentials may serve as an objective biomarker for disease progress in affected individuals.

Dimethyl fumarate dose-dependently increases mitochondrial gene expression and function in muscle and brain of Friedreich's ataxia model mice

Previously we showed that dimethyl fumarate (DMF) dose-dependently increased mitochondrial gene expression and function in cells and might be considered as a therapeutic for inherited mitochondrial disease, including Friedreich's ataxia (FA). Here we tested DMF's ability to dose-dependently increase mitochondrial function, mitochondrial gene expression (frataxin and cytochrome oxidase protein) and mitochondrial copy number in C57BL6 wild-type mice and the FXNKD mouse model of FA. We first dosed DMF at 0-320 mg/kg in C57BL6 mice and observed significant toxicity above 160 mg/kg orally, defining the maximum tolerated dose. Oral dosing of C57BL6 mice in the range 0-160 mg/kg identified a maximum increase in aconitase activity and mitochondrial gene expression in brain and quadriceps at 110 mg/kg DMF, thus defining the maximum effective dose (MED). The MED of DMF in mice overlaps the currently approved human-equivalent doses of DMF prescribed for multiple sclerosis (480 mg/day) and psoriasis (720 mg/day). In the FXNKD mouse model of FA, which has a doxycycline-induced deficit of frataxin protein, we observed significant decreases of multiple mitochondrial parameters, including deficits in brain mitochondrial Complex 2, Complex 4 and aconitase activity, supporting the idea that frataxin deficiency reduces mitochondrial gene expression, mitochondrial functions and biogenesis. About 110 mg/kg of oral DMF rescued these enzyme activities in brain and rescued frataxin and cytochrome oxidase expression in brain, cerebellum and quadriceps muscle of the FXNKD mouse model. Taken together, these results support the idea of using fumarate-based molecules to treat FA or other mitochondrial diseases.

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.

Altered neocortical tactile but preserved auditory early change detection responses in Friedreich ataxia

OBJECTIVE

To study using magnetoencephalography (MEG) the spatio-temporal dynamics of neocortical responses involved in sensory processing and early change detection in Friedreich ataxia (FRDA).

METHODS

Tactile (TERs) and auditory (AERs) evoked responses, and early neocortical change detection responses indexed by the mismatch negativity (MMN) were recorded using tactile and auditory oddballs in sixteen FRDA patients and matched healthy subjects. Correlations between the maximal amplitude of each response, genotype and clinical parameters were investigated.

RESULTS

Evoked responses were detectable in all FRDA patients but one. In patients, TERs were delayed and reduced in amplitude, while AERs were only delayed. Only tactile MMN responses at the contralateral secondary somatosensory cortex were altered in FRDA patients. Maximal amplitudes of TERs, AERs and tactile MMN correlated with genotype, but did not correlate with clinical parameters.

CONCLUSIONS

In FRDA, theamplitude of tactile MMN responses at SII cortex are reduced and correlate with the genotype, whileauditory MMN responses are not altered.

SIGNIFICANCE

Somatosensory pathways and tactile early change detection are selectively impaired in FRDA.

The importance of central auditory evaluation in Friedreich's ataxia

Objective To assess central auditory function in Friedreich's ataxia. Methods A cross-sectional, retrospective study was carried out. Thirty patients underwent the anamnesis, otorhinolaryngology examination, pure tone audiometry, acoustic immittance measures and brainstem auditory evoked potential (BAEP) assessments. Results The observed alterations were: 43.3% in the pure tone audiometry, bilateral in 36.7%; 56.6% in the BAEP test, bilateral in 50%; and 46.6% in the acoustic immittance test. There was a significant difference (p < 0.05) in the comparison between the tests performed. Conclusion In the audiological screening, there was a prevalence of the descending audiometric configuration at the frequency of 4kHz, and absence of the acoustic reflex at the same frequency. In the BAEP test, there was a prevalence of an increase of the latencies in waves I, III and V, and in the intervals of interpeaks I-III, I-V and III-V. In 13.3% of the patients, wave V was absent, and all waves were absent in 3.3% of patients.

ATP1A3 mutations can cause progressive auditory neuropathy: a new gene of auditory synaptopathy

The etiologies and prevalence of sporadic, postlingual-onset, progressive auditory neuropathy spectrum disorder (ANSD) have rarely been documented. Thus, we aimed to evaluate the prevalence and molecular etiologies of these cases. Three out of 106 sporadic progressive hearing losses turned out to manifest ANSD. Through whole exome sequencing and subsequent bioinformatics analysis, two out of the three were found to share a de novo variant, p.E818K of ATP1A3, which had been reported to cause exclusively CAPOS (cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss) syndrome. However, hearing loss induced by CAPOS has never been characterized to date. Interestingly, the first proband did not manifest any features of CAPOS, except subclinical areflexia; however, the phenotypes of second proband was compatible with that of CAPOS, making this the first reported CAPOS allele in Koreans. This ANSD phenotype was compatible with known expression of ATP1A3 mainly in the synapse between afferent nerve and inner hair cells. Based on this, cochlear implantation (CI) was performed in the first proband, leading to remarkable benefits. Collectively, the de novo ATP1A3 variant can cause postlingual-onset auditory synaptopathy, making this gene a significant contributor to sporadic progressive ANSD and a biomarker ensuring favorable short-term CI outcomes.

