Clinical, experimental, and theoretical investigations of the effect of superior semicircular canal dehiscence on hearing mechanisms.
OBJECTIVE
A superior semicircular canal dehiscence affects hearing by introducing a third window into the inner ear that 1) lowers cochlear input impedance, 2) shunts air-conducted sound away from the cochlea resulting in conductive hearing loss, and 3) improves bone-conduction thresholds by increasing the difference in impedance between the vestibule and the round window.


BACKGROUND
Superior semicircular canal dehiscence has been linked to a "conductive" hearing loss characterized by a decrease in the sensitivity to air-conducted sound and hypersensitivity to bone-conducted sound.


METHODS
Four investigations were performed: 1) laser-Doppler vibrometer measurements of sound-induced umbo velocity in patients with computed tomographic scan-confirmed superior semicircular canal dehiscence; 2) laser-Doppler vibrometry of sound-induced motions of the vestibular lymph (either perilymph or endolymph) exposed in a chinchilla model of superior semicircular canal dehiscence; 3) studies in chinchillas of the effect of superior semicircular canal dehiscence on the cochlea's sensitivity to bone-conducted sounds; and 4) anatomically based theoretical analyses of sound flow through the human cochlea and semicircular canals.


RESULTS
The low-frequency umbo velocity in superior semicircular canal dehiscence patients without previous middle ear surgery ranged from normal through high normal. This tendency toward hypermobility suggests a decrease in cochlear impedance. Measurements of sound-induced velocity of the lymph within a superior semicircular canal dehiscence in chinchillas demonstrated sound flow through the dehiscence. Measurements of the cochlear potential demonstrated a superior semicircular canal dehiscence-induced increase in response to bone-conducted sound in eight of nine chinchillas. An anatomically based model of the human ear predicts changes in auditory sensitivity similar to audiometric changes in superior semicircular canal dehiscence.


CONCLUSIONS
The results suggest that superior semicircular canal dehiscence can affect hearing function by introducing a third window into the inner ear.
Rosowski
John J
JJ
Eaton-Peabody Laboratory, Department of Otolaryngology, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts 02114, USA. jjr@epl.meei.harvard.edu
Songer
Jocelyn E
JE
Nakajima
Hideko H
HH
Brinsko
Kelly M
KM
Merchant
Saumil N
SN
D000818:Animals; D001300:Audiometry, Evoked Response; D001844:Bone Conduction / Q000502:physiology; D016022:Case-Control Studies; D002682:Chinchilla; D004195:Disease Models, Animal; D004427:Ear Diseases / Q000601:surgery; D006314:Hearing Loss, Conductive / Q000209:etiology*; D006801:Humans; D017078:Laser-Doppler Flowmetry; D012665:Semicircular Canals / Q000473:pathology*; D013201:Stapes Surgery / Q000009:adverse effects*; D013529:Surgical Wound Dehiscence / Q000150:complications* / Q000000981:diagnostic imaging / Q000601:surgery; D014057:Tomography, X-Ray Computed; D014433:Tympanoplasty / Q000009:adverse effects*