Clinical sensorineural deafness is a hearing impairment disorder that includes lesions of the sound sensory organ, the cochlear hair cells, the auditory nerve and the auditory nucleus. The etiology, pathology and clinical features of sensorineural deafness vary from site to site. The use of otoacoustic emissions (OAE) and auditory brainstem response (ABR) provides a reliable diagnostic basis for the diagnosis of cochlear and retrocochlear lesions. Auditory neuropathy (AN) has been recognized in recent years and is receiving more and more attention, and Starr et al. called it a symptom group of auditory neuropathy, which is a specific clinical manifestation of sensorineural deafness caused by damage to the auditory nerve of the cranial nerve VIII.
Scholars at home and abroad mostly name it according to its main site of damage and have different views on naming, such as central low-frequency hearing loss, auditory nerve synchronous distribution disease, and type I afferent neuron disease. Because auditory neuropathy may be accompanied by vestibular neuropathy, Sheykholeslami proposed that for only the auditory neuropathy of the auditory branch of the VIII cranial nerve (and its distribution) is called cochlear neuropathy. Due to the different etiologies of auditory neuropathies, the clinical manifestations have many differences in age of onset, coexistence of peripheral neuropathies, and lesion sites, in addition to the performance of audiometric tests. Hearing loss with abnormal ABR and normal OAE due to different etiologies and lesions (except for the cochlear nerve of the VIIIth cranial nerve between the cochlea and the brainstem) should be a group of symptom clusters (syndrome) rather than a disease name.
1.Characteristics of auditory neuropathy
In recent years, many scholars have noticed that the speech recognition rate of some deaf patients is significantly lower than the pure tone hearing threshold, and their cochlear electrograms and otoacoustic emissions are normal but ABR is unresponsive. 1996 Starr named this type of deafness as auditory neuropathy. The clinical features of auditory neuropathy are that patients who report significant hearing loss have a mild to moderate pure-tone hearing threshold; the ABR is unresponsive; most patients complain of inaudible speech and have varying degrees of difficulty communicating verbally, especially when talking on the phone. Poor speech hearing is an important feature of the disease, and its speech recognition rate is disproportionately lower than the pure tone hearing threshold. The diagnosis of the disease relies on characteristic audiological manifestations. absence or abnormality of ABR, normal DPOAE, TEOAE and cochlear microtonal potential (CM) are among the most important features of auditory neuropathy audiology.
Poor speech recognition, disproportionate to the pure tone hearing threshold, abnormal auditory brainstem response, and mild to moderate sensorineural deafness with a predominantly low-frequency hearing loss in a rising pattern are all characteristics of the audiological manifestations. However, there are individual differences. Sensorineural deafness can be severe or very severe, and the audiogram can be overlapping, falling or flat with high frequency hearing loss. Because of the individual differences in presentation, a comprehensive analysis of the hearing test results should be made. Since the disease is most often seen in infants and adolescents, a comprehensive audiological examination such as ABR, OAE and acoustic conductance test should be performed in patients with sensorineural deafness who are at high risk of neonatal, infant and adolescent hearing loss and some patients with contradictory subjective and objective findings of audiological examination to avoid misdiagnosis and missed diagnosis. In recent years, Duyle found that in addition to typical audiological manifestations, patients with auditory neuropathy have normal or enhanced responses to the brain nerve on imaging MRI.
2. Etiology and pathogenesis
There is no uniform understanding of the etiology and pathogenesis of AN. There are several etiologies that have been reported so far. Based on the associated diseases of the patient, it is presumed that the etiology may include.
(1) hyperbilirubinemia, riboflavin and perinatal asphyxia and hypoxia, which are mainly seen in infants: Mornt et al. described the diagnosis and management of two cases of infantile auditory neuropathy, a male infant with hyperbilirubinemia who was found to have hearing loss during a hearing screening test and a moderate hearing loss on rechecking 3 months later, diagnosed as auditory neuropathy . Tapia et al. studied a group of patients with auditory neuropathy, and the etiology included: hyperbilirubinemia erythrocythemia, perinatal asphyxia.
② hereditary diseases: such as Charcot-Marie-Tooth (CMT) syndrome; others are hereditary sensory autonomic disorders, Friedreich ataxic neuropathy, etc. The prevalence of hearing impairment in these patients has been reported in the literature to be up to 30%.
③ Idiopathic: Tapia et al. studied a group of patients with auditory neuropathy, some without a clear etiology, considered idiopathic.
④Infectious diseases: such as meningitis.
⑤ Auditory neuropathy with vestibular disease: the vestibular branch of the auditory nerve with its innervated structures may be involved at the same time as the onset of auditory neuropathy. Skeyhololeslamj et al. followed three patients with confirmed auditory neuropathy with balance disorders and abnormal vestibular evoked myogenic potentials (VEMP), suggesting that the inferior vestibular nerve may be involved.
(vi) Nerve demyelination: From the analysis of neurophysiological test results: the neuropathological changes of auditory neuropathy may be demyelination. There are two reasons for this: a. Certain peripheral neuropathies with demyelination as the main pathological change (Green-Barre syndrome), hereditary neurological diseases (such as Charcot-Marie-Tooth syndrome) and central primary demyelinating diseases (such as multiple sclerosis) can involve the auditory nerve and show manifestations of auditory neuropathy b. Demyelination can explain the neurophysiological abnormalities seen in patients with auditory neuropathy, such as abnormalities in auditory brainstem evoked potentials (manifested by poor wave repetition, delayed conduction time, partial or complete block, etc.) and slowed sensory nerve conduction velocity, reduced wave amplitude, and motor nerve changes in the peripheral nerves of the limbs.
(7) Autoimmune diseases of the inner ear: McCabe (1979) reported autoimmune sensorineural deafness, with a case holding point of bilateral hearing loss, decreased speech recognition, and end-organ lesions on localization. Gu R (2000) reported central low-frequency hearing loss twice, and some of the cases were positive for anti-membrane vagus protein antibodies and treated with steroids with some success, suggesting that these cases may be autoimmune diseases. The sites of lesions in auditory neuropathy, which have been inconsistently reported in the literature, include.
(i) Inner hair cells;
(ii) synaptic connections between the inner hair cells and cranial nerve fibers VIII;
(iii) spiral ganglion cells;
④Auditory nerve fibers in the cochlea;
Starr’s study of auditory neuropathy identified the cochlear branch of cranial nerve VIII between the cochlea and the brainstem, excluding the brainstem auditory pathway, and Tapia et al. found normal outer hair cells in a group of patients with auditory neuropathy by TEOAE and ABR, but were unable to determine whether the damage was to the inner hair cells, the auditory nerve, or both.
There is a lack of effective treatment for auditory neuropathy, and treatment should be tailored to the underlying disease. There are conflicting opinions on the effectiveness of cochlear implants and conventional hearing aid applications. tapia et al. found that the effectiveness of hearing aids varied from person to person, but concluded that speech rehabilitation should be started early. In conclusion, the etiology and pathogenesis of auditory neuropathy are still unclear, and it will be imperative to explore the etiology and pathogenesis of auditory neuropathy at the molecular level in terms of animal models, immunology, genetics, and neuroepidemiology.