Differentiation of Spiral Ganglion-Derived Neural Stem Cells into Functional Synaptogenetic Neurons

Spiral ganglion neurons (SGNs) are usually damaged in sensorineural hearing loss. SGN-derived neural stem cells (NSCs) have been identified and proposed to differentiate into neurons to replace damaged SGNs. However, it remains obscure whether SGN-NSC-derived neurons (ScNs) are electrophysiologically functional and possess the capability to form neural connections. Here, we found that SGN-derived cells demonstrated NSC characteristics and differentiated into SGN-like glutamatergic neurons. Neurotrophins significantly increased neuronal differentiation and neurite length of ScNs. Patch clamp recording revealed that ScNs possessed SGN-like NaV and HCN channels, suggesting electrophysiological function. FM1-43 staining and synaptic protein immunofluorescence showed ScNs possess the ability to form neural connections. Astrocyte-conditioned medium was able to stimulate ScNs to express synaptic proteins. These data suggested that neurotrophins are able to stimulate postnatal SGN-NSCs to differentiate into functional glutamatergic ScNs with the capability to form synaptic connections in vitro.

Clinical features of Friedreich's ataxia: classical and atypical phenotypes

One hundred and fifty years since Nikolaus Friedreich's first description of the degenerative ataxic syndrome which bears his name, his description remains at the core of the classical clinical phenotype of gait and limb ataxia, poor balance and coordination, leg weakness, sensory loss, areflexia, impaired walking, dysarthria, dysphagia, eye movement abnormalities, scoliosis, foot deformities, cardiomyopathy and diabetes. Onset is typically around puberty with slow progression and shortened life-span often related to cardiac complications. Inheritance is autosomal recessive with the vast majority of cases showing an unstable intronic GAA expansion in both alleles of the frataxin gene on chromosome 9q13. A small number of cases are caused by a compound heterozygous expansion with a point mutation or deletion. Understanding of the underlying molecular biology has enabled identification of atypical phenotypes with late onset, or atypical features such as retained reflexes. Late-onset cases tend to have slower progression and are associated with smaller GAA expansions. Early-onset cases tend to have more rapid progression and a higher frequency of non-neurological features such as diabetes, cardiomyopathy, scoliosis and pes cavus. Compound heterozygotes, including those with large deletions, often have atypical features. In this paper, we review the classical and atypical clinical phenotypes of Friedreich's ataxia.

Auditory processing deficits in children with Friedreich ataxia

Friedreich ataxia is a neurodegenerative disease with an average age of onset of 10 years. The authors sought to investigate the presence and functional consequences of auditory neuropathy in a group of affected children and to evaluate the ability of personal FM-listening systems to improve perception. Nineteen school-aged individuals with Friedreich ataxia and a cohort of matched control subjects underwent a battery of auditory function tests. Sound detection was relatively normal, but auditory temporal processing and speech understanding in noise were severely impaired, with children with Friedreich ataxia typically able to access less than 40% of the information available to controls. Use of FM-listening devices did, however, improve the speech perception performance of those with Friedreich ataxia to the level of their unaffected peers in conditions designed to replicate the listening environment of the average school classroom.

The genetic basis of auditory neuropathy spectrum disorder (ANSD)

OBJECTIVE

Auditory neuropathy is a hearing disorder where outer hair cell function within the cochlea is normal, but inner hair cell and/or the auditory nerve function is disrupted. It is a heterogeneous disorder which can have either congenital or acquired causes. Furthermore, the aetiology of auditory neuropathy is vast, which may include prematurity, hyperbilirubinaemia, anoxia, hypoxia, congenital brain anomalies, ototoxic drug exposure, and genetic factors. It is estimated that approximately 40% of cases have an underlying genetic basis, which can be inherited in both syndromic and non syndromic conditions. This review paper provides an overview of the genetic conditions associated with auditory neuropathy spectrum disorders (ANSDs) and highlights some of the defective genes that have been found to be linked to the pathological auditory changes.

METHOD

Literature search was conducted using a number of resources including textbooks, professional journals and the relevant websites.

RESULTS

The largest proportion of auditory neuropathy spectrum disorders (ANSDs) is due to genetic factors which can be syndromic, non-syndromic or mitochondrial related. The inheritance pattern can include all the four main types of inheritances such as autosomal dominant, autosomal recessive, X-linked and mitochondrial.

CONCLUSION

This paper has provided an overview of mutation with some of the genes and/or loci discovered to be the cause for auditory neuropathy spectrum disorders (ANSDs). It has been noted that different gene mutations may trigger different pathological changes in patients with this disorder. These discoveries have provided us with vital information as to the sites of pathology in auditory neuropathy spectrum disorders (ANSDs), and the results highlight the heterogeneity of the disorder.

The mitochondrial connection in auditory neuropathy

'Auditory neuropathy' (AN), the term used to codify a primary degeneration of the auditory nerve, can be linked directly or indirectly to mitochondrial dysfunction. These observations are based on the expression of AN in known mitochondrial-based neurological diseases (Friedreich's ataxia, Mohr-Tranebjærg syndrome), in conditions where defects in axonal transport, protein trafficking, and fusion processes perturb and/or disrupt mitochondrial dynamics (Charcot-Marie-Tooth disease, autosomal dominant optic atrophy), in a common neonatal condition known to be toxic to mitochondria (hyperbilirubinemia), and where respiratory chain deficiencies produce reductions in oxidative phosphorylation that adversely affect peripheral auditory mechanisms. This body of evidence is solidified by data derived from temporal bone and genetic studies, biochemical, molecular biologic, behavioral, electroacoustic, and electrophysiological investigations.

