Persistance of perilymph fistula mechanisms in a completely paretic posterior semicircular canal

Background: Perilymph fistula (PLF) are pathological connections between the perilymphatic space of the labyrinth and the middle ear (outer) or the CSF (inner fistula). These abnormal connections transmit pressure to excite the semicircular canal which clinically leads to paroxysmal vertigo of short duration. Previous PLF studies did not focus on the relation between the canal function and the mechanism that elicits PLF nystagmus. Specifically, how much canal and otolith function is required to still elicit PLF nystagmus?

Patient: A 32-year-old truck driver had a 9 months history of recurrent paroxysmal spells of rotational vertigo, oscillopsia, and nausea lasting for seconds. He noticed that the spells were typically elicited by coughing or with any kind of physical pressure applied on his left ear. High resolution CT revealed a mass of the left temporal bone which infiltrated the posterior semicircular canal (PC).

Methods: Otolith function was assessed by click-evoked myogenic potentials (CEMP), fundoscopy, and the subjective visual vertical, semicircular canal function by head thrust test using the scleral search coil technique (Remmel Labs, Maryland, USA) with 3D coils (Skalar, Delft, Netherland).

Results: Physical pressure on the left external ear canal elicited a vigorous upward and counterclockwise (ipsilesionally) beating nystagmus. Its slow phase velocity was on average 36.5 + 17.8 deg/s for the vertical, 27.5 + 10.2 deg/s for the torsional, and 1.7 + 5.9 deg/s for the horizontal component. The velocity vector orientation of the PLF nystagmus closely aligned to the left PC. Head thrust tests in the canal planes revealed a complete loss of left PC function. Fundoscopy and the subjective visual vertical (SVV) were normal.

Conclusion: Despite of a completely paretic left posterior semicircular canal PLF nystagmus could still be elicited that obeyed Ewald's first law. This might indicate that PLF can lead to canal plugging or that PLF mechanisms can also act on the ampulla by directly deflecting the cupula. It cannot be explained by otolith stimulation.