Dynamic changes of abnormal muscle response during decompression procedures in double compression-type hemifacial spasm.

Background: Hemifacial spasm (HFS) is a neurovascular movement caused by vascular compression of the facial nerve in its root exit zone (REZ). Cases of HFS caused by double compression (DC) in both REZ and the cisternal portion (CP) have been sporadically reported. The nature of DC-type HFS is still not fully understood. Compression in CP is often overlooked, resulting in reoperation in DC-type HFS cases.

Case description: A 48-year-old man with a 3-year history of left HFS was admitted to our department. Magnetic resonance imaging revealed that the vertebral artery (VA) passed around REZ of the facial nerve, and the anterior inferior cerebellar artery (AICA) was in contact with the facial nerve in CP. Microvascular decompression was performed while monitoring any abnormal muscle response (AMR). Although VA was dissected and detached from REZ, AMR showed only a transient decrease and the amplitude of the AMR wave soon recovered and subsequently increased. No other vessels compressing REZ beneath VA were found. AICA attached to the facial nerve in CP and was compressed upward by VA. When AICA was moved from the facial nerve in CP after the transposition of VA, AMR was immediately resolved. After surgery, the patient was completely free from HFS.

Conclusion: In DC-type HFS, precise preoperative diagnosis and intraoperative identification of the culprit vessel are difficult. In DC-type HFS, decompression of one side of a vessel may exacerbate the compression of the other side. In such a case, AMR helps us become aware of compressions in CP that we may preoperatively overlook. AMR is useful for identifying the exact culprit vessels and recognizing any compression changes caused by intraoperative manipulations.