Variations in the morphology of stylomastoid foramen: a possible solution to the conundrum of unexplained cases of Bell's palsy.

BACKGROUND

Stylomastoid foramen is the terminal part of facial canal and is the exit gateway for facial nerve from skull base. We hypothesized that anatomical variations of this foramen could be a risk factor for the injury of facial nerve resulting in unilateral facial nerve paralysis or Bell's palsy. Hence the present study was conducted to study the variations in size and shape of stylomastoid foramen in dry adult human skulls.

MATERIALS AND METHODS

The study was conducted on 37 dry adult human skulls of unknown age and sex. High resolution images of the skulls under study were processed by ImageJ software and observations were undertaken.

RESULTS

Total eight variations of stylomastoid foramen were observed in terms of shape. The common variants were round, oval and square (present in 83.79% skulls on right side and 81.07% skulls on left side), whereas the rare variants were triangular, rectangular, serrated, bean-shaped and irregular. It was noted that stylomastoid foramen were associated with extensions (45.95% skulls) and also adjacent foramen (18.92% skulls). Exclusively unilateral observations included bifurcation of foramen (16.22% skulls), foramen situated deep inside skull groove (5.41% skulls) and foramen interrupted by bony spur (2.7% skulls). No significant differences were observed between the mean diameters (antero-posterior and transverse) of the stylomastoid foramen.

CONCLUSIONS

The unilateral variations along with rare variations in terms of shape such as serrated, bean-shaped and irregular foramen (which were also unilateral findings) could be potential risk factors towards injury of facial nerve at the point of exit from skull base leading to Bell's palsy.