Superior semicircular canal dehiscence

Superior semicircular canal dehiscence (SSCD) is a disorder of the skull base that is gaining increasing recognition among neurosurgeons.

Superior semicircular canal dehiscence (SSCD) is an osseous defect of the arcuate eminence of the petrous temporal bone.

Since this was first reported in 1998 by Minor and colleagues, there has been much advancement made in terms of diagnosis and treatment 1).

The condition is confirmed on high-resolution computed tomography (CT) imaging.

High-resolution computed tomographic temporal bone images were imported into a freely available segmentation software. Dehiscence lengths and volumes were ascertained by independent authors. Inter-rater observer reliability was assessed using Cronbach's alpha. Correlation and regression analyses were performed to evaluate for relationships between dehiscence size and symptoms (pre- and post-operative).

Thirty-seven dehiscences were segmented using the novel volumetric assessment. Cronbach's alpha for dehiscence lengths and volumes were 0.97 and 0.95, respectively. Dehiscence lengths were more variable as compared to dehiscence volumes (σ 2 8.92 vs σ 2 0.55, F = 1.74). The mean dehiscence volume was 2.22 mm 3 (0.74, 0.64-0.53 mm 3 ). Dehiscence volume and headache at presentation were positively correlated ( R pb = 0.67, P = .03). Dehiscence volume and vertigo improvement after surgery were positively correlated, although this did not reach statistical significance ( R pb = 0.46, P = .21).

SSCD volumetry is a novel method of measuring dehiscence size that has excellent inter-rater reliability and is less variable compared to dehiscence length, but its potential as a predictor of symptom outcomes is not substantiated. However, the study is limited by low power 2).

MRI FIESTA scans have recently been used to image SSCD. Additionally, audiometry and vestibular evoked myogenic potential (VEMP) testing are useful screening tools.

Superior semicircular canal dehiscence (SSCD) is characterized by CT-confirmed bony erosion over the superior semicircular canal, creating vestibular and auditory symptoms. Endolymphatic hydrops (EH) is characterized by an MRI-confirmed excess of endolymph within the scala media that distorts the membranous labyrinth. While there is overlap in symptoms, the two diseases result from different pathophysiologies and require different interventions.

With this dehiscence, the fluid in the membranous superior canal (which is located within the tubular cavity of the bony canal) can be displaced by sound and pressure stimuli, creating certain vestibular and/or auditory signs and symptoms.

Concurrent SSCD and EH is a rare but treatable entity. Physicians should consider ordering an MRI of the IAC if SSCD patients' symptoms persist or recur after a successful surgery 3).

Currently, the middle fossa approach is the most common and standard surgical approach to repair SSCD. The transmastoid, endoscopic and transcanal or endaural approaches have also been recently utilized. Presently, there is no consensus as to the best approach, material or technique for repair of SSCD. As we learn more, newer and less invasive approaches and techniques are being used to treat SSCD 4).

Symptoms are often improved by surgical repair. Although a classic middle fossa craniotomy has been used with good results, recent advances in technique have allowed for modification of the traditional approach into a smaller skin incision and a minimally invasive middle fossa keyhole craniectomy roughly 1.7 cm in diameter.

To delineate this novel approach and describe the technique for accurate localization of the dehiscence using preoperative measurements and intraoperative image guidance, thereby minimizing the need for a larger skin incision and craniotomy.

Patients were independently diagnosed with SSCD by the senior authors. Once relevant imaging was acquired, the novel keyhole technique was performed. Patients' vestibular and auditory symptoms before and after the procedure were assessed. Outcomes from a series of patients treated with this keyhole approach were tabulated and reported.

Twelve cases from 11 patients were included in this series. Auditory symptoms had high rates of resolution with pulsatile tinnitus, internal amplification of sounds, and autophony being resolved in a majority of cases. Only 2 cases reported hearing decline. Sound/pressure induced vertigo and disequilibrium also demonstrated high rates of resolution. No complications were reported.

The minimally invasive middle fossa keyhole craniectomy is a novel approach for the repair of SSCD. This approach may contribute to resolved auditory and vestibular symptoms with low morbidity and quick recovery 5).

A analysis included 24 studies that described 230 patients that underwent either an middle cranial fossa (MCF) (n = 148, 64%) approach or a transmastoid approach (TM) (n = 82, 36%) for primary surgical repair of SSCD. A greater percentage of patients in the MCF group experienced resolution of auditory symptoms (72% vs 59%, p = 0.012), aural fullness (83% vs 55%, p = 0.049), hearing loss (57% vs 31%, p = 0.026), and disequilibrium (75% vs 44%, p = 0.001) when compared to the TM group. The MCF approach was also associated with higher odds of symptom resolution for auditory symptoms (odds ratio [OR] 1.79, 95% confidence interval [CI] 1.14-2.82), aural fullness (OR 4.02, 95% CI 1.04-15.53), hearing loss (OR 2.91, 95% CI 1.14-7.42), and disequilibrium (OR 3.94, 95% CI 1.78-8.73). The mean follow-up was 9 months.

The literature suggests that the MCF approach for the repair of SSCD is associated with greater symptom resolution when compared to the TM approach. This information could help facilitate patient discussions 6).

