Isolated lateral semicircular canal-facial nerve dehiscence as a cause of third window syndrome—an emerging phenomenon and outcomes of surgical management

Background: Third window syndrome (TWS) is a clinical entity that used to primarily encompass superior semicircular canal dehiscence (SSCD). However, there are an increasing number of patients who have TWS with Tullio phenomenon who are deemed to be “CT negative” who have been subsequently found to have lateral semicircular canal (LSCC)-facial (VII) dehiscence. This retrospective case series describes the clinical, radiological, and electrophysiological features of patients presenting with third window symptoms without SSCD and reports their surgical outcomes. Methods: The study assessed adult patients between January 2017 and June 2022 who presented with third window symptoms and were found not to have classical SSCD on initial imaging. Ten adult patients with non-SSCD TWS were reviewed. Patient symptoms, high resolution temporal bone computed tomography (CT) scans and comprehensive audiological and electrophysiological testing including cervical vestibular evoked myogenic potentials (cVEMPs) tests were collected. Patients underwent round window reinforcement (RWR) via an endaural surgical approach for disabling symptoms. Main outcomes measured included improvement in symptomatology and changes in electrophysiological studies. Results: Ten patients were identified. Radiologically, six patients had dehiscence at the lateral semicircular canal-facial nerve (LSCC-VII) segment, one patient had cochlear-carotid dehiscence, and three patients had no discernible radiological abnormalities. The mean preoperative air-bone gap (ABG) was 10.7 dB (SD 7.16 dB, range −1.5 to 15 dB). Five patients underwent RWR surgery, resulting in a mean postoperative ABG improvement of 9.98 dB (±7.46). All operative patients reported improvement in clinical symptoms. Conclusions: Patients with third window symptoms without SSCD may have alternative radiological findings such as LSCC-VII dehiscence. RWR via an endaural approach appears to offer symptomatic and audiometric improvement in selected patients.