Auditory neuropathy/Auditory dyssynchrony

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CASE REPORT

Auditory neuropathy/Auditory dyssynchrony - An underdiagnosed condition: A case report with review of literature Vinish Agarwal, Saurabh Varshney, Sampan Singh Bist, Sanjiv Bhagat, Sarita Mishra, Vivek Jha Department of E.N.T., Himalayan Institute of Medical Sciences, (HIHT University), Jollygrant, Doiwala, Dehradun, Uttaranchal, India

Abstract

Auditory neuropathy (AN)/auditory dyssynchrony (AD) is a very often missed diagnosis, hence an underdiagnosed condition in clinical practice. Auditory neuropathy is a condition in which patients, on audiologic evaluation, are found to have normal outer hair cell function and abnormal neural function at the level of the eighth nerve. These patients, on clinical testing, are found to have normal otoacoustic emissions, whereas auditory brainstem response audiometry reveals the absence of neural synchrony. Unlike space‑occupying lesions, radiologic evaluation reveals normal results. Patients with auditory neuropathy require a different management approach to their auditory and communication problems from approaches used with patients with usual peripheral hearing losses. Keywords: Auditory brainstem responses, Auditory neuropathy, Otoacoustic emission

Introduction Auditor y neuropathy (AN), also known as ‘neural dyssynchrony / auditory dyssynchrony (AD),[1] is a hearing loss due to abnormal neural processing of auditory stimuli. Patients with this disorder are able to respond to sounds appropriately, but their ability to decode speech and language is hindered. These patients have hearing loss ranging from mild to severe with poor speech perception abilities. AN/AD has only recently been described. In the late 1970s, clinical investigators began to describe groups of patients with normal or slightly elevated audiogram pure tone thresholds accompanied with absent or severely abnormal auditory brainstem responses (ABRs). With the advent of the otoacoustic emissions (OAEs) in the mid 1980s, these groups of patients were found to have normal cochlear function. The finding of normal cochlear function accompanied with abnormal brainstem responses was defined in 1996 as auditory neuropathy (AN). Whether this represents a true auditory nerve neuropathy is debatable. Further investigations led to the conclusion that AN may truly represent a dyssynchronous auditory nerve rather than a neuropathy. This finding gave rise to the newer term Address for correspondence: Dr. Saurabh Varshney,

Department of E.N.T., Himalayan Institute of Medical Sciences, (HIHT University), Jollygrant, Doiwala, Dehradun ‑ 248 140, Uttaranchal, India. E‑mail: [email protected] 156

of auditory dyssynchrony (AD).[1] For the purposes of this article, AN and AD are considered synonymous (i.e., AN/AD). AN can affect people of all ages, from infancy to adulthood. The purpose of this article is to discuss clinical presentation, audiological tests, and its management options, as AN is very often missed as a diagnosis in clinical practice.

Case Report A 16‑year‑old female student presented with complains of insidious, progressive hearing loss for 2 years in both ears. There was no history of any ear discharge, trauma, ototoxicity, or any other medical or surgical illness. There was no contributory antenatal, perinatal, and postnatal history. She was born as full term normal vaginal delivery in hospital. There was no history suggestive of hearing loss in the family. On clinical examination, bilateral otoscopy was normal. On performing Tuning fork tests, Rinne’s test was positive Access this article online Quick Response Code: Website: www.indianjotol.org

