Superior Canal Dehiscence

TITLE: Superior Canal Dehiscence
SOURCE: Grand Rounds Presentation, University of Texas Medical Branch,
= Dept. of Otolaryngology
DATE: September 30, 2010
MEDICAL STUDENT 4: Jill N. D&#= 8217;Souza, M= S4
FACULTY PHYSICIAN: Tomoko Makishima, M.D.
DISCUSSANT: Tomoko Makishima, M.D.
SERIES EDITOR: Francis B. Quinn, Jr., MD
ARCHIVIST: Melinda Stoner Qui= nn, MS(ICS)

&q= uot;This material was prepared by resident physicians in partial fulfillment of educational requirements established for the Postgraduate Training Program = of the UTMB Department of Otolaryngology/Head and Neck Surgery and was not intended for clinical use in its present form. It was prepared for the purpose = of stimulating group discussion in a conference setting. No warranties, either express or implied, are made with respect to its accuracy, completeness, or timeliness. The material does not necessarily reflect the current or past opinions of members of the UTMB faculty and should not be used for purposes= of diagnosis or treatment without consulting appropriate literature sources and informed professional opinion."

Introduction:

Superior Canal Dehiscenc= e has recently been described as an etiologic agent in vertigo.

History:

Superior Canal Dehiscenc= e as a cause of sound and/or pressure-induced vertigo was first postulated in 1998= , in a case series described by Minor et= al¹. This study described eight patient= s with vestibular symptoms (oscillopsia, vertigo), as = well as chronic disequilibrium. All patients were found to have evoked eye movem= ents with vertical and torsional components, with the direction corresponding to inhibition or excitation of the affected semicircular canal. Axial and coronal computed tomography (CT) of the tempo= ral bone and labyrinth revealed dehiscence of the bone overlying the superior circular canal. A study by Carey et= al² studied the incidence of superior canal dehiscence in a temporal bone archi= ve. Minimum thickness of the bone was measured at the floor of the middle crani= al fossa, and at the superior cranial fossa. Complete dehiscence was identifie= d in 0.5% of specimens, and thinning (<0.1mm) of the bone in an additional 1.= 4%. Thinning of the bone could also be detected on ultra-high-resolution CT scan and identified as dehiscent. Individuals with dehiscence or bone thinning o= ver the superior canal are thought to be prone to development of superior canal dehiscence syndrome with its associated clinical symptoms if trauma disrupts the fragile bone¹. Minor et al reported that up to 23% of patients describe an inciting event – = loud noise, heavy lifting, trauma to head – precedes the first symptomatic episode of superior canal dehiscence.

Embryologic Development:

At 15 weeks of gestation, ossification centers o= f the ear grow and fuse such that by 21 weeks, the bony labyrinth encases the membranous labyrinth. Each ossification center has a t= rilaminar structure: The outer periosteal layer continues to grow postnatally. The middle layer forms by invasion of osteoblast into cartilage, and eventually forms the petrous portion of the temporal bone. T= he innermost periosteal layer contacts the perilymphatic<= /span> space, and overlies the superior canal where it enters the middle cranial fossa. Postnatal failure to d= evelop outer/middle layer bone to cover the periosteal layer over the semicircular canal has been suggested as the cause of dehiscence. The other hypothesis, = that erosion of the bone occurs gradually over the individual’s lifespan, = is not supported by histologic evidence as ossific= ation patterns appear stable over time rather than altered²by an acute event.

Anatomy and Physiology:

The semicircular canals play an essential role in balance and perception of movement. They consist of three membranous interconnected tubes shaped as a to= rus, positioned orthogonally to each other. The membranous portion of the canal = is attached to temporal bone. The ampulla is locat= ed at one end of the torus, with a gelatinous cupula dividing one end of the ampulla from the other.= Each canal is filled with endolymph and contains cilia embedded in the cupula. Maximum sensitivity of the canal is rotation = in the plane of the torus³, with the horizontal canal detecting horizon= tal head movements, while the posterior and superior canals detect motion in the vertical direction.

