Perilymphatic Fistula Revisited: A Histopathologic Study of Inner Ear Dehiscence and Possible Third Window Syndrome

Cases historically presumed to be perilymphatic fistula of the round and/or oval window may be cases of third window syndrome due to inner ear dehiscence.

Perilymphatic fistula (PLF) is a condition where a pathologic external connection of the perilymphatic space is present, often with an associated otic capsule or stapes defect. Recently, it has become evident that otic capsule defects in locations that lack perilymphatic fluid leak can cause clinical symptoms. Such clinical entities are deemed "third window" syndromes-among which superior semicircular canal dehiscence is the most common.

Human temporal bone specimens underwent histopathologic study of cases previously suspected to have PLF at the oval or round window. These specimens were further scrutinized for the presence of an alternate site of inner ear dehiscence that may have potentially caused a third window syndrome.

Thirty-one out of 34 of the cases (61 ears) from a previously published study on PLF were reviewed. Altogether, dehiscences were noted at the following locations: cochlea-facial (11), superior semicircular canal (5), endolymphatic sac-jugular bulb (4), cochlea-internal auditory canal (3), posterior semicircular canal (2), and an enlarged/patent cochlear aqueduct (2). One patient with a histologic dehiscence had an audiogram consistent with third window syndrome.

The findings suggest that many clinical cases historically presumed to be PLF of the round and/or oval window may, in fact, be cases of third window syndrome due to inner ear dehiscence with pathology at sites other than the oval or round windows.