Brainstem excitability in hemifacial spasm and post-facial palsy synkinesias and effects of botulinum toxin

Background: Hemifacial Spasm (HFS) and Post-facial Palsy Syndrome (PFPS) are characterised by synkinesias which electrophysiological hallmark is the lateral spread of the blink reflex, but functional and anatomical evidence of this phenomenon is lacking.Objectives: To evaluate brainstem excitability in the facial and trigeminal pathways of the affected and unaffected side in HFS and PFPS and modifications induced by botulinum toxin treatment (BoNT-A).Methods: the Blink Reflex (BR), BR Recovery Cycle (BRRC) and Masseter Inhibitory Reflex (MIR) were assessed in HFS (n=10) and PFPS (n=7), before and after BoNT-A. Facial palsy (FP; n=4), used as lesional model, and and healthy controls (n=8), were also assessed.Results: BR and MIR were normal in all groups. In all patients BRRC analysis revealed a central hyperexcitability pattern following direct and indirect stimulation on both affected (p=0.000007) and unaffected (p=0.00007) sides, with highest effect at 500 ms interstimulus interval. BoNT-A injection did not affect this pattern in all patient groups.Conclusions: These preliminary results may indicate a brainstem compensatory attempt in response to a facial damage which is severe and temporary in PFPS and subtler but persistent in HFS. Central effects of BoNT-A should be searched at a higher level than the brainstem.