Disruption to higher order processes in Friedreich ataxia

Friedreich ataxia (FRDA), the most common of the genetically inherited ataxias, is characterised by ocular motor deficits largely reflecting disruption to brainstem-cerebellar circuitry. These deficits include fixation instability, saccadic dysmetria, disrupted pursuit, and vestibular abnormalities. Whether higher order or cognitive control processes involved the generation of more volitional eye movements are similarly impaired, has not been explored previously. This research examined antisaccade and memory-guided saccade characteristics in 13 individuals with genetically confirmed FRDA, and contrasted performance with neurologically healthy individuals. We demonstrate, for the first time, a broad range of deficits in FDRA consistent with disruption to higher order processes involved in the control of saccadic eye movement. Significant differences between FDRA and control participants were revealed across all movement parameters (latency, gain, velocity, position error), and across all saccade types, including alterations to velocity profiles. FDRA participants also generated significantly more erroneous responses to non-target stimuli in both saccade paradigms. Finally, a number of correlations between ocular motor and clinical measures were revealed including those between contrast acuity and saccadic latency (all saccade types), disease duration and measures of response inhibition (errors and relative latencies for antisaccades), and neurological scores and error latencies, arguably a reflection of difficulty resolving response conflict. These results suggest a role for the cerebellum in higher order cognitive control processes, and further support the proposal that eye movement markers, which can be measured with accuracy and reliability, may be a useful biomarker in FDRA.

Hearing impairment in children

This article describes the current standard for infant hearing loss identification and intervention. Since the standard of care was driven by the recommendations made by the Joint Committee on Infant Hearing, a summary of the most recent recommendations is provided, followed by illustrative case studies that highlight how implementation of these guidelines allow access to the critical window for auditory and speech-language development.

Speech perception ability in individuals with Friedreich ataxia

The aim of this study was to investigate auditory pathway function and speech perception ability in individuals with Friedreich ataxia (FRDA). Ten subjects confirmed by genetic testing as being homozygous for a GAA expansion in intron 1 of the FXN gene were included. While each of the subjects demonstrated normal, or near normal sound detection, 3 of the 10 showed electrophysiological evidence of auditory pathway disorder [presenting with the auditory neuropathy/dyssynchrony (AN/AD) result pattern], and 9 of the 10 showed abnormal speech understanding when tested with levels of background noise typical of everyday listening conditions. Information transmission analyses of the speech perception findings for the three FRDA subjects with AN/AD type hearing loss when compared with those of a cohort of individuals with peripheral [sensorineural (SN)] hearing loss, showed a distinct pattern of perceptual disruption. Where the listeners with SN loss confused sounds on the basis of frequency (pitch) differences, the FRDA subjects with AN/AD made errors that reflected an inability to perceive temporal (timing) cues in the speech sounds.

Auditory nerve is affected in one of two different point mutations of the neurofilament light gene

OBJECTIVE

To define auditory nerve and cochlear functions in two families with autosomal dominant axonal Charcot-Marie-Tooth (CMT).

METHODS

Affected members in two families with different point mutations of NF-L gene were screened with auditory brainstem responses (ABRs). Those with abnormal ABRs were further investigated with clinical, neurophysiological and audiological procedures. The point mutations of NF-L gene involved were Glu397Lys in 8 affected members of the family with AN, and Pro22Ser in 9 affected members of the family without AN.

RESULTS

ABRs and stapedial muscle reflexes were absent or abnormal in affected members of only one family consistent with auditory neuropathy (AN). In them, audiograms, otoacoustic emissions, and speech comprehension were normal. Absent or abnormal ABRs were consistent with slowing of conduction along auditory nerve and/or brainstem auditory pathway. Wave I when present was of normal latency.

CONCLUSIONS

Auditory nerve involvement in the presence of normal cochlear outer hair cell activity is asymptomatic in one of two families with CMT disorder with different point mutations of the NF-L gene. The nerve disorder is consistent with altered synchrony and slowed conduction.

SIGNIFICANCE

The absence of "deafness" may reflect the ability of central mechanisms to compensate for the slowly developing auditory nerve abnormalities.

Rebuilding lost hearing using cell transplantation

OBJECTIVE

The peripheral auditory nervous system (cochlea and auditory nerve) has a complex anatomy, and it has traditionally been thought that once the sensorineural structures are damaged, restoration of hearing is impossible. In the past decade, however, the potential to restore lost hearing has been intensively investigated using molecular and cell biological techniques, and we can now part with such a pessimistic view. In this review, we examine an important field in hearing restoration research: cell transplantation.

METHODS

Most efforts in this field have been directed to the replacement of hair cells by transplantation to the cochlea. Here, we focus on transplantation to the auditory nerve, from the side of the cerebellopontine angle rather than the cochlea.

