71 Hypertrophic olivary degeneration after dentate nucleus involvement in two patients with Central Nervous System Lymphoma: a potential mimic of relapse

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Abstract Introduction Hypertrophic olivary degeneration (HOD) is a rare transsynaptic degeneration caused by disruption of the Guillain-Mollaret triangle. In neuro-oncology, it may mimic tumour recurrence, particularly during post-treatment surveillance of posterior fossa disease. We describe two cases of contralateral HOD following dentate nucleus involvement in central nervous system lymphoma (CNSL), highlighting an important diagnostic pitfall. Method We reviewed clinical, radiological and follow-up data from two patients who developed HOD in the context of CNSL. The first had an isolated CNS relapse of previously treated diffuse large B-cell lymphoma involving the right dentate nucleus. The second presented following a steroid-responsive primary CNSL involving the right cerebellum. Following radiological resolution of the dentate lesions, serial brain magnetic resonance imaging (MRI) was performed over about two and four years respectively. Results The interval between radiological diagnosis of CNSL and detection of HOD ranged from approximately three weeks to three months. In both cases, dentate involvement resulted in contralateral HOD, with MRI demonstrating a new non-enhancing T2-weighted hyperintensity of the left inferior olivary nucleus. These findings remained stable on long-term follow-up. In the first patient, HOD was associated with rhythmic palatal tremor, left upper limb hypertonia and cerebellar cognitive affective syndrome (CCAS). In the second, HOD remained radiologically stable despite a later distant supratentorial relapse of primary CNSL, limiting the clinical assessment of typical HOD manifestations. Conclusion HOD may occur following dentate nucleus injury in CNSL and can closely mimic disease progression on surveillance imaging. Awareness of its temporal evolution and associated clinical features, including CCAS, is essential to support multidisciplinary decision-making and to avoid misdiagnosis and unnecessary treatment escalation.