Northeast Florida Medicine
Vol. 66, No. 1 2015
47
Otolaryngology
Vestibular Disease Update –
Superior Semicircular Canal Dehiscence
Introduction
In 1892 Ewald
1
discovered that individual labyrinthine
semicircular canal stimulation with a “pneumatic hammer”
resulted in specific movements of the eyes of pigeons. This
was a major discovery, as the resultant nystagmus was spe-
cific with respect to the particular vectors of specific canals.
Tullio
2
discovered in 1929 that loud sound could result in
dizziness if there was a fenestration of a semicircular canal,
a perilymph fistula, Meniere’s syndrome, post surgery, and
vestibulofibrosis. Tullio created openings in the semicircular
canals of pigeons and demonstrated that sound waves spread
primarily into the canals at the site of the opening.The “Tullio
phenomenon” is therefore dizziness due to exposure of loud
sound. Hennebert’s sign
3
is dizziness and nystagmus from
pressure such as nose blowing, straining or applying pressure
to the ear canal. Fistulas, or dehiscences, of the labyrinth
historically have been considered a rare cause of dizziness.
However, in 1998, Minor
4
described the dehiscent superior
semicircular canal syndrome, which has become recognized
as a relatively frequent cause of dizziness.
Clinical Example
A 29 year old female presented with imbalance beginning
in the fall of 2012. She described her sensations as listing to
the right when she walked, like a rug was moving under her,
unsteadiness, feeling off balance, and being cloudy headed.
In retrospect, she noted right-sided pulsatile tinnitus since
2007. At that time she had a computerized tomography
(CT) scan of her head, which was negative. As her pulsatile
tinnitus persisted, she had a CT scan of temporal bones
2009 (standard axial and coronal views), which was normal.
With her onset of disequilibrium, she underwent a magnetic
resonance imaging (MRI) scan of the brainwith andwithout
contrast on December 30, 2012, which was also normal.
She presented to us in February 2013. Her audiogram
revealed normal hearing. Her vestibular testing did show
an abnormal oVEMP (ocular vestibular evoked myogenic
potential) for the right ear, with increased amplitude and a
low threshold. Her FGA (functional gait assessment) score
was 15 out of a possible 30. An FGA score should be at
least 24 or greater, with the mean for normal patients being
28.9. She underwent a repeat CT scan using protocol for
obtaining sub-millimeter slice reconstruction in the oblique
axial and oblique coronal planes, which revealed a dehiscent
right superior semicircular canal. She underwent surgery on
March 18, 2013, a transmastoid cartilage cap procedure, and
on formal follow-up on June 5, 2013, she reported feeling
remarkably better with no dizziness, no imbalance, no lim-
itation to her physical activities, and resolution of her right
pulsatile tinnitus. Her only symptom was mild autophony
when singing loudly. Her postoperative FGA score was 29.
By communication inMarch 2014, she indicated continued
resolution of her symptoms.
Pathology and Pathophysiology
The pathologic correlate is a bony defect, or dehiscence,
of the superior semicircular canal. The most commonly
accepted theory is that of postnatal failure of skull bone
development over the superior semicircular canal in the floor
of the middle cranial fossa.
5,6
Although histologic temporal
bone studies are limited, one review of 1,000 temporal bones
from 596 adults found a complete dehiscence in 0.5 percent
of specimens (0.7percent of individuals), with another 1.4
percent having abnormally thin bone overlying the supe-
rior semicircular canal.
5
This estimation of 0.7 percent of
individuals equates to approximately one in 142 people.
Pathophysiology
The symptoms are due to a “third” mobile window.
The normal bony cochlea/labyrinth complex is filled with
perilymph, within which is suspended the membranous
By Larry B. Lundy, MD and David A. Zapala, PhD
Abstract:
Most causes of dizziness are non-vestibular in etiology,
and most vestibulogenic dizziness is benign, self-limiting, and
managed medically. There are relatively few causes of dizziness that
potentially benefit from surgery. A relatively newly recognized cause of
vestibulogenic dizziness is superior semicircular canal dehiscence. The
diagnosis and management can present a challenge to all physicians.
Address Correspondence to:
Larry Lundy, MD, Department of Otolaryngology,
Mayo Clinic Florida
4500 San Pablo Road
Jacksonville, FL 32224
