Objective: To present a case of hemimasticatory spasm (HMS) secondary to an anterior inferior cerebellar artery (AICA) loop treated with botulinum injection.

Background: HMS is a rare disorder characterized by unilateral paroxysmal spasms of the jaw-closing muscles. The motor branch of the trigeminal nerve is likely involved with typical involvement of the masseter and less commonly the temporalis and medial pterygoid. The mechanism of HMS is unclear, though vascular compression or focal demyelination of the trigeminal nerve has been hypothesized. The AICA can cause hemifacial spasms due to facial nerve compression at the root exit zone or rarely at the distal, cisternal portions. Neurovascular contact of the AICA and the trigeminal nerve is reported in trigeminal neuralgia, though no cases of hemimasticatory spasms have been reported secondary to an AICA loop.

Method: NA

Results: A 64-year-old male presented with paroxysmal left masseter spasms since the eighth grade. The episodes last a few hours, during which he feels a throbbing in his left cheek that aligns with his heart rate. He was initially diagnosed with temporomandibular joint dysfunction by his dentist and trialed on a splint with no improvement. On physical exam, he had brief twitches of his left masseter muscle, with no other facial muscles involved. Brain magnetic resonance imaging (MRI) was unremarkable, specifically no visible displacement or abnormal enhancement along the cranial nerves or within the internal auditory canals. Head computed tomography angiography (CTA) showed an AICA loop coursing into the left internal auditory canal and an anomalous normal variant venous structure coursing in the left cerebellopontine angle cistern region. He was diagnosed with hemimasticatory spasms likely from compression from the vascular anomalies and was treated with intramuscular botulinum toxin injections in his left masseter muscle. He had a favorable response to the botulinum toxin injection and the frequency of his hemimasticatory spasms improved.

Conclusion: HMS can be secondary to an AICA loop and can present with isolated masseter muscle involvement. Brain MRI and head CT angiography should be done in patients with HMS to assess for neurovascular compression. Botulinum toxin injections are the nonsurgical treatment of choice and have favorable outcomes in patients with HMS.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/hemimasticatory-spasm-secondary-to-an-anterior-inferior-cerebellar-artery-loop-treated-with-botulinum-toxin-injection/