Introduction
The superior semicircular canal dehiscence (SSCD) syndrome was described by Lloyd
B. Minor et al[1] in 1998. This syndrome is characterized by the formation of a “third opening” or
“third window” between the superior semicircular canal and the middle cranial fossa,
secondary to a bony defect in the canal. This abnormal communication can result in
vertigo and oscillopsia induced by loud sounds (Tullio's phenomenon), by Valsalva
maneuvers, or by changes in pressure in the external auditory canal (Hennebert's sign).
The patients may also present autophony, hyperacusis, pulsatile tinnitus and hearing
loss. In patients with debilitating symptoms and low health utility scores, a surgical
management may be offered, since the manifestations of the disease and the utility
score may improve with surgery. The different surgical techniques used to approach
and repair the SSCD include the middle cranial fossa approach, the transmastoid approach,
the endoscopic-assisted middle cranial fossa approach, and the round window tissue
reinforcement. In this review of the literature we analyze this clinical syndrome
and its diagnosis, as well as the aforementioned surgical approaches and techniques
performed to repair the dehiscence.
Review of the Literature and Discussion
There are two functional windows in the cochleovestibular system, the oval window,
which allows the passage of soundwaves into the inner ear's scala vestibuli through
the stapes footplate, and the round window, involved in the release of sound and mechanical
energy from de scala tympani out of the inner ear. This is a hydraulically closed
system. When a dehiscence in the superior semicircular canal is created, the hydro-acoustic
waves flowing through the cochlea are inadvertently transmitted throughout the labyrinthine
system. This causes the activation of the vestibular system, and thus vertigo is perceived.
In addition, the intracranial pressure may be transmitted through the dehiscent superior
canal, also causing the stimulation of vestibular end organs until the round window
releases this pressure.[2] In a more detailed manner, a SSCD alters the hydrodynamic stability of this system,
enabling an exaggerated movement of the endolymph. Since a third window is present,
the transmission of pressure through the inner ear may result in an outward bulging
of the membranous canal at the bony defect to the side of the middle cranial fossa.
Hydraulically speaking, it would serve as a pressure release valve and, therefore,
less pressure would be distributed in the inner ear, as it would in a closed hydraulic
system.[3] When this outward bulging of the membranous labyrinth is produced, the disturbed
endolymph fluid motion causes the ampullofugal deflection of the cupula of the superior
semicircular canal, which the brain perceives as body movement, thus resulting in
vestibular symptoms.[4] Intracranial pressure fluctuations or negative pressure in the external or middle
ear may lead to the contrary, an abnormal reversal of energy through the dehiscence
into the labyrinth, resulting in an inward bulging of the membranous canal with a
consequent ampullopetal deflection of the cupula.[3]
[4] These theories support the noise or pressure-induced vertigo observed in the SSCD
syndrome.
The increased compliance of the inner ear explained earlier also contributes to the
conductive hearing loss and the perception of the pulsatile tinnitus observed in these
patients.[2] The pathologic opening from the SSCD may weaken the energy transmission produced
by the movement of the stapes footplate, resulting in the reduction of sound transmission
to the cochlea and hearing loss. On the other hand, the normal impedance inequality
of the oval and round windows may be altered by the third window, which results in
bone hyperconduction that the patient perceives as autophony or hyperacusis.[5]
The diagnosis is suspected by the clinical manifestations mentioned earlier. Nystagmus
evoked by sound or pressure, with eye movements oriented in the same plane as the
dehiscent semicircular canal, is a classic feature. Audiometric findings typically
include supernormal bone thresholds and a low-frequency conductive hearing loss. The
tympanometric examination and acoustic reflexes are normal.[6] Cervical vestibular evoked myogenic potential (cVEMP) testing in patients with SSCD
syndrome will often have responses with lower thresholds (less than 70 decibels [dB]
to tone burst testing) than in people without the syndrome.[7] In 2012, Zuniga et al[8] showed that cVEMP threshold results showed a sensitivity and a specificity ranging
from 80% to 100% for the diagnosis of SSCD syndrome. In contrast, the ocular cVEMP
amplitudes demonstrated a sensitivity and a specificity greater than 90%. Finally,
when a SSCD syndrome is suspected, a temporal bone computed tomography (CT) scan confirms
the diagnosis. The best images are seen in a high resolution 0.5 mm collimation, and
projections in the plane of the superior canal (Pöschl views) and in a perpendicular
plane to this same canal (Stenvers views).[9] Two types of SSCD have been described after a CT examination: dehiscence of the
arcuate eminence or dehiscence in the region of the superior petrosal sinus. Studies
have found that magnetic resonance imaging (MRI) is also very useful for diagnosing
SSCD syndrome. In 2013, Browaeys et al[10] described that when diagnosing SSCD syndrome, the MRI had a sensitivity of 100%,
a specificity of 96.5%, a positive predictive value of 61.1%, and a negative predictive
value of 100% in comparison with the CT.
