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

Abstract

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 8 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 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 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.

Morphology and variability of the facial nerve trunk depending on the branching pattern, gender, anthropometric type and side of the head in Moldovan population

“…Three main directions of the FNT were established in the current study and according to our observation they depended on variation of the mastoid segment of the facial canal, but it is not excluded, that those variants may even depend on variability of the stylomastoid foramen [5].…”

Evaluation of the Morphometry of the Stylomastoid Foramen in the Turkish Population: A Retrospective Study Using Cone-Beam Computed Tomography (CBCT)

“…SMF lies between the mastoid process and the base of the styloid process of the temporal bone [1,2,3]. The SMF is the terminal portion of the facial canal (FC) and is the exit gateway of the facial nerve from the skull base [2,[4][5][6]. The posterior auricular artery and the stylomastoid branch of the facial nerve pass through the SMF [7].…”

“…Information on the morphometry of the SMF and FC is of great importance to surgeons and anesthesiologists in a variety of procedures to avoid neural complications [1,2]. SMF's anatomical variations may be a risk factor resulting in unilateral facial nerve palsy or Bell's palsy [4]. As far as we know, researches have been done on the mastoid, labyrinthine, and tympanic segments for the etiology of Bell's palsy, but the stylomastoid foramen has not been investigated so far [8].…”

Morphological and Morphometric Analysis of Stylomastoid Foramen in Dry Human Skulls and its Clinical Implications

“…The facial nerve trunk blockage is at risk of nerve injury and neurological complications at the position of the facial nerve passes [8]. The facial nerve blockage of stylomastoid foramen is also called a Nadbath block [9].…”

“…Complications reduced by localization of foramen [6]. Anatomical variation in stylomastoid foramen leads to nerve injury which is the major risk factor in Bell's palsy (unilateral facial nerve paralysis) [8].…”