|Year : 2015 | Volume
| Issue : 2 | Page : 171-180
Neurological associations in auditory neuropathy spectrum disorder: Results from a tertiary hospital in South India
Anjali Lepcha Associate Professor 1, Reni K Chandran2, Mathew Alexander3, Ann Mary Agustine1, K Thenmozhi1, Achamma Balraj1
1 Department of Ear, Nose and Throat, Christian Medical College and Hospital, Vellore, Tamil Nadu, India
2 Department of Otolaryngology, Head and Neck Surgery, Hamad Medical Corporation, Al Wakra Hospital, Doha, India
3 Department of Neurology, Christian Medical College and Hospital, Vellore, Tamil Nadu, India
|Date of Submission||21-Mar-2014|
|Date of Decision||03-Jun-2014|
|Date of Acceptance||06-May-2014|
|Date of Web Publication||8-May-2015|
Department of Ear, Nose and Throat, Unit IV, Christian Medical College and Hospital, Vellore - 632 004, Tamil Nadu
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Aims: To find out the prevalence and types of neurological abnormalities associated in auditory neuropathy spectrum disorder in a large tertiary referral center. Settings and Design: A prospective clinical study was conducted on all patients diagnosed with auditory neuropathy spectrum disorder in the ear, nose, and throat (ENT) and neurology departments during a 17-month period. Patients with neurological abnormalities on history and examination were further assessed by a neurologist to determine the type of disorder present. Results: The frequency of auditory neuropathy spectrum disorder was 1.12%. Sixty percent were found to have neurological involvement. This included cerebral palsy in children, peripheral neuropathy (PN), spinocerebellar ataxia, hereditary motor-sensory neuropathy, spastic paresis, and ponto-bulbar palsy. Neurological lesions did not present simultaneously with hearing loss in most patients. Sixty-six percent of patients with auditory neuropathy spectrum disorder were born of consanguineous marriages. Conclusions: There is a high prevalence of neurological lesions in auditory neuropathy spectrum disorder which has to be kept in mind while evaluating such patients. Follow-up and counselling regarding the appearance of neuropathies is therefore important in such patients. A hereditary etiology is indicated in a majority of cases of auditory neuropathy spectrum disorder.
Keywords: Auditory neuropathy, Charcot- marie- tooth disease, consanguinity, Friedreich ataxia, genetic counseling, hereditary sensory, motor neuropathy, neurologic dysfunction, Refsum disease, sensorineural hearing loss
|How to cite this article:|
Lepcha A, Chandran RK, Alexander M, Agustine AM, Thenmozhi K, Balraj A. Neurological associations in auditory neuropathy spectrum disorder: Results from a tertiary hospital in South India. Ann Indian Acad Neurol 2015;18:171-80
|How to cite this URL:|
Lepcha A, Chandran RK, Alexander M, Agustine AM, Thenmozhi K, Balraj A. Neurological associations in auditory neuropathy spectrum disorder: Results from a tertiary hospital in South India. Ann Indian Acad Neurol [serial online] 2015 [cited 2021 Sep 27];18:171-80. Available from: https://www.annalsofian.org/text.asp?2015/18/2/171/150578
| Introduction|| |
Auditory neuropathy (AN) is an uncommon disorder characterized by five integral features that distinguish it from other types of hearing loss. These are:
- Sensorineural hearing loss of any degree,
- With preserved otoacoustic emissions (OAEs) and /or cochlear microphonics (CM),
- Auditory nerve dysfunction as evidenced by abnormal auditory brainstem evoked responses (ABR),
- Poor speech perception with poor speech recognition score that seems out of proportion to the degree of hearing loss depicted by pure tone thresholds, and
- Absent stapedial reflexes to the ipsilateral and contralateral tones at a 110-dB hearing level.  prematurity (n = 10 [45%]
Peripheral neuropathy (PN) is not a constant finding in all patients with this hearing disorder. Some authors emphasize the dyssynchrony of temporal processing as the main pathology.  As new light is shed on AN, investigators find it is instead a syndrome (auditory neuropathy spectrum disorder or ANSD) and does not represent a single disease entity.  ANSD is an extremely varied disease in terms of age of onset and etiology which may range from neonate to adulthood; etiology from prematurity, kernicterus, birth hypoxia, ototoxic drug exposure, infection, and genetic. The mode of inheritance may be autosomal dominant, autosomal recessive, X-linked or mitochondrial. 
