Objective: We aimed to test the value of Blink Reflex (BR) as an electrophysiological tool in differentiating functional facial spasm (F-FS) from hemifacial spasm (HFS).

Background: Diagnosis between F-FS and HFS may be challenging despite a few clinical distinguishing features are supportive of F-FS diagnosis.
Blink reflex (BR) is a valid tool for the diagnosis of HFS as well as for understanding the underlying pathophysiology.
One of the classic features of HFS is spread of the BR to muscles other than the orbicularis oculi. These early response in lower facial muscles are named R1-like responses because of the resemblance to the R1 response recorded in the orbicularis oculi. R1-like responses in lower facial muscles are likely due to ephaptic responses in the facial nerve after antidromic activation of facial nerve axon terminals by the electrical stimulus at the supraorbital region. As abnormal movements in subjects with functional movement disorders are generated by disconnection of cortical areas involved in movement preparation, emotion processing and sensory filtering, we hypothesized that people with F-FS would not have R1-like responses, which are produced by ephaptic transmission.

Method: We included 15 patients with HFS, 8 patients with clinically definite F-FS and 15 healthy controls (HC). BR was obtained by bilateral stimulation of the supraorbital nerve and recording with surface electrodes using 4 channel recordingfrom the orbicularis oculi (O-OC) and orbicularis oris (O-OR) muscles of both sides. The primary endpoint was to compare the presence of early R1-like response in ipsilateral O-OR in the 3 groups.

Results: In all subjects (HFS, F-FS, HC), we obtained the expected R1 response in ipsilateral O-OC and the bilateral R2 responses in both O-OC. In HFS, the early R1-like response was recorded in ipsilateral O-OR in all subjects and in both O-OR in 8 subjects.
Early R1-like responses in O-OR were recorded neither ipsilaterally nor bilaterally in any of F-FS and HC subjects.

Conclusion: The findings of this pilot study suggest that BR might be a reliable and valid electrophysiological tool to support the diagnosis of F-FS.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/differential-diagnosis-between-functional-facial-spasm-and-hemifacial-spasm-role-of-blink-reflex/