RESULTS

Delivery of cells to the cochlea is potentially damaging, and nerve cells transplanted distally to the Schwann-glial transitional zone (cochlear side) may become inhibited when they reach the transitional zone. The auditory nerve is probably the most suitable route for cell transplantation.

CONCLUSION

The auditory nerve occupies an important position not only in neurosurgery but also in various diseases in other disciplines, and several lines of recent evidence indicate that it is a key target for hearing restoration. It is familiar to most neurosurgeons, and the recent advances in the molecular and cell biology of inner-ear development are of direct importance to neurorestorative medicine. In this article, we review the anatomy, development, and molecular biology of the auditory nerve and cochlea, with emphasis on the advances in cell transplantation.

Auditory neuropathy/dys-synchrony and its perceptual consequences

Auditory neuropathy/dys-synchrony is a form of hearing impairment in which cochlear outer hair cell function is spared but neural transmission in the auditory pathway is disordered. This condition, or group of conditions with a common physiologic profile, accounts for approximately 7% of permanent childhood hearing loss and a significant (but as yet undetermined) proportion of adult impairment. This paper presents an overview of the mechanisms underlying auditory neuropathy/dys-synchrony-type hearing loss and the clinical profile for affected patients. In particular it examines the perceptual consequences of auditory neuropathy/dys-synchrony, which are quite different from those associated with sensorineural hearing loss, and considers currently available, and future management options.

A dominantly inherited progressive deafness affecting distal auditory nerve and hair cells

We have studied 72 members belonging to a large kindred with a hearing disorder inherited in an autosomal dominant pattern. We used audiological, physiological, and psychoacoustic measures to characterize the hearing disorders. The initial phenotypic features of the hearing loss are of an auditory neuropathy (AN) with abnormal auditory nerve and brainstem responses (ABRs) and normal outer hair cell functions [otoacoustic emissions (OAEs) and cochlear microphonics (CMs)]. Psychoacoustic studies revealed profound abnormalities of auditory temporal processes (gap detection, amplitude modulation detection, speech discrimination) and frequency processes (difference limens) beyond that seen in hearing impairment accompanying cochlear sensory disorders. The hearing loss progresses over 10-20 years to also involve outer hair cells, producing a profound sensorineural hearing loss with absent ABRs and OAEs. Affected family members do not have evidence of other cranial or peripheral neuropathies. There was a marked improvement of auditory functions in three affected family members studied after cochlear implantation with return of electrically evoked auditory brainstem responses (EABRs), auditory temporal processes, and speech recognition. These findings are compatible with a distal auditory nerve disorder affecting one or all of the components in the auditory periphery including terminal auditory nerve dendrites, inner hair cells, and the synapses between inner hair cells and auditory nerve. There is relative sparing of auditory ganglion cells and their axons.

Auditory neuropathy: a potentially under-recognized neonatal intensive care unit sequela

Auditory neuropathy (AN) is a hearing disorder that affects newborns. Those with high-risk neonatal histories, family history of childhood hearing loss, and hyperbilirubinemia are at greatest risk. Current neonatal intensive care unit (NICU) hearing screening methods that rely only on otoacoustic emissions will fail to detect this disorder. Auditory neuropathy differs from conductive hearing loss and sensorineural hearing loss; a specific constellation of findings on audiologic evaluation are diagnostic of this disorder. The pathophysiology of AN is unclear; however, it may be caused by demyelinization or degeneration at points along the auditory pathway. The actual incidence of AN is unknown; it is more prevalent in high-risk infants. The course of AN varies widely among patients. Current management ranges from close monitoring of the child's development to cochlear implantation. Neonatal intensive care unit nurses need to be aware of this disorder to help support and educate at-risk families and to alert them of the need to monitor hearing and language development in their infants.

Myelin-deficiency in the cochlear nerve of the 'bt' mutant hamster

In a previous report, we showed abnormal auditory evoked potentials in the mutant hamster, 'black tremor (bt)', with significantly prolonged wave latencies of auditory brainstem responses and prolonged N1 latencies of compound action potentials, but normal cochlear microphonics. In this report, we present the results of morphological studies supporting the results of our electrophysiological studies of the auditory pathway in bt. Observation by transmission electron microscopy revealed an abnormal myelin sheath surrounding the spiral ganglion cells, and a thinner compact myelin sheath surrounding the axons in bt than in normal hamsters. The bt hamster has a myelin deficiency not only in the brainstem, but also in the cochlear nerve.

Electrical stimulation of the auditory nerve via cochlear implants in patients with auditory neuropathy

Auditory neuropathy (AN) is a term used to describe an auditory disorder in which there is evidence of normal outer hair cell function (otoacoustic emissions and/or cochlear microphonics) and poor function of the auditory nerve (absent or highly distorted auditory brain stem response starting with wave I). Many of these patients have evidence of generalized peripheral nerve disease, leading to an assumption that the peripheral portion of the auditory nerve is the most likely site of lesion. A small group of these patients has received cochlear implants, and the majority of them achieve average to above-average performance. Although this outcome may seem incongruous with neural disease, average performance by patients with AN may be a result of the reintroduction of neural synchrony by electrical stimulation and/or the fact that most deaf patients have poor nerve survival. Although cochlear implants are promising for deaf patients with AN, more study of the disorder is needed.