Studies have shown the association of neuropathies like Charcot-Marie-Tooth disease More Details (CMT), Mohr-Tranebjaerg syndrome, Refsum's disease, and Friedreich's ataxia with AN. ,,,
Starr et al. have reported that ANSD was associated with a hereditary neurological disorder in 42% of their patients.  AN without the involvement of the neurological system has been termed as primary auditory neuropathy (PAN).  In this article we have highlighted the frequency and types of neurological involvement in ANSD.
| Materials and Methods|| |
The study was carried out in the ear, nose, and throat (ENT) and neurology departments of a tertiary referral center in southern India after approval by the institutional review board and followed guidelines set by the 1964 Declaration of Helsinki as revised in 2008. Adults and children presenting to the ENT outpatient during a 17-month period with hearing loss and complaints of difficulty in understanding speech and children referred for suspected hearing loss were included in this prospective study after obtaining informed written consent from all adults and guardians in case of minors. All patients underwent a detailed structured questionnaire and a full ear, nose, throat, and neurotological examination. Patients above 5 years then underwent pure tone audiometry, impedance audiometry, and speech audiometry. Those who had sensorineural hearing loss with absent acoustic reflexes and poor speech scores underwent an ABR and distortion product otoacoustic emissions (DPOAE). Children below 5 years underwent an ABR and DPOAE. Those with present DPOAE and absent or abnormal ABR were diagnosed as AN. If there was a suggestion of any neurological involvement a neurologist carried out a detailed examination and appropriate tests were ordered.
Pure tone audiometry was done using GSI-61, Clinical two channel audiometer from Grason-Stadler, USA, MA-53 audiometers, MAICO Diagnostic GmbH, USA. Hearing thresholds up to 20 dB across the frequencies 250-4000 Hz were considered normal. Siemens SD-30 impedance audiometer was used to obtain immitance measurements. The test was performed using a probe tone frequency of 226 Hz. An ipsilateralstapedial reflex at 1000 Hz was elicited and considered normal if the level at which it was obtained fell between 70 dB and 100 dB. DPOAE testing was done with the smart DPOAE of Intelligent Hearing Systems, USA. DPOAE were recorded using a test protocol where preliminaries were fixed at L1 = 65 dB sound pressure level (SPL), L2 = 55 dB SPL with an f2/f1 ratio of 1.22. The f2 frequencies were carefully selected to correspond closely to audiometric test frequencies of 1000 Hz, 2000 Hz and 4000 Hz. A DPOAE response was considered to be present if the SNR >6.13 dB. The DPOAEs were recorded with DPOAE amplitude in dB SPL as a function of stimulus frequency. ABR was done using Intelligent Hearing systems launch pad version 1.0×. High intensity clicks were used presented at a rate of approximately 10 per second. Potential difference between an electrode on the vertex and ipsilateral stimulated mastoid were amplified in a band pass of approximately 30-30,000 Hz and averaged across a few thousand sweeps of 10-15 ms. Sampling rate was kept not lower than 20 kHz in order to avoid wave form distortions and to enhance latency resolution. The normal waveform included a series of 5-7 voltage oscillations approximately 1 millisecond apart during the first 6-10 ms after stimulus onset which confirmed the sequential transmission of synchronous action potentials of the eight nerve through the auditory pathway. The waves I to VII were labelled where V wave was the most positive in the complex. Patients with ANSD had an absent or abnormal ABR represented by waves I, III, and V when their level of hearing would typically result in measurable ABR responses. All patients with dizziness or imbalance also underwent a battery of tests with the computerized digital Electronystagmography Nystagmorite Mark II, Recorders and Medicare Systems (P) Ltd. 2007-2008.
