Excerpt from Inner Ear, Sudden Hearing Loss

Synonyms, Key Words, and Related Terms: sudden sensorineural hearing loss, idiopathic sudden sensory hearing loss, ISSHL, sudden deafness

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Background: Definitions of sudden hearing loss have been based on severity, time course, audiometric criteria, and frequency spectrum of the loss. Abrupt as well as rapidly progressive losses have been included under a single definition of sudden hearing loss. Awakening with a hearing loss, hearing loss noted over a few days, selective low- or high-frequency loss, and distortions in speech perception have all been classified as sudden hearing losses. A commonly used criterion to qualify for this diagnosis is a sensorineural hearing loss of greater than 30 dB over 3 contiguous pure-tone frequencies. Fortunately, the vast majority of cases of sudden hearing loss are unilateral, and the prognosis for some recovery of hearing is good. Usually it presents as unilateral loss of hearing; bilateral involvement is rare, and simultaneous bilateral involvement is very rare.

Sudden deafness or sudden SNHL and has many possible etiologies.

Pathophysiology: The postulated pathophysiology for idiopathic sudden sensory hearing loss (ISSHL) has 4 theoretical pathways. These are labyrinthine viral infection, labyrinthine vascular compromise, intracochlear membrane ruptures, and immune-mediated inner ear disease. A disease process involving any of these theoretical possibilities could have sudden hearing loss as a symptom. Each theory may explain a fraction of the episodes of sudden sensory hearing loss, but none of the existing theories individually could account for all episodes.

Viral infection

The evidence to implicate viral infection as one cause of sudden idiopathic sensory hearing loss is circumstantial. Studies of patients with ISSHL show a moderate prevalence of recent viral-type illness. Sometimes, evidence of recent viral seroconversion or inner ear histopathology consistent with viral infection is present.

The weakest of these links is the history of a recent viral illness. Noncontrolled studies report that 17-33% of patients recall a recent viral illness. Should those numbers seem significant, 25% of patients without hearing loss visiting an otolaryngology clinic had experienced a viral-like illness within a month.

Comparing patients experiencing ISSHL with control patients has produced some evidence of viral seroconversion. Rates of seroconversion for the herpesvirus family were significantly higher in the population of patients with sudden hearing loss.

Finally, temporal bone histopathologic studies of patients who experienced ISSHL found damage in the cochlea consistent with viral injuries. Loss of hair cells and supporting cells, atrophy of the tectorial membrane, atrophy of the stria vascularis, and neuronal loss were observed. These patterns were similar to findings in documented cases of hearing loss secondary to mumps, measles, and maternal rubella. Viral infection can be implicated as a cause of ISSHL, but this cannot, as yet, be proven. Infections with mumps virus provide the best model for a virally induced sensorineural hearing loss. In one study of ISSHL, subclinical mumps infections were documented in 9 of 130 patients by positive immunoglobulin M (IgM) mumps antibodies.

Vascular compromise

The cochlea is an end organ with respect to its blood supply, with no collateral vasculature. Cochlear function is exquisitely sensitive to changes in blood supply. Vascular compromise of the cochlea due to thrombosis, embolus, reduced blood flow, or vasospasm seems to be a likely etiology for ISSHL. The time course correlates well with a vascular event, a sudden or abrupt loss. A reduction in oxygenation of the cochlea is the likely consequence of alterations in cochlear blood flow. Alterations in perilymph oxygen tension have been measured in response to changes in systemic blood pressure or intravascular carbon dioxide partial pressure (pCO₂).

Histologic evidence of cochlear damage following occlusion .....