Progression of a pontine cavernous malformation and delayed bilateral hypertrophic olivary degeneration: illustrative case.

backgroundHypertrophic olivary degeneration (HOD) is a rare form of transsynaptic degeneration caused by disruption of the dentato-rubro-olivary pathway (Guillain-Mollaret triangle). It is most described following surgical intervention or acute hemorrhagic injury affecting that pathway. However, longitudinal imaging documenting the natural progression of HOD in patients with brainstem cerebral cavernous malformations (CCMs) remains sparsely described at the individual patient level. OBSERVATIONS: The authors present the case of a 70-year-old woman with a pontine CCM followed longitudinally over 4 years. Initial imaging demonstrated a right-sided pontine lesion with unilateral HOD in the absence of symptoms. Over time, the CCM enlarged and extended across the midline, coinciding with the development of bilateral HOD and delayed onset of gait imbalance and palatal myoclonus. No surgical intervention was performed. LESSONS: In this case, the radiographic evolution from unilateral to bilateral involvement occurred concurrently with the delayed clinical manifestation of HOD. https://thejns.org/doi/10.3171/CASE26577.