Patients who were found to have neurologic involvement on history and examination by a neurologist were subjected to tests based on clinical findings; Nerve conduction studies (NCV), needle electromyography (EMG), multi-mode evoked potentials including visual evoked potentials (VEP) and somatosensory evoked potentials (SSEP), magnetic resonance imaging (MRI) of brain and spinal cord and sural nerve biopsies of sites affected were done. NCVs were done using Nicolet Viking Select, Natus Medical Incorporated, USA. The motor nerves tested were the median, ulnar, peroneal, and tibial nerves. The sensory nerves tested were the median, ulnar, sural, and superficial peroneal nerves. Needle electrode examination was done to look for spontaneous activity in the form of fibrillation, positive sharp wave discharges, and fasciculations. The motor unit potentials were assessed and the recruitment pattern noted. Multimode evoked potential studies included the VEP and SSEP. In VEP, the P100 latency was measured. SSEP was measured using surface electrodes at various points over the spine and scalp for recording the sequential wave forms reflecting the normal function/dysfunction of the afferent somatosensory pathway in median and tibial nerves. MRI was done appropriate to the clinical context.
| Results|| |
Of the total number of patients attending the ENT outpatient department at our tertiary referral center during 17 months (March 2007 to July 2008), an estimated 18,579 underwent hearing assessment. Among them there were 3120 with sensorineural hearing loss. Of these, 35 had AN. The frequency of AN over a 17-month period was 1.12% (35/3120). There were 22 males and 13 females, 5 were under-fives. The mean age of onset of ANSD was 13 years with a range of 3 months-26 years. The distribution of the 35 patients with ANSD is shown in [Figure 1]
|Figure 1: Flow chart showing distribution of patients with ANSD into different groups, ANSD = Auditory neuropathy spectrum disorder|
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Twenty one (60%) of the 35 patients with AN had abnormal neurological findings and were further assessed by the neurologist and additional tests done. The neurological profiles of the patients are given in [Table 1] and [Table 2].
|Table 1: Patient profi le with regards to neurological evaluation in the 21 patients|
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Four of the 5 children less than five-years of age (patients numbered 8, 14, 16, and 21) had associated neurological abnormalities. Of the four, one was diagnosed as evolving central motor delay with a generalized tonic clonic seizure disorder. Another child, 3 months of age had signs of pyramidal involvement. Two children were diagnosed to have cerebral palsy with pyramidal signs.
Of the 21 patients with neurological abnormalities, nine patients had evidence of PN; NCVs were done which showed pure sensory involvement in six, pure motor involvement in one and sensorimotor involvement in two. Among the 3 patients who underwent sural nerve biopsies, one had marked myelin loss while the other two had moderate loss of nerve fibres. Quadriceps muscle biopsy showed atrophic fibres in the patient with motor neuropathy.
Blood tests showed low Vitamin B12 levels in two of the 21 with neurological abnormalities and elevated blood lactate levels in all 4 of the family of BVVLS siblings and another two of the 21 patients. Six had a family history of similar neurological symptoms and four had history of neonatal insults like kernicterus, prematurity, low birth weight, and birth asphyxia.
Clinical study of a few representative cases
Patient number 6, an 18-year-old female with a family history (maternal uncle also having similar complaints) had features of bulbar palsy, anterior horn cell involvement, pyramidal tract involvement with signs of hypertonia, hypereflexia, and clonus, clinical features of cerebellar involvement, posterior column involvement both clinically, and confirmed by SSEP. Brain and spine imaging was normal. Optic nerve dysfunction was found on VEP [Figure 2] and a diagnosis of complicated hereditary spastic paresis was made.
|Figure 2: Visual evoked potential tracings showing bilateral anterior visual pathway dysfunction|
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Patient numbers 10, 11, 12, and 13 were siblings from the same family and their ages were 22, 20, 17, and 14 respectively at presentation. They had developed hearing loss around 13-years of age. Their electronystagmograms (ENG) were suggestive of bilateral peripheral vestibular pathology. Clinically all had temporalis muscle wasting with two having tongue fasciculation at rest. Their NCV showed mildly reduced sensory nerve action potentials (SNAP) in both sural nerves. Needle EMG showed spontaneous activity (fibrillation) from mentalis (Vll Nerve), temporalis (V Nerve), and tongue (Xll nerve). The study also showed evidence of bulbar muscle denervation, chronic denervation of C8/T1 myotomes, and a coexistent mild sensory neuropathy. Clinically these siblings had features consistent with Brown-Vialetto-Van Laere Syndrome (BVVLS).
