Measurement of the depth of facial nerve at the level of stylomastoid foramen using MR imaging in Bell's palsy.

Objectives: To investigate whether the depth of the facial nerve from the skin surface as it exits the stylomastoid foramen differs between the paralyzed and unaffected sides in patients with Bell's palsy.

Methods: Forty-three patients (23 females, 20 males; mean age 43.8 ± 15.2 years) diagnosed with Bell's palsy between January 2014 and June 2017 were retrospectively reviewed and those who had a cranial MR imaging performed within 10 days upon admission to hospital were included in the study. The axial postcontrast CUBE sequence was utilized for the measurement of the facial nerve depth. Age, gender, and body mass index (BMI) as well as concomitant chronic diseases, were also noted. The severity of facial paralysis was graded using the House-Brackmann (HB) scoring system.

Results: The facial nerve depth was significantly lower on the paralytic side compared to the unaffected side (32.9 ± 5.4 mm vs. 36.9 ± 5.1 mm, respectively; p = 0.007). The facial nerve depth on the paralytic side was not statistically different in the female patients compared to male patients (31.2 ± 4.6 mm vs. 34.7 ± 5.7 mm, respectively; p = 0.270). The facial nerve depth on the paralytic side was not correlated with patients' age (r = 0.288; p = 0.999), BMI (r = 0.215, p = 0.999), and HB scores (r = 0.031; p = 0.999).

Conclusion: In our study cohort of patients with Bell's palsy, the facial nerve in the paralytic side is located more superficially as it exits the stylomastoid foramen when compared to the contralateral side. Therefore, the depth of the facial nerve may potentially play a key role in the etiology of Bell's palsy, which should be further evaluated.