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Superior Canal Dehiscence
Mechanisms of Pressure Sensitivity in a Chinchilla Model
Timo P. Hirvonen, MD;
John P. Carey, MD;
Cindy J. Liang;
Lloyd B. Minor, MD
Arch Otolaryngol Head Neck Surg. 2001;127:1331-1336.
Background Patients with superior canal dehiscence syndrome may experience vertigo
and nystagmus when pressure changes occur in the external auditory canal,
the middle ear, or the intracranial space. The cause is a defect in the bone
of the superior canal.
Objective To study the mechanisms of pressure sensitivity of the labyrinth in
superior canal dehiscence syndrome and its surgical repair in a chinchilla
model.
Methods We investigated the changes in firing rates of vestibular nerve afferents
in the chinchilla in response to changes in external auditory canal pressure
before and after fenestration of the superior canal, and after repair of the
fenestra.
Results Before superior canal fenestration, external auditory canal pressure
changes caused no responses in horizontal canal or otolith afferents, and
only 1 of 9 superior canal afferents responded to pressure. After fenestration,
all superior canal afferents were excited by positive pressure and inhibited
by negative pressure. Half of 18 otolith and most (21 of 33) horizontal canal
afferents were unaffected by pressure. The superior canal afferents had higher
pressure gain than the horizontal canal afferents (P
= .03). Pressure responses could be abolished only by applying a rigid seal
to the fenestra.
Conclusions Fenestration of the superior canal rendered all superior canal afferents
sensitive to pressure, whereas less than half of the other afferents became
pressure sensitive. The direction of the superior canal afferent responses
agreed with the predictions of our model of endolymph flow within the superior
canal. A rigid seal applied to the fenestra abolished pressure sensitivity
while maintaining physiologic rotational sensitivity.
From the Departments of Otolaryngology–Head & Neck Surgery
(Drs Hirvonen, Carey, and Minor) and Biomedical Engineering (Ms Liang and
Dr Minor), The Johns Hopkins University School of Medicine, Baltimore, Md;
and the Department of Otolaryngology–Head & Neck Surgery, Helsinki
University Central Hospital, Helsinki, Finland (Dr Hirvonen).
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