Patient number 18 was a 16-year-old female with features of bulbar palsy, anterior horn cell involvement with tongue wasting and fasciculations, pyramidal tract involvement with signs of plasticity, hypereflexia and clonus, posterior column involvement with asymmetrical sensory neuropathy as shown on nerve conduction studies. On tibial SSEP, she had delayed cortical potentials indicative of dorsal cord dysfunction [Figure 3]. MRI showed features of cerebellar atrophy. She did not have a positive family history and was diagnosed to have a sporadic onset spinocerebellar ataxia.
|Figure 3: Posterior tibial somato-sensory evoked potential of a patient revealing bilateral central pathway dysfunction|
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Patient number 20, a 15-year-old boy with an uncle having similar symptoms and hearing loss had features of pyramidal tract involvement, bilateral distal muscle wasting, spasticity of lower limbs, bilateral optic nerve dysfunction and sensory motor PN on NCV was diagnosed to have hereditary motor-sensory neuropathy type 2 variant (CMT/ HMSN type 2).
Involvement of various subdivisions of the neural axis in the 21 patients is given in [Table 2].
| Discussion|| |
Our frequency of ANSD (1.12% ) in the tertiary care setting is within what is reported by Cummings et al. Vestibular lesions are known to be associated with ANSD but the extent of involvement and site of lesion is variable; though most often they tend to be of peripheral type. ,
Named neurodegenerative conditions like Friedreich's ataxia, CMT, Madras motor neuron disease (MMND) and Refsum's disease are found to be associated with AN. ,
The neuropathology in these conditions appears to be due to specific mutations causing abnormalities in different neuronal mitochondrial proteins. ,,,, More research points towards mitochondrial defects as fundamental pathology in these diseases. 
In our series, the presence of neurological abnormalities in AN was found to be 60% (21/35). ANSD can be associated with involvement of all regions of the neuroaxis like the cranial nerves, peripheral nerves, cerebellum, anterior horn cells and dorsal columns of the spinal cord. Patients with AN having PN seem to be a feature unique to the adult population. ,
Nine subjects older than fifteen years demonstrated the presence of PN whereas no one younger than five years had any clinical evidence of PN. The timing of onset of PN associated with AN is unpredictable. In some, the hearing loss preceded the symptoms of peripheral weakness or sensory loss, while in others, the neuropathy came first. Starr et al. report that sural nerve biopsies from patients with clinical evidence of PN have rapidly progressive mixed axonal and demyelinating neuropathy.  Starr also reports 3 out of 10 who had apparent HMSN (CMT) and 5 had slight evidence of neuropathy clinically as well as on nerve conduction studies, two young patients (4 and 16 years old) had no sign of neuropathy.
Due to the insidious onset and slow progress of vestibular lesions, most patients are symptom free. Gait disturbances, falls and imbalance in these patients have been associated with neurological involvement. All 10 of our patients who complained of imbalance were found to have abnormalities of the neuroaxis suggesting that it could be an important clinical symptom indicative of neurological associations. Apart from vestibular testing for these patients it is important to look for signs of other central involvement.