DOI: 10.4103/0971-7749.103445

Indian Journal of Otology | July 2012 | Vol 18 | Issue 3 |


Agarwal, et al.: Auditory neuropathy

bilaterally, Weber’s test was centralized, and absolute bone conduction test was reduced bilaterally. Rest of the ENT and head neck examination was normal. Pure tone audiometry (PTA) showed moderately severe sensorineural hearing loss (Avg. 65 dB) on right side and mild to moderate sensorineural hearing loss (Avg. 40 dB) on left side, which was more for lower frequencies [Figure 1]. Speech discrimination scores were out of proportion with suspected hearing loss. Tympanometry showed bilaterally ‘A’ type of curve with absence of ipsilateral and contralateral reflexes [Figure 2]. Otoacoustic emissions (DPOAE/TPOAE) were present for both ears [Figure 3]. On ABR, no definite Vth waves were seen on either side, even after 90 dB and 96 dB [Figure 4]. The patient underwent magnetic resonance imaging (MRI) of brain and HRCT temporal bone, which were normal. Hence, a diagnosis of AN was made as patient had history of bilateral hearing loss; PTA showed bilateral sensorineural hearing loss, OAE was present bilaterally, but ABR showed severe sensorineural hearing loss in both ear. The patient was advised cochlear implant, but she could not afford it. Hence, patient was advised use of hearing aid for both ears, which she is using with some gain.

Pathophysiology The term auditory neuropathy/auditory dyssynchrony (AN/ AD) describes a diagnosis that affects a small group of patients with hearing loss and speech intelligibility scores out of proportion with their presumed hearing loss. Many authors have suggested that the abnormalities that cause AN reside within the lower auditory system. Specifically, the spiral

ganglion cells, auditory nerve, or the auditory brainstem nuclei have all been implicated. The combination of a dysfunctional auditory nerve with preservation of cochlear function can theoretically be caused at several different points along the lower auditory pathway. The following abnormalities have been proposed: • Injury to the synaptic junction between inner hair cells of the cochlea and dendrites of spiral ganglion neurons • Direct damage to the dendrites of the spiral ganglion neurons • Direct injury to the spiral ganglion neurons • Direct axonal damage to the auditory nerve that causes a cascade of damage to the lower auditory nuclei

Discussion Auditory neuropathy was first reported in late 1970’s as paradoxical findings between ABR and OAE and was defined as auditory neuropathy (AN) in 1996 by Arnold Starr.[2] Madden et al. found that 22 out of 428 children with hearing loss had AN.[3] Some authors have suggested that the prevalence is 2‑15% of children with known hearing loss. In a 2002 review of the prevalence, Sininger suggested that approximately 1 in 10 children with hearing loss and severely affected ABR test results have AN. Overall, AN is rare and can be found in an estimated 1‑3 children per 10,000 births.[4] In Southern India, prevalence of auditory dys‑synchrony was reported around 1 in 183 in individuals with sensory neural hearing loss.[5] Auditory neuropathy / auditory dyssynchrony

Figure 1: Pure Tone Audiogram‑ Right Ear‑moderate to severe SN hearing loss; Left Ear‑mild to moderate SN hearing loss. (Note: Conductive component could be seen on low frequency) Indian Journal of Otology | July 2012 | Vol 18 | Issue 3 |

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Figure 2: Tympanometry‑ Bilateral “A” type of curve, without ipsi and contralateral acoustic reflex. (Impression: No indication of middle ear pathology)

a

b

Figure 3 (a, b): Otoacostic emission (O.A.E.)‑ TEAOE and DPOAE‑ showing bilateral normal cochlear function

(AN) has no racial bias and occurs with near‑equal frequency in males and females. The patients range in age from infant to adult and display auditory characteristics consistent with normal outer hair cell function and abnormal neural function at the level of the VIIIth nerve. Because AN is a relatively newly described condition, many adults may have not obtained the 158

proper audiologic testing to reach a diagnosis of AN. With the advent of newborn hearing screening tests, the delay in diagnosis of AN should be minimized, which will expedite intervention. Spiral ganglion cells, auditory nerve, or auditory brainstem Indian Journal of Otology | July 2012 | Vol 18 | Issue 3 |



Figure 4: ABR‑ Auditory Brain Response‑ no definite Vth wave were seen on either side even after 90 dB and 96 Db.

nuclei have been implicated for the abnormalities that cause AN. Two forms of AN have been suggested (i) pre‑synaptic type and (ii) post‑synaptic version.[6] AN may also affect vestibular function.[7] Risk factors for AN include neonatal history of anoxia, hyperbilirubinemia, mechanical ventilation, hypoxia, congenital brain abnormalities, low birth weight, extremely premature birth (