Bone dehiscence of the facial canal during transmastoid decompression surgery is a poor prognostic factor in severe Bell's palsy.

Objective: To investigate the incidence of facial canal dehiscence and its association with the recovery rate of facial palsy in patients with severe Bell's palsy who underwent transmastoid facial nerve decompression surgery.

Methods: This retrospective study included 142 patients with severe Bell's palsy who underwent transmastoid decompression surgery. The presence of a facial canal dehiscence, the site of the dehiscence, the protrusion of the nerve from the dehiscence, and the recovery rate of facial palsy were investigated.

Results: A bone dehiscence of the facial canal was observed in 50 patients (35.2 %) during surgery, with most (92.5 %) being located in the tympanic segment. Protrusion of the facial nerve from the dehiscence was observed in 25 patients (17.6 %). The recovery rate of facial palsy was significantly better in patients without dehiscence of the facial canal (57.6 %) compared to those with bone dehiscence but lacking protrusion of the nerve (32.0 %; p = 0.044) or those having a bone dehiscence with protrusion of the nerve (28.0 %; p = 0.011).

Conclusions: Patients exhibiting dehiscence of the facial canal during transmastoid decompression surgery for severe Bell's palsy demonstrate a diminished recovery rate in comparison to those without dehiscence of the canal.