The fluidity of endolymph allows it to move within the semicircular canal; however, its inertial prop= erty causes it to initially oppose the turning motion. The elastic component of the cupula allows it to detect slight changes in pressure= and deflect accordingly. As the cupula deflects, stereocilia move toward or away from the kinocilium, stimulating or inhibiting the vestibular = nerve, respectively. Kinocilia are positioned in a par= allel fashion to the long axis of each semicircular canal; in the posterior and superior semicircular canal, ampullofugal flow = of endolymph is excitatory as the hair cells are organiz= ed so that kinocillia are located further away from t= he vestibule. In the horizontal canal, the excitatory direction is reversed as= the kinocillia are closer to the vestibule. Thus, <= span class=3DSpellE>ampullopetal flow stimulates vestibular nerve firing = in the horizontal canal.

Pathophysiology:

Supe= rior canal dehiscence syndrome results when the temporal bone overlying the supe= rior circular canal is absent. Under normal conditions, sound pressure enters the inner ear via the stapes footplate and oval window, rotates through the coc= hlea and exits via the round window⁴. In superior canal dehiscence, t= he lack of bone overlying the area results in greater compliance⁵, creating a third window space and thus allowing abnormal movement of endolymph within the canal in the presence of certain stimuli. These stimuli increase the pressure gradient within the membranous labyrinth: stimuli that result in inward pressure at the round or oval wind= ows (such as Valsalva against closed nostrils, or <= span class=3DSpellE>tragal compression) cause outward bulging at the dehi= scence site⁶, with resulting ampullofugal deflection of superior canal cupula. Inward bul= ging at the bony dehiscence results in ampullopetal deflection of the cupula; this can be caused by= Valsalva against a closed glottis or jugular venous compression.

Evoked eye movements that are seen in patients w= ith superior canal dehiscence syndrome are related to Ewal= d’s first law: stimulation of the superior semicircular canal produces eye rota= tion in the vertical-torsional plane. Torsional movements are counterclockwise if canal dehiscence is seen on the right, clockwise if it is present on the left⁵.

Clinical Findings and Diagnosis:

Superior canal dehiscence is a primarily clinical diagnosis. Suspicious historical findings include Tull= io’s phenomenon, which is dizziness/vertigo induced by sounds, and oscillopsia, a visual disturbance where objects in the field of vision seem to oscillate. Autophony, w= here the patient reports hearing their own voice louder in the affected ear, is another common finding in this disorder. When hearing loss is present, it i= s a conductive hearing loss secondary to dissipation of acoustic energy transmi= tted through air conduction – bone conduction is increased in patients suffering from superior canal dehiscence. This increased bone conduction is apparent on physical exam, where Rinne test wil= l be negative in the affected ear. Weber test will lateralize toward the affected ear, which is also consistent with the finding of conductive hearing loss. Vestibular testing shows some variability among subjects.

However, most have a negative Dix-Hallpike test (in the absence of concurrent benign paroxysmal positional vertigo). <= span class=3DSpellE>Videonystagmography (VNG) testing, Frenzel goggles, tragal compression or pneumatic otoscopy can all cause eye movements in the plane of = the superior canal as well as concurrent dizziness. An audiometer fitted to pat= ient during evoked eye movements can also show nystagmus to low-frequency, high-intensity sounds. Though vestibular testing can aid = in the confirmation of a vertiginous disorder, the gold standard of diagnosis = of superior canal dehiscence is a high resolution 0.5 mm collimation or less C= T of the temporal bones with images that are reformatted to include cuts in the plane of (Pöschl), as well as perpendicular (Stenver) to, the superior canals.

Medical and Surgical Interventions:

Severity of a patient’s symptoms and impac= t of these symptoms in the patient’s daily life determine the degree of intervention. Patients who are found to have dehiscence of the superior can= al as an incidental finding on a CT scan can be followed until such a time as symptoms develop. Patients with mild symptoms can be managed conservatively= with avoidance of causative stimuli. Myringotomy tubes can be placed in patients with primarily pressure-related symptoms. The decision to insert surgical plugging of the canal to alleviate symptoms should be made based on symptoms and patients’ comorbid conditions as well= as ability to undergo surgical procedure.

