To describe a diagnostically challenging case of hemimasticatory spasm (HMS) due to neurovascular compression by an anterior inferior cerebellar artery (AICA) loop, and highlight successful management with botulinum toxin.

HMS is a rare movement disorder causing unilateral, paroxysmal spasms of jaw-closing muscles, most often the masseter. It results from dysfunction of the trigeminal motor branch. While vascular compression is well established in trigeminal neuralgia and hemifacial spasm, it is rarely implicated in HMS. The AICA is known to cause hemifacial spasm through facial nerve compression and has been linked to trigeminal neuralgia, but AICA compression of the trigeminal motor root leading to HMS has not been previously reported.

We present a 64-year-old man with left-sided jaw spasms since adolescence, misdiagnosed for decades as temporomandibular joint dysfunction. His spasms, pulsatile and confined to the masseter, were observed on exam without other facial involvement. MRI was unremarkable; however, CT angiography revealed an AICA loop extending into the internal auditory canal with an anomalous venous structure near the left cerebellopontine angle cistern, consistent with neurovascular compression. The patient underwent botulinum toxin injection to the left masseter.

The patient experienced complete resolution of spasms, remaining symptom-free at follow-up with no adverse effects. This outcome highlights the role of botulinum toxin for symptom control in HMS, including cases with structural or vascular etiologies.

This case emphasizes considering neurovascular compression in HMS, particularly in atypical or treatment-resistant presentations. CTA may reveal subtle vascular anomalies missed on MRI. Botulinum toxin is a safe, effective, minimally invasive therapy for HMS, even when compression is the underlying mechanism. Recognition of vascular loops as contributors to HMS may broaden understanding of its pathophysiology and guide future treatment strategies.