Timing of Onset and Recovery of Delayed Facial Palsy after Vestibular Schwannoma Resection: Insight into Mechanisms

Background: Preservation of facial nerve function during resection of vestibular schwannomas (VS) remains a primary goal of surgery, with a significant impact on patient quality of life. Delayed facial palsy (DFP) is described as a noticeable worsening of facial nerve function after an initially normal post-operative result, though many different specific definitions are used in the literature. Several mechanisms have been postulated to explain this delayed onset, including post-surgical edema, compression at the meatal foramen, vasospasm and ischemia, mechanical traction and heat or vibration injury, and viral reactivation; however, less attention has been paid to the recovery rates as a defining feature of these patients. Objective: The aim of this study was to retrospectively review our institution’s series of VS resections to evaluate the timing of DFP onset and recovery, and to quantitatively test our hypothesis that later onset DFP patients do better than patients who exhibit facial palsy immediately or in the early post-operative period. Methods: 403 consecutive cases of VS resection taking place between Nov 2001 and Jun 2015 were identified and retrospectively reviewed. Patients with any pre-operative facial palsy were excluded from our study. Patients who developed significant facial palsy (House-Brackmann (HB) grade ≥3) post-operatively were categorized into three groups based on the timing of onset: immediate facial palsy (IFP), “early-onset” DFP (on post-operative day 2 or prior), and “late-onset” DFP (on post-operative day 3 or later). Furthermore, IFP patients were subdivided by the severity of their weakness into “minor” (HB grade 3) and “major” (HB grade ≥4) groups. These four facial palsy subgroups were compared with respect to patient age and sex, surgical approach, tumor size and laterality, intraoperative neurophysiological monitoring, hearing preservation, and time-course of recovery. Results: Of the 385 cases of VS resection meeting inclusion criteria, 6.8% developed minor IFP, 4.9% major IFP, 2.6% early-onset DFP, and 5.7% late-onset DFP, with 80.0% not exhibiting any significant weakness post-operatively. The late-onset DFP group demonstrated the quickest recovery to HB grade 1 or 2 (3.0 weeks), followed by the minor IFP group (8.5 weeks), then the early-onset DFP group (40.8 weeks), and with the major IFP group exhibiting the poorest recovery with only 21% of patients recovering to HB grade 1 or 2 within one year of onset. Conclusion: An examination of the time-course of recovery in the different facial palsy subgroups in this study suggests that early-onset and late-onset DFP likely involve different pathophysiological mechanisms. While the literature would suggest a multitude of different ultimate causes as listed above, based on the uniformly complete and extremely rapid recovery seen in our late-onset DFP patients, we propose that apoptosis of facial nerve Schwann cells after surgery results in delayed demyelination and dysfunction of the facial nerve, as the proximate cause that best explains the subsequent rapid recovery seen in these late-onset DFP patients.

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