Lost in Cognition: Paraneoplastic Rhombencephalitis With Anti-Ri/Antineuronal Nuclear Antibody Type 2 (ANNA-2) Antibodies Presenting as Rapid-Onset Cognitive Decline and Ophthalmoplegia-Ataxia With Reversible Brainstem Lesion

Anti-Ri/antineuronal nuclear antibody type 2 (ANNA-2) autoantibodies have emerged as significant markers within the realm of paraneoplastic neurological syndromes (PNS), which are recognized for their connection with onconeural antibodies and their potential to cause neurological manifestations in the presence of an underlying malignancy. If left untreated, PNS involving anti-Ri antibodies can result in substantial morbidity and mortality, and the heterogeneous range of associated syndromes and clinical presentations often complicates the diagnosis. We report the case of a 67-year-old Asian female patient with a history of hypertension and diabetes mellitus who developed progressive gait ataxia, diplopia, dysphagia, facial twitching, and cognitive decline over three months. Neurological examination revealed conjugate horizontal gaze palsy, partial ptosis, oromandibular dystonia, hyperreflexia, and ataxia. Brain magnetic resonance imaging (MRI) showed non-enhancing T2/fluid-attenuated inversion recovery (FLAIR) patchy signal changes in the brainstem, and cerebrospinal fluid (CSF) analysis revealed mildly elevated protein and pleocytosis. ANNA-2 antibodies were positive in both serum and CSF, prompting a malignancy workup that uncovered an invasive, poorly differentiated ductal carcinoma of the breast. The patient was started on high-dose corticosteroids, resulting in partial radiological and clinical improvement. In conclusion, a subacute gait ataxia, ophthalmoparesis, and jaw dystonia should raise suspicion for anti-Ri/ANNA-2 rhombencephalitis, warranting prompt malignancy evaluation and early immunotherapy to optimize outcomes.