Impact of flocculus medial extension on the root exit zone visibility during microvascular decompression for hemifacial spasm.

Microvascular decompression (MVD) for hemifacial spasm (HFS) involves retraction of the cerebellar flocculus to monitor any neurovascular compression at the facial nerve root exit zone (REZ). Even so, identification of neurovascular compression may be challenging in some cases, due to a "large" or "hypertrophic" flocculus. We have frequently encountered cases where the REZ remains poorly visible despite the flocculus appearing normal in size on MRI. Accordingly, we hypothesized that the anatomical relationship between the flocculus and the REZ, rather than flocculus size, is the key factor affecting REZ visibility. In this retrospective observational study, we examined the correlation between preoperative MRI findings and REZ visibility during MVD for HFS. We measured the size of the flocculus and evaluated its degree of medial extension, i.e. whether its medial border extended beyond the facial nerve REZ, in three-dimensional images. REZ visibility was assessed through retrospective review of surgical videos. We also examined whether REZ visibility affected the surgical outcome using grading scores. Sixty-five patients were included in the study. The REZ was successfully visualized under a surgical microscope in 53 cases and not visualized in 12 cases, the flocculus volume being 0.37 ml and 0.41 ml, respectively, with no significant intergroup difference (P =.23). Medial extension of the flocculus was observed in 1 (1.9%) and 5 (41.7%) of these cases, respectively, the difference being significant (P =.0002). There were no significant intergroup differences in MVD effectiveness (P =.88) or complications (P =.41). We have revealed a novel anatomical variation, "medial extension of the flocculus," that significantly impacts facial nerve REZ visibility during MVD surgery. Although this feature did not affect outcomes in our study cohort, it needs to be borne in mind for avoiding excessive flocculus retraction, especially by less experienced surgeons. Clinical trial number Not applicable.