Influence of GAA expansion size and disease duration on central nervous system impairment in Friedreich's ataxia: contribution to the understanding of the pathophysiology of the disease

OBJECTIVE

To verify if GAA expansion size could account for the severity of the central nervous system involvement in Friedreich's ataxia (FA).

METHODS

Retrospective study of 52 FA patients (mean age 26.9+/-12.1 years; mean disease duration 10.6+/-7.6 years) homozygous for GAA expansion. Median nerve somatosensory evoked potentials (SSEPs) were available in 36 FA patients, upper limb motor evoked potentials (MEPs) to transcranial magnetic stimulation in 32, brainstem auditory evoked potentials (BAEPs) in 24, and visual evoked potentials (VEPs) in 34. N20, P100, MEP amplitude, SSEP and MEP central conduction time (CCT and CMCT), P100 latency and I-III and I-V interpeak latency, and a BAEP abnormality score were correlated with disease duration and GAA expansion size on the shorter (GAA1) and larger (GAA2) allele in each pair.

RESULTS

The GAA1 size inversely correlated with the N20 amplitude (r = -0.49; P<0. 01). Disease duration directly correlated with CMCT (r = 0.57; P<0.01) and BAEP score (r = 0.61; P<0.01) and inversely with MEP (r = -0.40; P<0.05) and P100 amplitude (r = -0.39; P<0.05).

CONCLUSIONS

Our data suggest that central somatosensory pathway involvement in FA is mainly determined by GAA1 expansion size. Vice versa, degeneration of pyramidal tracts, auditory and visual pathways seems to be a continuing process during the life of FA patients.

Cochlear implantation in auditory neuropathy

OBJECTIVE

Auditory neuropathy is a recently described clinical entity characterized by sensorineural hearing loss in which the auditory evoked potential (ABR) is absent but otoacoustic emissions are present. This suggests a central locus for the associated hearing loss. In this study the results observed in a child with auditory neuropathy who received a cochlear implant are presented and compared with those of a matched group of children who were recipients of implants.

METHODS

A single-subject, repeated-measures design, evaluating closed-set and open-set word recognition abilities was used to assess the subject and a control group of matched children with implants who had also experienced a progressive sensorineural hearing loss.

RESULTS

The subject demonstrated improvements in vowel recognition (82% correct) by 1 year after implantation, which were only slightly lower than the control group. Consonant recognition and open-set word recognition scores were significantly lower.

CONCLUSION

Caution should be exercised when considering cochlear implantation in children with auditory neuropathy. As with conventional hearing aids, less than optimal results may be seen.

Auditory brain-stem responses and MRI findings in patients with olivo-ponto-cerebellar (Dejerine-Thomas)-type and cerebello-olivary (Holmes)-type spino-cerebellar degeneration

Auditory brain-stem responses (ABRs) and magnetic resonance imaging (MRI) findings were investigated in 10 patients with olivo-ponto-cerebellar atrophy (OPCA: Dejerine-Thomas type) and in 5 patients with Holmes-type cerebello-olivary degeneration. In patients with OPCA, (1) the III-V interpeak latency (IPL) was shortened significantly (P < 0.05) as compared with normal subjects when the decrease in longitudinal distance of the pons was within 20% of that of normal controls, (2) the I-III IPL was elongated (P < 0.05), the III-V IPL was shortened (P < 0.01), and the amplitude of the V wave was reduced (P < 0.05) when the decrease in longitudinal distance of the pons was more than 20% of that of normal controls. Patients with Holmes-type degeneration showed no significant difference in any component of ABRs or in the size of any part of the brain-stem, as compared with normal subjects. We conclude that abnormalities in the I-III IPL or the III-V IPL in patients with OPCA were produced by changes in the volume conduction in the pons and midbrain. This study also suggests that differences of IPL appear between OPCA and Holmes-type degeneration.

Evoked potentials in hereditary spastic paraplegia

Pattern reversal visual evoked response (VER) and monaural stimulation of brainstem auditory evoked responses (BAER) were recorded from both sides in 25 patients (males 19; females 6) with hereditary spastic paraplegia (HSP). Their age ranged from 15-52 (mean +/- SD; 25.2 +/- 22.5) years and duration of symptoms 6 months-9 (mean +/- SD; 4.2 +/- 3.6) years. A prolonged P100 latency was seen in 6 patients and BAER abnormality in 13. None of the patients had clinical evidence of brainstem involvement. It is suggested that VER and BAER abnormalities are due to segmental demyelination and fiber loss in central conduction pathways and could serve as an important tool for the diagnosis of this disorder.

Brain-stem auditory evoked potentials and blink reflex in Friedreich's ataxia

The brain-stem involvement in Friedreich's ataxia (FA) was studied by using brain-stem auditory evoked potentials (BAEPs) and the blink reflex. Ten out of 18 patients had abnormal BAEPs, the main abnormality being complete absence of responses and disappearance of wave V. Combined degeneration of the peripheral and central acoustic pathways probably accounts for these findings. The blink reflex was abnormal in 50% of the cases. The outstanding abnormality was bilateral delay of late responses with normal early response, which could be correlated with the known pallor of the descending trigeminal tracts. In contrast with BAEP findings, blink reflex abnormalities did not correlate with either the age of patients or the severity and duration of the disease. These data suggest a difference in susceptibility to degeneration between the auditory system and neuronal system subserving the blink reflex. We conclude that systematic BAEP and blink reflex recording is useful in the electrophysiological evaluation of FA patients.