Thirty-three dehiscences (30 patients) involving the superior semicircular canal were identified. The average age at the time of presentation was 52.5 years (median, 56.9; range, 4.9-75.3 yr), and 53.3% of patients were men. Three patients had bilateral SPS associated SSCD. The most common associated symptoms at presentation were episodic vertigo (63.6%), subjective hearing loss (60.6%), and aural fullness (57.6%). Four distinct types of dehiscence were identified: class Ia. SSCD involving a single dehiscence into an otherwise normal appearing SPS; class Ib. SSCD involving a single dehiscence into an apparent venous anomaly of the SPS; class IIa. SSCD involving two distinct dehiscences into the middle cranial fossa and the SPS; class IIb. SSCD involving a single confluent dehiscence into the middle cranial fossa and the SPS.

SSCD involving the SPS represents a small but distinct subset of SSCD cases. This scenario can create a unique set of symptoms and surgical challenges when intervention is sought. Clinical findings and considerations for surgical intervention are provided to facilitate effective diagnosis and management 7).

A total of 72 cases of SSCD in 60 patients were repaired via a middle fossa approach at a single institution. Main Outcome Measures  The distance from the proposed reference point to the dehiscence was statistically analyzed using Shapiro-Wilk's goodness-of-fit test and Student's t -test. Results  Average distance for all patients was 28.84 ± 2.22 mm (range: 22.96-33.43). Average distance for females was 29.08 mm (range: 24.56-33.43) versus 28.26 mm (range: 22.96-32.36) for males. There was no difference in distance by sex ( p  = 0.174). The distance measurements followed a normal distribution with 95% of the patients between 24.49 and 33.10 mm.

This study analyzed a potential reference point during a middle fossa approach for SSCD surgery. The distance from this reference point to the SSCD was found to be consistent and may serve as a readily identifiable landmark in localizing the dehiscence 8).

A 35-year-old man with superior semicircular canal dehiscence treated by a joint neurosurgical and otolaryngological team 9).

1) , 4)
Mau C, Kamal N, Badeti S, Reddy R, Ying YM, Jyung RW, Liu JK. Superior semicircular canal dehiscence: Diagnosis and management. J Clin Neurosci. 2018 Feb;48:58-65. doi: 10.1016/j.jocn.2017.11.019. Epub 2017 Dec 7. Review. PubMed PMID: 29224712.
Lagman C, Beckett JS, Chung LK, Chen CHJ, Voth BL, Gaonkar B, Gopen Q, Yang I. Novel Method of Measuring Canal Dehiscence and Evaluation of its Potential as a Predictor of Symptom Outcomes After Middle Fossa Craniotomy. Neurosurgery. 2017 Aug 9. doi: 10.1093/neuros/nyx430. [Epub ahead of print] PubMed PMID: 28945893.
Johanis M, De Jong R, Miao T, Hwang L, Lum M, Kaur T, Willis S, Arsenault JJ, Duong C, Yang I, Gopen Q. Concurrent superior semicircular canal dehiscence and endolymphatic hydrops: A novel case series. Int J Surg Case Rep. 2020 Dec 26;78:382-386. doi: 10.1016/j.ijscr.2020.12.074. Epub ahead of print. PMID: 33421957.
Vanessa T, Pelargos PE, Spasic M, Chung LK, Voth B, Ung N, Gopen Q, Yang I. Minimally Invasive Middle Fossa Keyhole Craniectomy for Repair of Superior Semicircular Canal Dehiscence. Oper Neurosurg (Hagerstown). 2017 Jun 1;13(3):317-323. doi: 10.1093/ons/opw046. PubMed PMID: 28521355.
Nguyen T, Lagman C, Sheppard JP, Romiyo P, Duong C, Prashant GN, Gopen Q, Yang I. Middle cranial fossa approach for the repair of superior semicircular canal dehiscence is associated with greater symptom resolution compared to transmastoid approach. Acta Neurochir (Wien). 2017 Oct 11. doi: 10.1007/s00701-017-3346-2. [Epub ahead of print] Review. PubMed PMID: 29022108.
Sweeney AD, O'Connell BP, Patel NS, Tombers NM, Wanna GB, Lane JI, Carlson ML. Superior Canal Dehiscence Involving the Superior Petrosal Sinus: A Novel Classification Scheme. Otol Neurotol. 2018 Oct;39(9):e849-e855. doi: 10.1097/MAO.0000000000001965. PubMed PMID: 30199501.
Beckett JS, Chung LK, Lagman C, Voth BL, Jacky Chen CH, Gaonkar B, Gopen Q, Yang I. A Method of Locating the Dehiscence during Middle Fossa Approach for Superior Semicircular Canal Dehiscence Surgery. J Neurol Surg B Skull Base. 2017 Aug;78(4):353-358. doi: 10.1055/s-0037-1601886. Epub 2017 Apr 18. PubMed PMID: 28725523; PubMed Central PMCID: PMC5515662.
Martin JE, Neal CJ, Monacci WT, Eisenman DJ. Superior semicircular canal dehiscence: a new indication for middle fossa craniotomy. Case report. J Neurosurg. 2004 Jan;100(1):125-7. PubMed PMID: 14743924.
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