Another Rare Cause of Hypertrophic Olivary Degeneration Following Cavernous Malformation Hemorrhage: A Case Report

<b>Introduction</b>: Hypertrophic olivary degeneration (HOD) is a rare form of trans-synaptic degeneration involving the Guillain-Mollaret triangle, characterized by enlargement of the inferior olivary nucleus-unlike the atrophy typical of most neurodegenerative processes. It is usually associated with stroke, surgical injury, or demyelination, but rarely follows hemorrhage from a cavernous malformation (CM). This report presents a case of HOD secondary to a mesencephalic CM hemorrhage, with emphasis on imaging findings and diagnostic considerations. <b>Case Description:</b> A 55-year-old woman presented with acute-onset, right-sided facial, torso, and limb hypoesthesia, along with gait instability. Neurological examination revealed sensory impairment in the right maxillary (V2) and mandibular (V3) trigeminal territories, as well as diminished pain and temperature sensation throughout the right hemibody. MRI revealed a hemorrhage in the posterior mesencephalon near the left red nucleus, leading to the diagnosis of a CM with an associated venous angioma. She was managed conservatively and improved clinically. Six months later, MRI showed hypertrophy and T2/FLAIR hyperintensity of the left inferior olive, consistent with developing HOD. At 1.5 years follow-up, olivary enlargement had progressed-now consistent with stage 2 HOD-and a bilateral palatal tremor was observed, more pronounced on the right side. DTI revealed asymmetric volume loss in the left brainstem fiber pathways at the level of the medulla oblongata, confirming trans-synaptic degeneration. <b>Conclusions</b>: This case highlights HOD as a rare but important complication of mesencephalic CM hemorrhage. Recognition of its characteristic imaging features-olivary hypertrophy with persistent T2/FLAIR hyperintensity-is essential for accurate diagnosis. DTI supports the trans-synaptic mechanism, helping distinguish HOD from other pathologies and preventing unnecessary investigations.