Current surgical options include canal resurfaci= ng and canal occlusion. The site of dehiscence is located on the medial aspect of = the superior semicircular canal, which contacts the superior petrosal sinus or, less frequently, the middle ear fossa⁴. Procedure can enter through transmastoid approach, or via middle fossa craniotomy. Dural elevation over arcuate eminence to avoid damage= to membranous labyrinth is performed, and dehiscence is either closed with fas= cia and bone dust or sculpted cortical bone, or canal is occluded completely wi= th bone paté. Superior canal resurfacing ha= s the advantage that it preserves the physiologic function of the superior semicircular canal, but may result in recurrence should the fascia or bone graft move out of place. Canal plugging has a lower rate of recurrence, but results in more trauma to the membranous labyrinth.

Current Research:

The surgical approach to superior canal dehiscen= ce continues to be in development, with no long-term follow-up studies availab= le to evaluate benefit to risk ratio of canal plugging vs. canal resurfacing at this time. Canal plugging is currently the most used procedure due to its u= se by the surgeons who first described the syndrome. Another area of research delves into whether the size of the dehiscence will impact symptom severity= ; in a study by Pfammater et al⁷, results showed that patients with large dehiscences have significantly increased vestibulocochlear symptoms, lower VEMP thresholds, and increased rate of objective vestibular findings as compared to patients with smaller dehiscences.

In summary,

Superior Canal Dehiscence is a relatively new diagnosis in the field of Otolaryngology. It is characterized by pressure and/or sound-induced vertigo, evoked eye movements in the plane of the affe= cted semicircular canal, as well as increased bone conduction and conductive hea= ring loss. Medical and surgical management is continually evolving as more accur= ate understanding of the syndrome emerges.

Discussant's Remarks: Tomoko Ma= kishima, M.D

This was a well designed, concise review on a relatively new disease entity, "superior canal dehiscence". I believe t= hat in most cases, the presenting symptoms and signs are very subtle in the beginn= ing. Therefore, the key to diagnos= is is to suspect "superior canal dehiscence". Now that is more widely known, more= and more cases are being reported, which will in turn enrich our knowledge base= on this condition. The treatment options and the accompanied prognosis is still mostly uncertain. This presentation covered most of what is known today. I am looking forward to revisit this topic again in a few years.

References

1. Minor L.B., Solomon= , D. & Zinreich, J.S. et al. (1998) Sound= - and/or pressure induced vertigo due to bone dehiscence of the superior semicircular canal. Arch. Otolaryngol. Head Neck Surg. 124, 249 – 25= 8

2. Carey J.P., Minor, L.B. & Nager, G.T. (2000) Dehisence or thinning of bone overlying the superior semicircular canal in a temporal bo= ne survey. Arch. Otolaryngol. Head Neck Surg. 126, 137 = 211; 147

3. Bailey, B; Johnson, J; Newlands, S. – Head and Neck Surgery – Otolaryngology. Fourth Edition. Lippincott Williams and Wil= kins 2006

4. Flint, PW. Cummings Otolaryngology – Head and Neck Surgery. Fifth edition. S= t. Louis: Mosby 2010

5. Banerjee, A; Whyte, A; Atlas, M.D. (2005) Superior Canal Dehiscence: review of a new condition. Clinical Otolaryngology<= /i> Vol 30 Issue 1, 9-15

6. Minor, L (2000) Superior Canal Dehiscence Syndrome. The American Journal of Otology 21= (1), p9-19

7.= = Pfammatter, A; Darrouze= t, V; Gärtner, M; Somers, T; Van Dinther, J; Trabalzini, F= ; Ayache, D; Linder, T (2010) A Superior Semicircular C= anal Dehiscence Syndrome Multicenter Study: Is There an Association Between Size= and Symptoms? Otology and Neurotology 31(3), 447-454

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