Visualization of the nervus intermedius during microvascular decompression in hemifacial spasm: anatomical study

OBJECTIVE The surgical anatomy of the nervus intermedius (NI) is highly variable. The aim of this study was to describe the anatomy of the NI during endoscope-assisted microvascular decompression (MVD) in hemifacial spasm (HFS), and the involvement of the nerve in the vascular conflict. METHODS The authors reviewed a prospectively maintained database for MVDs performed between 2002 and 2022 and extracted clinical data including patient demographics, symptoms, and offending vessel(s). Operative videos and photographs were analyzed retrospectively in an attempt to identify the NI. RESULTS Endoscopic identification of the NI was possible in 139 of 435 MVDs. The anatomy is very variable. In 79 (56.8%) patients, a single-bundle pattern was detected, whereas a multiple-bundle pattern was identified in 60 (43.2%) patients. Overall the most common pattern was a single-bundle type A (49.7%). In 20.1%, a multiple-bundles type A was identified. In 4.3%, a single-bundle type B was detected. In 2.9% a single-bundle type C was found, and in just 0.7% a multiple-bundles type C was detected. A multiple-origin pattern (type D) was found in 31 patients (22.3%). The NI was frequently involved in the neurovascular conflict (approximately 85%). The type of NI or vascular compression pattern did not affect the results regarding the outcome or recurrence of HFS. CONCLUSIONS The anatomy of the NI is for the first time evaluated endoscopically in MVD for HFS. The nerve had various anatomical patterns that were clearly identified. Further studies to evaluate the compression patterns in relation to NI neuralgia are warranted.