Pathophysiology

Perilymph fistula refers to a pathologic condition in which there is a persistent communication between the inner ear and the middle ear. Perilymph fistula implies that there is an egress of perilymph, either constantly or intermittently from the inner ear, and it is this loss of perilymph from the inner ear that produces symptoms. The precise effect of this change in the volume of perilymph is not understood. Temporary loss of perilymph occurs often in stapedectomy with little or no permanent effect on inner ear function. Chronic perilymphatic leak, as seen in fistulization of the labyrinth by cholesteatoma, can result in permanent auditory and vestibular dysfunction. Although not proved, the chronic loss of fluid from the peri-lymphatic space should lead to expansion of the endolymphatic spaces, producing a condition similar to endolymphatic hydrops. This most likely explains the reason that the symptoms of perilymph fistula and endolymphatic hydrops are at times indistinguishable.

The mechanisms for the development of perilymph fistula are credited to Goodhill.3 He described both implosive and explosive causes. Implosive fistulae occur when a force is exerted on the oval and round windows through the middle ear. This might involve severe excursion of the tympanic membrane or ossicular chain, such as in acoustic trauma, or by direct positive pressure on the oval and round windows due to severe change in middle ear pressure as in a sudden decompression syndrome (scuba diving). The explosive mechanism occurs by transmission of a sudden increase in intracranial pressure to the inner ear. This occurs primarily through the cochlear aqueduct, theoretically, when the aqueduct is abnormally patent, but it also can occur through the internal auditory canal in congenital anomalies such as Mondini dysplasias. This increase in intracranial pressure can occur as a result of head trauma, coughing, or any type of straining activity. The existence of microfissures or patent tracts in the fissula ante fenestra have been postulated as sources for the perilymph fistula by various investigators. The precise pathophysiologic mechanism has not been described, obviously lending concern to the actual existence of this disorder.

Perilymph fistulae are either traumatic, iatrogenic, idio-pathic, or congenital. Trauma may be blunt, such as a head injury with a temporal bone fracture, barotrauma, increased intracranial pressure; acoustic, as occurs with a blast injury; or penetrating, as in ossicular fracture due to foreign body injury. Iatrogenic causes of perilymph fistula most commonly occur after stapedectomy but may occur in any case of inadvertent ossicular disruption. Idiopathic, or spontaneous, perilymph fistulae occur in cases in which no identifiable preceding condition can be recognized. Congenital malformations such as Mondini dysplasias carry a high risk of perilymph fistulae because of the more extensive communication with intracranial cerebrospinal (CSF) pressure due to the foreshortened patent internal auditory canals that often open directly into the otic capsule.

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