Severe sensorineural deafness that usually occurs in only one ear and lasts for a few hours or less. Sudden deafness affects approximately 1 in 5000 people each year. Although the sudden onset suggests a vascular cause (embolism, thrombosis, or hemorrhage), similar to vascular accidents of the central nervous system, there is evidence in a large number of patients to support an etiology due to a viral infection. Sudden deafness tends to occur in children without evidence of vascular disease and in young or middle-aged adults. Histopathologic findings of temporal bone in patients with sudden deafness do not resemble those seen in animals with experimental embolism or inner ear changes after vascular obstruction, but approximate sudden deafness caused by viral infections of the inner ear in humans (viral endolymphatic vaginitis), such as mumps and measles, influenza, varicella and mononucleosis viruses, adenovirus and other viruses that can also cause sudden deafness. The pathologic findings of permanent deafness caused by viral endolymphatic vaginitis are similar regardless of the virus causing it. The Corti apparatus of the basal gyrus of the cochlea shows progressive damage, with a decrease in spiral ganglion cells, a tendency for individual hair cells to disappear, and atrophy of the vascular lines. The lamina cribrosa often curls upwards and sits in the syncytium. The vestibular membrane may atrophy and adhere to the basement membrane. Exolymphatic fistulas between the inner and middle ear sometimes occur due to drastic changes in external air pressure or forceful activities such as weight lifting. Fistulas with round or oval windows cause sudden or fluctuating sensorineural deafness and vertigo. When a fistula occurs, the patient may feel an explosion in the affected ear. The presence of an exolymphatic fistula can be verified by a combination of pressure changes in the ear canal using an acoustic conductance meter and nystagmography. Nystagmus resulting from altered pressure in the external auditory canal can be recorded by nystagmography and indicates the presence of an exolymphatic fistula.