The etiology is unknown. The first manifestations of the syndrome usually occur during
adulthood; nevertheless, a congenital anomaly is suspected, since thinning of the
bone overlying the superior semicircular canal occurs bilaterally in almost one-third
of the patients, and there are some cases that began having symptoms during childhood.[11]
[12] In contrast to adults, children with SSCD syndrome usually present with auditory
symptoms first, and conservative management should be carried out as often as possible.[12]
Another possibility for the pathophysiology of this disease is that it may be an acquired
condition. Repeated, low intensity cranial trauma from combat sports or diving, for
instance, has been related to this disease. A defect in the floor of the middle cranial
fossa, at the level of the canal, may also occur secondary to increments in the pressure
of the cerebrospinal fluid.[11]
Patients with mild symptoms should be treated with conservatively. For patients with
debilitating symptoms and lower health utility scores, surgical management may be
offered. Both the clinical manifestations of the disease and the so-called health
utility scores improve after surgery.[13] When surgery is necessary, the bony dehiscence can be resurfaced, plugged or capped
by different surgical approaches.[14]
[15]
[16]
[17]
[18]
Middle Cranial Fossa Approach
This approach to treat SSCD syndrome was first described by Minor et al.[14] A 4 × 4 cm craniotomy is drilled. The temporal lobe is retracted, and the arcuate
eminence is identified. At this point, the dehiscence may be visualized. The canal
is opened using a diamond drill, and then it is plugged. The canal may also be capped
or resurfaced with bone pate, bone wax, hydroxyapatite cement or soft tissue.
The advantages of the middle fossa craniotomy include the direct access to the arcuate
eminence defect without the need for labyrinthine bone removal and the exposure of
the surrounding cranial base if the repair is needed for other associated tegmen defects.
Concomitant resurfacing of the tegmen mastoideum and tympani is performed, since the
floor of the middle cranial fossa is usually thin. Resurfacing of the dehiscent canal
also prevents chronic stimulation from the pulsating temporal lobe.
Transmastoid Approach
The surgical technique begins with a post auricular incision, as it usually performed
for a mastoidectomy. Supra and subperiosteal flaps are made, and then a mastoidectomy
is performed using a cutting burr with suction irrigation. The sigmoid sinus, the
posterior and middle fossa dura, and the pre-sigmoid area are all skeletonized. The
horizontal, posterior and superior semicircular canals are identified and skeletonized
with a diamond burr. The area of dehiscence is identified, and the middle fossa dura
is carefully elevated from de dehiscent superior semicircular canal. In patients with
dehiscence at the superior petrosal sinus, the sinus was exposed at the sinodural
angle, posterior to the solid angle, and followed to the superior canal.[16] Two points of the bony labyrinth are fenestrated with a 1 mm diamond burr, and the
endosteum is opened just inferior to the fenestrated apex of the superior canal, on
the ampullated and non-ampullated portions of the canal.[19] Care is taken to avoid suction or manipulation of the membranous labyrinth to prevent
hearing loss or chronic disequilibrium after surgery.[20] Bone dust, bone wax, bone pate, fascia and even bone chips may be used to fill the
lumen of the superior canal at the points of fenestration (plugging the canal). If
bone wax is used, Cheng et al[21] recommend that the applications of two wax spheres, 2 mm in size, is sufficient
to occlude the canal without risk of damaging the neuroepithelium, which prevents
vestibular hypofunction and hearing loss after the repair. Conchal cartilage is harvested
and placed in an intracranial extradural position, repairing the middle cranial fossa
floor defect, and finally the wound is closed in a regular fashion.[19]
When the transmastoid middle fossa resurfacing is selected, after dissecting the middle