Cross-correlation function in the analysis of auditory brainstem response in spinocerebellar degeneration

Cross-correlation functions were derived from the analysis of auditory-evoked brainstem response (ABR) and compared with measurements of wave latency and computed tomography findings in the assessment of ABR findings in spinocerebellar degeneration (SCD). Gender-specific normative ABR templates were produced from 30 normal males and 30 normal females separately. The cross-correlation indices used were the correlation coefficient at time 0, the maximal correlation coefficient and the latency delay in milli-seconds. The technique was applied to 33 patients with SCD. The incidence of abnormal cross-correlation functions (81.8%) was greater than the incidence of abnormal ABR peak latencies assessed according to gender (75.8%) which, in turn, was more common than the abnormal peak latencies assessed conventionally (69.7%). Moreover, the incidence of abnormal cross-correlations and latencies in Menière's disease was much lower (less than 8%). These results suggest that the evaluation of ABR waveform characteristics with cross-correlation functions using normative ABR templates of the same gender contributes to the precise detection of abnormality in the brainstem auditory pathway.

BAEP changes in Leber's hereditary optic atrophy: further confirmation of multisystem involvement

A neurophysiological study of 11 patients belonging to 5 families affected by Leber's hereditary optic atrophy is reported. Electromyography, nerve conduction velocities and somatosensory evoked potentials were normal. Visual evoked potentials were absent or delayed, desynchronized and reduced in amplitude. Brainstem auditory evoked potentials were anomalous in 64% of subjects all without hearing defects. These changes which have never before been reported, confirm multisystem involvement in this disease.

Utility of short-latency evoked potentials in the classification of progressive, early onset cerebellar ataxias

In recent years, several authors have proposed new classifications of inherited ataxias, some of them being based on systematic clinical studies of large groups of patients. This methodic approach has led to the identification of new types of ataxias and helped the development of molecular biology research in these diseases. Up to now, nerve conduction velocity and evoked potential studies have not been considered in the classification of hereditary ataxias. We have studied the results of short latency evoked potentials in 102 patients affected by a early onset, progressive cerebellar ataxia. Based on the results of this study and a review of the literature on this subject, we will evaluate the utility of nerve conduction velocity and evoked potential recordings in the classification of this group of diseases.

Neurophysiological study in chronic GM2 gangliosidosis (hexosaminidase A and B deficiency), with motor neuron disease phenotype

We report the electrophysiological investigation of two adult cases with GM 2 gangliosidosis with hexosaminidase A and B deficiency. Superficial peroneal biopsy was obtained from one patient. The electrophysiological alterations of the peripheral nervous system were fasciculations, signs of collateral reinnervation and loss of motor units, decrease in sensory potential amplitude and increase in distal motor latency. Increase in N9-N13 interpeak latency of the somatosensory evoked potentials and an increase I-V interpeak latency of the brain-stem auditory potentials were evident in both cases. Visual evoked potentials were normal. Nerve biopsy showed a severe loss of myelinated fibers, especially of those with the largest diameter, with no signs of segmental demyelination, or remyelination. A tentative interpretation of our findings is given.

Evoked potential studies in Friedreich's ataxia and progressive early onset cerebellar ataxia

We recorded somatosensory evoked potentials (SEP) in 15 patients affected by Friedreich's ataxia (FA) and in 9 patients with progressive early onset cerebellar ataxia (PEOCA). Brainstem auditory evoked potentials (BAEP) were also recorded in 14 FA patients and in five PEOCA patients. SEP results showed clear differences between groups of FA, evidence of peripheral involvement was seen in all patients, with absence of the N9 potential or a major reduction of its amplitude. In patients in whom central responses could be recorded, conduction velocity was normal or near normal up to the brainstem but was reduced from brainstem to cerebral cortex. Four patients with PEOCA had SEP abnormalities similar to those seen in FA. In the five other patients, the amplitude and latency of N9 were normal but conduction velocity was reduced from brainstem to cerebral cortex. In FA, BAEP were abnormal in all patients with a disease duration of four years or more but were normal in four of the five PEOCA patients. Systematic evoked potential recording is useful in the investigation of hereditary ataxias.

Present criteria for the diagnosis of in vivo of olivopontocerebellar atrophy

We present 6 cases of olivopontocerebellar atrophy (OPCA) of Dèjérine-Thomas type. The diagnosis was based on an appropriate clinical picture, anomalous brainstem auditory evoked potentials (BAEP) and computerized tomography (CT) and/or magnetic nuclear resonance (MNR) imaging which demonstrated atrophy of the cerebellum and the pons. We believe that these methods will provide greater uniformity in the diagnosis of such an heterogeneous entity as OPCA.

Involvement of somatosensory and visual pathways in late onset ataxia

Nerve conduction studies, median nerve somatosensory (SEPs) and pattern-reversal visual evoked potentials (VEPs) were recorded in 10 patients with late onset ataxia. Nine patients had dysfunction of somatosensory pathways, eight of them axonal polyneuropathy in nerve conduction measurements, and four had also impaired impulse conduction along the visual pathways. Abnormalities in records of evoked potentials occurred as prolonged latencies rather than reduced amplitudes. The observed dysfunction of peripheral and central pathways was not related to the duration or the severity of the clinical involvement by the disease. It is emphasized, however, that there seemed to be a relationship between the prolonged SEP latencies and the impaired peripheral nerve conduction values and between the diminished VEP amplitudes and the most delayed VEP latencies.

