Posterior Canal Dehiscence

Man 49yr with tinnitus and hearing loss at right ear lasting for 5-6years. Two times per year he has episodes of positional vertigo usually lasting about 7 days (last time a month ago). He doesn’t have vertigo on sounds.
pseudoconductive hearing loss
tympanograms type A
But cVEMP shows good vestibulo-cervical reflex at both sides.
pseudoconductive hearing loss
Because of present cVEMP, the hearing loss is pseudoconductive. Also, the bone conductive tone thresholds are elevated. We’re looking for dehiscence at temporal bone CT. And here we can see high jugular bulb at right side and dehiscent posterior canal to jugular bulb. Please, click on a picture for better view.

posterior canal dehiscence posterior-canal-dehiscence-coronal

Dehiscence is usually on superior semicircular canal. But dehiscence of the posterior canal is described in the literature as well and usually is a consequence of high jugular bulb like in this patient.
Audiological parameters which suggest the dehiscence are conductive hearing loss with good tympanograms, elevated bone conduction threshold and finaly present cVEMP which confirms that hearing loss isn’t conductive but pseudoconductive. In patient with conductive hearing loss, cVEMPs are absent.
The only unusual thing is that amplitude of the vestibulo-cervical reflex is not larger than on the opposite side (almost the same). In patients with third window lesions we expect larger amplitude and decreased VEMP threshold on the affected side.

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