cranial fossa dura from the dehiscent canal, tragal perichondrium is placed in the
space between the dura and the dehiscence by folding it over the top of an annulus
elevator and inserting it over the superior canal. The dura and overlying temporal
lobe or superior petrosal sinus will stabilize the position of the graft, with no
tendency to extrude.[16]
It is well known that the transmastoid approach avoids the risks of performing a craniotomy
in the middle fossa, resulting in lower morbidity and a shorter hospital stay. The
disadvantage of transmastoid procedures is that the dehiscence is visualized with
more difficulty compared to the middle fossa approach.[22] Another limitation is when the tegmen hangs too low, precluding the safe exposure
and manipulation of the dura just lateral to the superior semicircular canal. A preoperative
CT scan is very important in order to anticipate this difficulty, and it could precisely
identify the size, extension and location of the dehiscence.
Endoscopic-assisted Middle Cranial Fossa Repair
The endoscopic-assisted middle fossa procedure appears to be advantageous when compared
with binocular microscopy, since it provides a high definition view of the middle
cranial fossa. In this surgical technique, following the incision, a temporalis fascia
graft is harvested. A periosteal flap is made, and a minicraniotomy (3 × 2 cm) is
performed. Under microscopy, the dura is gently elevated off the tegmen mastoideum
and tegmen tympani until the arcuate eminence is visualized. Then, a 3 mm wide, 14
cm long, 30° rigid endoscope is introduced in order to visualize the defect. The House-Urban
middle fossa retractor is used to maintain the retraction of the temporal lobe while
working with the endoscope. The dura is dissected from the medial aspect of the defect
under endoscopic view. At this point, a gentle occlusion of the ampullated and non-ampullated
limbs of the defect is performed using bone wax.[15] Peng et al[23] reported 10 cases managed with the endoscopic-assisted middle fossa technique in
which they used hydroxyapatite bone cement to resurface the superior canal with good
results. In 2014, Carter et al[18] reported their experience with 5 patients managed endoscopically. They mentioned
that endoscopy enhances the visualization of the superior canal defect, allows for
transillumination, and reduces temporal lobe retraction, making it a useful adjunct
in craniotomies used to repair SSCD defects.
Round Window Reinforcement
This surgical technique is performed using a traditional tympanomeatal flap approach
under general or local anesthesia. If necessary, a drill or curettage is used to enlarge
the posterior auditory canal wall in order to gain adequate exposure of the round
window niche. This niche and the promontory are denuded of mucosa, and the round window
is reinforced with temporalis fascia, tragal cartilage, perichondrium, fat or connective
tissue.[24] The tympanomeatal flap is repositioned and the external auditory canal is packed.
The round window reinforcement is a procedure with low risk compared to the middle
fossa or transmastoid approaches, and it may be offered as a first procedure in patients
with mild symptoms. Complete occlusion of the round window is another technique that
has been used for SSCD syndrome, but some authors suggest that it should not be recommended,
since it has been noticed that symptoms may become worse in the late postoperative
period.[24]
Plugging or Capping versus Resurfacing
Different studies have shown that the combination of plugging and resurfacing achieves
better long-term control of the symptoms than resurfacing alone. When only resurfacing
is performed, a complete sealing of the defect is not guaranteed, and this area may
remain sensitive to pressure changes.[25]
[26] Goddard and Wilkinson,[22] in 2014, showed excellent hearing outcomes and a reduction of the preoperative symptoms
in 24 ears in which the plugging technique was used.
A meta-analysis of 64 surgical procedures for SSCD syndrome indicated that capping
and plugging had a significantly higher success rate when compared to resurfacing.[27]