Friedreich's ataxia: electrophysiologic and histologic findings in patients and relatives

Peripheral nerve conduction velocity and cortical evoked potentials were investigated in 48 patients with Friedreich's disease and in 35 relatives. There were 14 patients and 2 relatives who underwent sural nerve biopsy. In the patients sensory conduction velocity was moderately slowed, whereas sensory responses were markedly reduced. Nerve biopsy showed a severe loss of large myelinated fibers and no demyelination. On teased nerve fiber preparations, most fibers presented uniformly short internodes. No correlation was seen between sensory conduction findings or histologic abnormalities and clinical disability. In patients SSEP changes, which were constant, and VEPs, which were frequently involved, were unrelated to the severity or duration of clinical disability. There were 14 relatives who showed clinical signs of Friedreich's disease. Slightly decreased distal conduction velocity along sensory fibers was observed in more than half of the relatives. Nerve biopsy was noncontributory. In conclusion, we could not determine whether the abnormalities observed in the siblings were an expression of a heterozygotic condition, or whether they were early signs of the disease.

Brainstem auditory evoked responses and CT findings in multiple system atrophy

Brainstem auditory evoked responses (BAERs) and CT findings were comparatively studied in 11 patients with Shy-Drager syndrome and 10 patients with olivopontocerebellar atrophy. The I-III interpeak latencies (IPLs) were prolonged in 6 patients with Shy-Drager syndrome and in 6 patients with olivopontocerebellar atrophy. Mean values of the I-III IPLs were 2.54 +/- 0.28 ms (Shy-Drager syndrome) and 2.62 +/- 0.15 ms (olivopontocerebellar atrophy). In each disease, the I-III IPLs correlated well with the degree of the pontine atrophy estimated from the CT scan. The patients with Shy-Drager syndrome could be clinically divided into two varieties. In addition to autonomic dysfunction, one variety (4 patients) was linked with parkinsonism, and the other (7 patients) with signs of multiple nervous systems involvement. Prolongation of I-III IPLs and pontine atrophy were noted in 6 out of the latters patients, whereas the former patients did not show such abnormalities. The combination of BAERs and CT scan provides useful clinical information on multiple system atrophy.

Spinocerebellar ataxia associated with localized amyotrophy of the hands, sensorineural deafness and spastic paraparesis in two brothers

Two brothers aged 75 and 61 years, born from non-consanguineous non-affected parents, had spinocerebellar ataxia and impaired proprioception in the lower limbs, associated with sensorineural deafness, amyotrophy of the hands and spastic paraparesis. The older patient also had vitiligo of the right hand, and both displayed likely dysendocrine features. The disease was present since the second-third decade and showed a slow course. An EMG confirmed a neurogenic disorder strictly localized to the distal upper limbs. This new phenotype of heredoataxia is compared with previously reported similar syndromes and, in particular, with diseases featuring localized amyotrophy of the hands.

Brain stem auditory evoked potentials in patients with multiple system atrophy with progressive autonomic failure (Shy-Drager syndrome)

Brain stem potentials from three groups of patients, namely those with pure progressive autonomic failure, Parkinson's disease and multisystem atrophy with progressive autonomic failure (Shy-Drager syndrome) were compared with each other and a group of normal subjects. In virtually all the patients with multisystem atrophy with progressive autonomic failure the brain stem potentials were abnormal in contrast to normal findings with Parkinson's disease. The closely associated group of patients with progressive autonomic failure alone also revealed no abnormalities of the BAEP. This separation of the two groups, Parkinson's disease and progressive autonomic failure from multisystem atrophy with progressive autonomic failure is important clinically as multiple system atrophy of the Shy-Drager type has extra-pyramidal features closely resembling Parkinsonism or a late onset cerebellar degeneration. From the abnormalities of the brain stem response in multisystem atrophy with progressive autonomic failure, it is clear that some disruption of the auditory pathway occurs in the ponto-medullary region as in nearly all patients there is a significant delay or reduction in the amplitude of components of the response generated beyond this region. The most likely area involved is the superior olivary complex.

Brainstem auditory-evoked responses and clinical picture in a one year follow-up of 18 patients with Friedreich ataxia

Brainstem auditory-evoked responses (BAER) and behavior audiometry were evaluated on 18 patients with Friedreich ataxia. Behavior audiometry showed a normal pure tone test, normal performance intensity and phonetically balanced functions in all patients. BAER were abnormal in all patients. The data obtained show a good correlation between BAER and clinical picture and suggest that BAER could be useful in monitoring the clinical condition of FA patients.

