Perilymphatic Fistula

A labyrinthine, or perilymphatic, fistula is an abnormal communication between the inner ear and a neighboring space, most often the middle ear or mastoid cavity. Connections with the intracranial space are less common, with fistulas into the carotid and facial nerve canals being rarer still. Most cases of perilymphatic fistula (PLF) are associated with some traumatic event, but 20% to 40% remain idiopathic even after a thorough evaluation. Idiopathic PLF may be especially challenging to recognize, as the symptoms, particularly mild or intermittent manifestations, are otologically nonspecific, including hearing loss, disequilibrium, aural fullness, autophony, and tinnitus. Of note, some authors differentiate between the terms "labyrinthine fistula" and "perilymphatic fistula," defining the former as a defect in the dense otic capsule bone that surrounds the inner ear, thus exposing the membranous labyrinth, while the latter is a subset of the former that also involves disruption of the membranous labyrinth and leakage of perilymph. In this activity, however, the 2 terms will be used interchangeably. Clinical interest in the PLF phenomenon arose initially in the 1960s as the stapedectomy procedure for otosclerosis gained popularity. Escape of perilymph through the oval window during manipulation of the stapes footplate, poor sealing of the oval window membrane around the stapes piston prosthesis, and malposition or postoperative migration of the prosthesis have all been implicated as potential causes of PLF. During the subsequent decade, however, reports of similar clinical presentations to those of postoperative stapedectomy patients began to appear, but occurring in patients without prior otologic surgery. Further research resulted in Goodhill's early hypothesis of "implosive" and "explosive" routes of PLF development, with the former including increased intracranial pressure and barotrauma, eg, Valsalva maneuvers, pinched-nose sneezing, and straining. While the oval and round windows (or vestibular and cochlear fenestrae, respectively) may be the most common locations for PLFs to occur, several other sites have been reported as well, eg, the fissula ante fenestram and the horizontal semicircular canal, depending upon the patient's medical history and anatomy (see . Internal Ear). The inner ear consists of both hearing and balance elements, with membrane-bound endolymphatic and perilymphatic compartments spanning the 2 sections (see . Cochlea, Cross Section and Internal Ear or Labyrinth, Transverse Section). For this reason, a decrease in perilymph volume relative to that of endolymph may result in auditory and/or vestibular symptoms. Within the cochlea, the endolymph is contained in the scala media, while the perilymph resides inside the scala tympani and the scala vestibuli, which are contiguous via the helicotrema at the cochlear apex (see . Cochlea and Vestibule). The scala tympani ends at the round window and the scala vestibuli at the oval window, beneath the stapes footplate. Anterior to the oval window lies the fissula ante fenestram, a small bony cleft filled with connective tissue that, if incompletely closed during fetal development or traumatically disrupted, may provide a connection between the middle ear space and the vestibule of the inner ear (see . View of the right middle ear.). Perilymph efflux may also occur via bony microfissures that have been observed extending between the ampulla of the posterior semicircular canal and the round window. Beyond these normal anatomical structures, however, a broad range of variations may predispose patients to PLF formation, including congenital enlargement of the vestibular aqueduct, Mondini malformation, and erosion of the horizontal semicircular canal by cholesteatoma.