When Degeneration Masquerades as Disease: Diagnosing Hypertrophic Olivary Degeneration on a Single Magnetic Resonance Imaging Examination in an Elderly Patient with Posterior Fossa Hemorrhage.

Hypertrophic olivary degeneration (HOD) is a delayed trans-synaptic consequence of injury to the dentato-rubro-olivary pathway - the Guillain-Mollaret triangle (GMT) - and represents one of the few conditions in which the inferior olivary nucleus (ION), a pivotal node of the olivopontocerebellar system, paradoxically enlarges rather than atrophies following disconnection. Its peak magnetic resonance imaging (MRI) appearance closely mimics a primary brainstem neoplasm, and failure to recognize it has led to unnecessary biopsy in documented cases. We report a case of a 79-year-old man with uncontrolled hypertension who developed palatal tremor and contralateral cerebellar ataxia 7 months after a spontaneous left cerebellar hemorrhage. Brain MRI demonstrated T2/fluid attenuation inversion recovery hyperintensity with hypertrophy of the right ION, blooming at the left dentate nucleus, and no restricted diffusion - the full diagnostic triad of HOD, established on a single examination. Neoplasm was confidently excluded without biopsy. This case illustrates that systematic, anatomy-guided interpretation of multi-sequence MRI - anchored by knowledge of GMT connectivity - is sufficient for definitive HOD diagnosis from a single study, with particular relevance for elderly patients and resource-limited clinical settings in which olivary degeneration may otherwise be misattributed to recurrent cerebrovascular disease or primary malignancy.