Sudden Hearing Loss
- Author: Neeraj N Mathur, MBBS, MS; Chief Editor: Arlen D Meyers, MD, MBA more...
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 occurring within 3 days' period. 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 sensorineural hearing loss (SNHL) has many possible etiologies.
The postulated pathophysiology for idiopathic sudden sensory hearing loss (ISSHL) has 4 theoretical pathways, as follows:
Labyrinthine viral infection
Labyrinthine vascular compromise
Intracochlear membrane ruptures
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.
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.
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 of the labyrinthine vessels was documented in temporal bone studies in animals and humans. Intracochlear hemorrhage was noted as an early development; subsequently, fibrosis and ossification of the cochlea evolved.
In one study, a partial overlap was found between classical coronary risk factors and risk factors for sudden hearing loss. Hypercholesterolemia and hypoalphalipoproteinemia (low HDL cholesterol levels) were not found to be apparent major risk factors for sudden hearing loss, whereas the GPIa C807T polymorphism, elevated fibrinogen levels, and smoking were associated with an increased risk for ISSHL.
In contrast to the above study, a historical prospective cohort study by Chang et al indicated that hypercholesterolemia is associated with an increased risk for idiopathic sudden sensorineural hearing loss (ISSNHL). Comparing nearly 74,000 patients with hypercholesterolemia with the same number of age-matched controls, the investigators found the incidence of ISSNHL to be 1.62 times greater in the hypercholesterolemia group.
Altogether, the various study findings suggest a vascular involvement in the pathogenesis of ISSHL. This may have important implications for the development of therapeutic and preventive strategies for ISSHL.
Intracochlear membrane rupture
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. The theory of intracochlear membrane rupture was favored by Simmons and Goodhill, and histologic evidence has been documented by Gussen.[4, 5, 6]
Immune-mediated inner ear disease
Sensorineural hearing loss induced by an immune process has gained greater and greater notoriety since the concept was introduced in 1979. Progressive sensorineural loss is observed with this condition. Whether or not sudden hearing loss occurs with immune-mediated inner ear disease is unclear, but immunologic activity in the cochlea is supported by greater and greater evidence. The association of hearing loss in Cogan syndrome, systemic lupus erythematosus, and other autoimmune rheumatologic disorders has been well documented. With better markers for inner ear autoimmunity, perhaps a greater linkage with ISSNHL will be found. A recent prospective study on 51 patients with ISSNHL supported the existence of multiple immune-mediated disorders in these patients.
A study by Chung et al indicated that iron-deficiency anemia increases the risk for sudden sensorineural hearing loss. The report, which involved about 4000 persons with sudden sensorineural hearing loss and approximately 12,000 controls, found that 4.3% of the group with hearing loss had previously been diagnosed with iron-deficiency anemia, compared with 3.0% of the control group. The link between hearing loss and anemia seemed to be strongest in persons aged 44 years or younger.
Estimates of the annual incidence of sudden sensory hearing loss range from 5-20 cases per 100,000 persons. Many cases likely go unreported, and the incidence may be higher. A sudden hearing loss may resolve before the patient can be evaluated medically, making it unlikely for that individual to seek care.
The female-to-male distribution appears to be equal. Combined data from several studies show a slight male preponderance, at 53%. However, a single large study of 1220 patients had slightly more females. Sex does not seem to be a risk factor. An equal distribution of right- and left-sided cases should be expected. As with sex, no greater risk for right-sided losses compared with left-sided losses seems to exist. Bilateral sudden hearing loss occurs in approximately 1-2% of cases.
People of all age groups are affected by sudden hearing loss, but fewer cases are reported in children and the elderly. The peak incidence appears to be in the sixth decade of life. Young adults have incidence rates similar to those of middle-aged adults. The median age at presentation ranges from 40-54 years. The occurrence of sudden hearing loss across all age groups is an indication of the multifactorial nature of this clinical problem.
A literature review by Sara et al indicated that the onset of bilateral sudden sensorineural hearing loss tends to occur at a younger age than the unilateral form.
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