Friedreich's ataxia: clinical involvement and evoked potentials

In 15 patients definitely affected by Friedreich's ataxia, precocious SEP abnormalities were constantly recorded independent of the duration of symptoms and the severity of clinical involvement; VEPs were frequently involved, but neither VEP abnormalities, nor visual impairment could be correlated with the severity or duration of Friedreich's ataxia clinical involvement; BAEPs were, to different extents, abnormal. These were completely dissociated from hearing disorders and significantly correlated (P less than 0.001) with Friedreich's ataxia clinical disability. Our observation of increased latency of the VEP P100 component and the early disappearance of BAEP Wave V with persistence of Wave I is in contrast with the hypothesis that changes are related to primary axonal degeneration. For practical purposes SEPs could help in the early diagnosis of Friedreich's ataxia, and BAEPs could be used for the electrophysiological monitoring of its clinical progression.

Familial periodic ataxia responsive to acetazolamide

Two cases, a father and son, of recurrent cerebellar ataxia in the same family are reported, suggesting a familial trait for the dysfunction. In the older male the onset of each episode (30-90 min.) was signalled by dysarthria which then progressed towards gait ataxia; the son presented closely similar clinical symptoms. Physical examination and blood chemistry revealed no obvious neurological deficit or biochemical abnormalities, with the exception of I-III and III-IV evoked auditory wave interpeak latencies, which were found markedly abnormal on the left side in the father but not in the son; the EEG of both individuals showed some diffuse, slow wave abnormalities. A low dose of acetazolamide, 250 mg daily, has successfully repressed recurrence of the attacks over the past six months. Temporary withdrawal for 14 days of the carbonic anhydrase inhibitor in the father coincided with two observed ataxic episodes.

Visual evoked potentials and brain stem auditory potentials in Friedreich's ataxia--a longitudinal study

Six patients with Friedreich's ataxia, 4 males and 2 females, their ages ranging from 13 to 33 years, were studied. The early manifestations started between age 7 and 13 with an evolution time between 6 and 20 years. Serial visual and brain stem auditory evoked potential recordings were made. A progressive increase in latency, reduction in amplitude and in latency inter-ocular difference of P100 were observed. The pattern of the reversal checker-board visual evoked potential was preserved. A disorganized BAEP pattern, a well defined potential I, a very small potential V and a delay in the interpeak latency were constant findings. The assumption is made of a progressive involvement of both visual and central auditory pathways. Pathophysiological mechanisms are discussed.

Specific impairment of BAER's in Friedreich's ataxia. Auditory evoked responses in clinical evaluation and differential diagnosis

We have investigated brainstem and cortical auditory responses (BAERs and CAERs) in 16 cases of Friedreich's ataxia (FA) and have compared the findings with those obtained in 2 cases of familial spastic paraplegia (FSP), in 5 cases of Charcot-Marie-Tooth disease (CMTD), and in 6 cases of atypical FA of uncertain classification. BAERs could not be elicited in 11 FA patients and constantly disappeared at a higher intensity threshold than in normal subjects in the remaining 5 patients. BAERs were normal or only slightly abnormal in FSP and CMTD patients. CAERs were normal in all 29 patients. BAERs tended to disappear with the progression of FA and BAER thresholds were correlated with the Inherited Ataxias Clinical Rating Scale score, which is an index of the severity of illness. BAERs contributed to the diagnosis, or exclusion of FA in patients with an atypical picture. It is suggested that in FA myelinated fibers in the spiral ganglion are partially affected, resulting in the decrease of wave amplitude such as occurs for peripheral sensory potentials.

Auditory evoked potentials (early, middle, late components) and audiological tests in Friedreich's ataxia

A functional assessment of the acoustic pathways in a group of 9 patients with Friedreich's ataxia was performed. In none of them was symptomatic auditory impairment noticeable. All patients were submitted to audiological tests and AEP recording. AEPs included early (BAEPs), middle (MLCs) and long (LLCs) latency components. The absence of one or more BAEP waves was observed in 5 out of 9 patients. N85 latency was significantly increased in all but one patient. MLCs, particularly P12, N15, P25 were often normal in amplitude and latency, scarcely showing a positive relationship with abnormalities of preceding and following waves. Pure tone audiometry revealed cochlear or neural impairment only in 2 patients. Speech audiometry showed deficits in all patients. These investigations, taken as a whole, showed a prevalent involvement of the brain-stem acoustic pathways, presumably at different levels, seldom associated with eighth nerve impairment. In some cases LLC (N85) abnormalities could not be interpreted as a consequence of impaired function in peripheral and brain-stem acoustic sites and might suggest a cortical dysfunction. Major BAEP abnormalities were observed in patients with longer disease duration, while N85 changes were unrelated to the duration and the severity of the illness. The former may prove to be a reliable means of monitoring the progression of the disease, the latter could be employed as a complementary procedure in its early diagnosis.

Electrophysiological studies in five cases of abetalipoproteinemia

Auditory brainstem responses (ABRs), visual and somatosensory evoked responses (VEPs and SEPs) and nerve conduction studies were conducted in 5 patients with abetalipoproteinemia. The ABRs were normal in all cases. The VEPs were of normal amplitude but of increased latencies in two patients. The four eldest patients had delayed cortical SEPs but normal peripheral sensory nerve conduction studies. The peripheral motor conduction velocities were normal in all cases. The peripheral sensory studies showed normal velocity when a response was seen; however, the amplitude of the response was often reduced or it was absent. The electrophysiological studies reported here support a model of axonal loss of large myelinated fibres with secondary demyelination in abetalipoproteinemia.