78 Perilymph and Labyrinthine Fistula

Patients with perilymph and labyrinthine fistula may present with Ménière’s syndrome and be incorrectly diagnosed with Ménière’s disease.

78.1 Definitions

Labyrinthine fistula This is a defect or abnormal opening in the bony labyrinth, including the bone or membranes of the medial wall of the middle ear and semicircular canals, that either exposes or ruptures the endosteum. An example is a lateral semicircular canal fistula and a perilymph fistula (PLF). If rupture occurs, a perilymph leak occurs.

Perilymph fistula This is a perilymph leak from the bone or membranes of the medial wall of the middle ear into the middle ear cavity. Usually, it arises from a defect in the oval or round window. A PLF has also been described arising from the fissula ante fenestram, a small connective tissue–filled cleft sited immediately anterior to the anterior margin of the oval window but posterior to the processus cochleariformis. Microfissures extending from the ampulla of the posterior semicircular canal to the round window have also been described as a site of a PLF.

One should also be aware of two other entities, namely, the cochlear aqueduct and the vestibular aqueduct.

Cochlear aqueduct It is a tiny bony passage arising from the basal turn of the cochlea and opening just behind and medial to the carotid canal at the skull base. It is lined on the cochlea side by endosteum and on the skull base side by a prolongation of dura. There is therefore a direct communication between the perilymph of the cochlear aqueduct and the subarachnoid space. Usually the aqueduct is occluded with arachnoid tissue and this tissue becomes denser with age. Therefore, raised intra-cranial pressure does not usually cause raised perilymph pressure but the density of the arachnoid tissue is thought to be a factor in the risk of developing a sensorineural hearing loss with meningitis and why some drugs that enter the cerebrospinal fluid (CSF) may cause ototoxicity. Sometimes, however, the cochlear aqueduct is thought to be patent and a patent cochlear aqueduct will allow direct transmission of CSF pressure to the cochlea and to the adjacent vestibule. Over time, this can dilate the vestibular aqueduct. Most patients with an enlarged vestibular aqueduct have a normal-sized cochlear aqueduct, so it is the patency of the cochlear aqueduct that is the important factor and not the size. That said, it is thought that an enlarged cochlear aqueduct is a risk factor for developing a patent cochlear aqueduct.

Of further interest, a patent cochlear aqueduct is not inevitably associated with an enlarged vestibular aqueduct, perhaps because fibrous tissue can occlude the ductus endolymphaticus. Such cases, one might presume, should be associated with fluctuating hearing loss and indeed there have been several case reports of an enlarged cochlear aqueduct, without a coexisting enlarged vestibular aqueduct, in children with a progressive sensorineural hearing loss.

Vestibular aqueduct It is a tiny bony passage arising from the posteromedial wall of the vestibule and extends to open on the posterior surface of the petrous temporal bone. It is lined by endosteum forming the ductus endolymphaticus. On reaching the posterior surface of the petrous temporal bone the ductus expands to form a blind ending sac, the saccus endolymphaticus. Enlargement of the vestibular aqueduct is often stated to be an aqueduct that is greater than 1.5 mm at its widest point, but recent studies have suggested the correct definition should be a vestibular aqueduct that is greater than 1 mm at the midpoint of the aqueduct and 2 mm at the operculum.

Enlargement of the vestibular aqueduct is associated with enlarged vestibular aqueduct syndrome, where there is fluctuating and often progressive sensorineural hearing loss and sometimes vertigo, usually in both ears. It may be associated with Pendred’s syndrome. The diagnosis is made by high-resolution computed tomography (HRCT) or magnetic resonance imaging (MRI) scanning of the petrous temporal bone. As a guide, the membranous labyrinth in the adjacent posterior semicircular canal (PSSC) is 1 mm in width.

When the diagnosis is made in young patients, the avoidance of contact sports is important because the evidence is that even minor head trauma, such as heading a football or playing rugby, may be associated with a progression of hearing loss. One of the hypotheses of this is that, in such patients, there is a patent cochlear aqueduct that allows direct transmission of CSF pressure to the perilymph and which, over time, may dilate the vestibular aqueduct. Minor head trauma, in such patients, can cause a momentary increase in intra-cranial pressure which is directly transmitted to the cochlear perilymph through the patent cochlear aqueduct. This may injure cochlear receptors. In such cases, an enlarged vestibular aqueduct is merely evidence of a pre-existing patent cochlear aqueduct.

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