Estimates of the annual incidence of sudden SSNHL range from 5 to 20 cases per 100,000 persons (2). Because many patients do not seek medical attention after spontaneous resolution, and others are misdiagnosed initially, this incidence figure is an underestimate. Likely, there is an equal distribution of female-to-male cases. Cumulative data from several studies show a slight male preponderance at 53% (1,530/2,864) (2,3,4,5,6,7,8); however, another large study of 1,220 patients noted a slight female preponderance, without specifying numbers (9). Gender does not seem to be a risk factor. Several studies have not delineated right versus left ears affected, other than to state an equal distribution. Curiously, from combined study data, it was found that hearing loss affected more left ears (55%) (2,3,4,5,7,10). An equal distribution between ears would be expected. Bilateral sudden hearing loss occurs in about 1% to 5% of cases (3,4,5,7,9,10,11). Sudden hearing loss occurs in all age groups, but fewer cases are reported in children or the elderly (2,5,6). Middle-aged and young adults experience similar incidence rates (2,5,6,9). The median age at presentation ranges from 40 to 54 years (3,4,5,6,8). Acute tinnitus accompanies the hearing loss in most cases, and vestibular symptoms are present in one-fourth to half of patients (3,4,5,6,8).

Considerable circumstantial evidence implicates viral infection as one cause of ISSNHL. From studies of patients with ISSNHL, this trail of evidence can be traced from reports of a high prevalence of a recent viral type of illness, through evidence of recent viral seroconversion, to temporal bone histopathology. The weakest of these links is the associated history of a recent viral illness. Noncontrolled studies report that 17% to 33% of patients recall a recent viral illness (3,9). Lest those numbers seem significant, 25% of healthy patients visiting an otolaryngology clinic had experienced a potential viral illness within a month preceding the visit (14). Wilson provided good evidence of viral seroconversion by comparing patients experiencing ISSNHL with control patients. Rates of seroconversion for the herpesvirus family were significantly higher in the sudden-hearing-loss population (15). Finally, temporal bone histopathologic studies of patients who experienced ISSNHL found damage in the cochlea consistent with viral injuries (16,17,18,19). Loss of hair cells and supporting cells, atrophy of the tectorial membrane, atrophy of the stria vascularis, and neuronal loss were seen; these patterns were similar to findings in documented cases of mumps-, measles-, and maternal rubella-related hearing loss. Viral infection can be implicated as a cause of ISSNHL, but it cannot as yet be proved.

The cochlea derives its blood supply from the labyrinthine artery, with no collateral vasculature. Cochlear function is exquisitely sensitive to changes in blood supply (20). Thus, vascular compromise of the cochlea caused by thrombosis, embolus, reduced blood flow, or vasospasm would seem a likely etiology for ISSNHL. The abrupt or rapid time course in sudden hearing loss correlates well with a vascular event. 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) (21). Histologic evidence of cochlear damage following occlusion of the labyrinthine vessels was documented in temporal bone studies in both animals and humans (19,22). Intracochlear hemorrhage was noted as an early development; subsequently, fibrosis and ossification of the cochlea evolved. It remains unproven whether SSNHL is associated with common cardiovascular risk factors. An association between SSNHL and hypercholesterolemia was revealed in a few case-control series (23,24,25). SSNHL may be more common and severe in patients with diabetes (26).

Thin membranes separate the inner ear from the middle ear, and within the cochlea, delicate membranes separate the perilymphatic and endolymphatic spaces. Rupture of either or both sets of membranes theoretically could produce a sensory hearing loss. A leak of perilymph fluid into the middle ear via the round window or oval window has
been postulated to produce hearing loss by creating a state of relative endolymphatic hydrops or by producing intracochlear membrane breaks. Rupture of intracochlear membranes would allow mixing of perilymph and endolymph, effectively altering the endocochlear potential. Simmons (27) favored the theory of intracochlear membrane rupture, as did Goodhill and Harris (28); histologic evidence was documented by Gussen (29).