Congenital ANSD is mainly caused by genetic disorders and they may be expressed in isolation or manifest with a range of associated abnormalities. Non syndromic autosomal dominant inheritance of ANSD has been reported by Kim et al.  A few instances of non syndromic autosomal recessive type of ANSD have been described in literatures. , Genetic studies have shown mutations in OTOF gene which encodes otoferlin to cause AN associated with temperature dependant hearing loss.  Wang et al. identified a region on the X chromosome responsible for this disorder and called it the AN X-linked recessive locus 1.  There is a strong underlying genetic basis for ANSD associated with neurological abnormalities as seen in patients six and 20, and siblings belonging to one family; patients 10, 11, 12, and 13. These last four siblings were diagnosed to have BVVLS with an autosomal recessive type inheritance from disease-free parents of consanguineous marriage (uncle-niece).  Some cases of this entity are caused by mutations in the SLC52A3 gene which encodes the intestinal (hRFT2) riboflavin transporter.  Out of the 35 patients of ANSD diagnosed during this study 23 were born to parents of consanguineous marriage (10 of the 21 with neurological lesions). The custom of maternal uncle marrying his niece and marriage between first cousins is widely prevalent in this region of southern India in Hindu as well as in Muslim communities.  Further genetic tests are needed to study this in more detail among our patients. There is a strong association of neonatal hyperbilirubinemia in infants with acquired AN and conditions like low socioeconomic status and exposure to toxic chemicals have been postulated to play a role in late onset AN. ,
Shivshanker et al. described 47 cases with no neuropathy clinically as well as on nerve conduction studies and they termed it as PAN.  In our study, out of five children with ANSD, four had an associated neurological disorder. Such a high association is likely because of the referral pattern to this tertiary center. India does not have a mandatory national universal neonatal hearing screening programme in place. Most children younger than five with hearing loss are presumably fitted with conventional hearing aids in smaller centers and the type of hearing loss remains undetected. When such children continue to have poor speech outcomes, they eventually get referred to tertiary centers and the referred subset is more likely to have ANSD.
Among the 30 adults with AN, we found 17 to have neurological abnormalities. These abnormalities were subclinical in most, with a positive history elicited only on probing and pointed questioning (eight patients). Further examinations revealed 21 patients with neurological findings. Although the common presentation of such patients is with hearing loss and difficulty in understanding speech, a detailed neurological history and examination results in identification of subtle neurological abnormalities and hence early referral to a neurologist. The hearing loss and neurological deficits also do not develop at the same time; one symptom may precede the other by several years. Gait abnormalities developed on an average of a year from the onset of hearing loss with a range of 0-5 years. Follow up of newly diagnosed cases of PAN is important in order to detect the neurological deficits as and when they manifest. Management of the hearing loss in these patients needs to be done in conjunction with the other abnormalities present and hence a multidisciplinary approach for rehabilitation is recommended. Counselling of patients and their families become important in the light of the strong associations with neural abnormalities.
There has been controversy surrounding the exact site of lesion in patients with AN. The area of involvement can be presynaptic (inner hair cells), synaptic or post synaptic.  The coexistence of other neuropathies in our patients suggests neuronal damage of peripheral or cranial nerves pointing to the site of lesion as postsynaptic. 
Management of these patients is supportive. Genetic testing and counselling should be offered. Physiotherapy to prevent limb contractures can minimize disability. There is no specific disease modifying therapy available yet that alters the course of disease. Riboflavin supplementation has been shown to improve outcomes and long term survival in some patients with BVVLS irrespective of whether plasma flavin and acylcarnitine levels were abnormal. In situations of resource crunch, a trial of riboflavin supplementation may be warranted in patients with ANSD and neurological involvement, pending further genetic tests.
Hearing rehabilitation should be started early for better communication outcomes. Conventional hearing aids as well as cochlear implantation have been found to be useful in patients with elevated hearing thresholds.  Recent studies have predicted good cochlear implant outcomes for children with ANSD. , One reason attributable to this is that even nerves that fire dyssynchronously in AN are better stimulated electrically with increased repeatability and precision than acoustically stimulated nerves.  There are relatively fewer reports of adult patients with ANSD undergoing cochlear implants as more commonly these patients have associated neuropathies and hence reservations about outcomes may be present. De Leenheer et al. reported three adults with ANSD, two of whom had associated neuropathies. These two patients with PN had improved outcomes compared to pre-implant scores. 
| Conclusion|| |
In our study we found neurological involvement in patients with AN to be quite high (60%). The range of abnormalities associated varied from cerebral palsy and seizure disorder in young children to PN, hereditary motor-sensory neuropathy, spinocerebellar ataxia, spastic paresis, and pontobulbar palsy in older people. A follow up of patients with AN should be done to detect any abnormalities which may appear later in life and patients should be advised regarding the appearance of such symptoms. Majority of the patients, who had abnormal neurological findings, had subclinical presentation and symptoms were elicited only with careful history taking and probing questions. A hereditary etiology is associated with a significant number of patients with ANSD. Genetic counselling and testing should be offered wherever feasible.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2]