Acoustic responses of vestibular afferents in a model of superior canal dehiscence.
OBJECTIVE
Afferents innervating the superior semicircular canal are rendered especially sensitive to acoustic stimulation when there is a dehiscence of the superior canal. Other vestibular end organs are also more sensitive to acoustic stimulation.


BACKGROUND
Dehiscence of the superior semicircular canal is associated with vertigo and nystagmus caused by loud sounds (Tullio phenomenon) or changes in middle ear or intracranial pressures. The mechanisms by which acoustic stimuli act on the vestibular end organs are unclear. The nystagmus caused by acoustic stimuli generally aligns with the affected superior canal.


METHODS
Responses to acoustic stimuli in the superior vestibular nerves of anesthetized chinchillas were recorded before and after fenestration of the superior canal.


RESULTS
Two acoustic response patterns were seen: rapid phase locking and slow tonic changes in firing rate. Phasic responses principally occurred in irregular afferents and tonic responses in regular afferents. Afferents from all of the vestibular end organs encountered could respond to acoustic stimuli, even before fenestration. However, fenestration lowered the thresholds for acoustic stimulation in superior canal afferents with phasic responses and increased the magnitude of tonic responses.


CONCLUSIONS
Superior canal dehiscence may render the irregular afferents innervating the superior canal particularly sensitive to loud sounds. Rapid phase-locking responses may explain the short latency of nystagmus seen in patients with superior canal dehiscence syndrome. The mechanisms by which acoustic stimuli activate the vestibular end organs may differ from the damped endolymph motion associated with head acceleration.
Carey
John P
JP
Department of Otolaryngology--Head and Neck Surgery, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21287-0910, USA. jcarey@jhmi.edu
Hirvonen
Timo P
TP
Hullar
Timothy E
TE
Minor
Lloyd B
LB
D000161:Acoustic Stimulation; D000344:Afferent Pathways / Q000502:physiology; D000818:Animals; D002682:Chinchilla; D004195:Disease Models, Animal; D005072:Evoked Potentials, Auditory / Q000502:physiology*; D005260:Female; D005276:Fenestration, Labyrinth; D006198:Hair Cells, Auditory / Q000502:physiology; D007759:Labyrinth Diseases / Q000503:physiopathology; D008297:Male; D009759:Nystagmus, Pathologic / Q000503:physiopathology; D012665:Semicircular Canals / Q000294:innervation / Q000502:physiology*; D013529:Surgical Wound Dehiscence / Q000503:physiopathology*; D014717:Vertigo / Q000503:physiopathology; D014722:Vestibule, Labyrinth / Q000294:innervation / Q000502:physiology*