Objective: To describe an anatomical mechanism of reverse hemifacial spasm (RHFS) inducible with neck extension.

Background: Hemifacial spasm (HFS) is a movement disorder often caused by vascular compression of the facial nerve as it leaves the brainstem, and it is described as involuntary contractions starting in the orbicularis oculi and spreading to the lower face. RHFS is a rare presentation of this condition without a well established mechanism and is defined by lower facial muscle contractions progressing to upper facial muscle contractions. Bell’s Palsy is a known cause of HFS due to synkinesis and aberrant nerve regeneration, but neck extension is not described in the literature as a mechanism for RHFS.

Method: n/a

Results: 58 year old male with five episodes of Bell’s Palsy in the last 10 years presented with intermittent right RHFS for 6 years. Exam was notable for spasms starting in the right platysma and spreading to the depressor angularis oris and risorius. There was no involvement of the orbicularis oculi. The spasms were inducible with neck extension. Work up including head imaging and EEG were unremarkable. His spasms improved with botulinum toxin in the right platysma, depressor angularis oris, and risorius.

Conclusion: A possible mechanism for this unique case of RHFS is an anastomosis of the cervical plexus and facial nerve via synkinesis from multiple episodes of Bell’s Palsy. After an episode of Bell’s Palsy, during the regeneration process, there is aberrant nerve regeneration leading to unintended co-contraction of muscles. The muscles of lower facial expression and the platysma are innervated by the mandibular branch and cervical branch of CN 7. Uncommonly, the cervical plexus forms an anastomosis with the marginal mandibular branch of the facial nerve via the transverse cervical nerve (C2, C3).  If our patient is one of these rare individuals with an anastomosis of his cervical plexus and facial nerve, then neck extension could result in co-contraction of the platysma. This aberrant contraction could then continue to spread from the platysma to the lower facial muscles of expression resulting in a reverse hemifacial spasm.

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2.Anatomical Dissection and Modified Sihler Stain of the Lower Branches of the Facial Nerve.
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3. Chopade TR, Lui F, Bollu PC. Hemifacial Spasm. [Updated 2024 Jan 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK526108/

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