Anti-Ma2 Encephalitis as a Treatable Cause of Narcolepsy Type 1: A Case Report.

BACKGROUND Secondary narcolepsy type 1 (NT1) is an uncommon but increasingly recognized disorder, often associated with autoimmune etiologies that disrupt hypothalamic hypocretin/orexin-producing neurons. Anti-Ma2 encephalitis, a paraneoplastic syndrome commonly linked to testicular tumors, typically involves the limbic system and diencephalon, leading to diverse neuropsychiatric manifestations. While hypersomnia has been reported in a subset of these patients, full syndromic NT1 - characterized by cataplexy, hypocretin deficiency, and objective sleep study abnormalities - remains underrecognized as a presentation. This gap in awareness can delay diagnosis, particularly when sleep-related symptoms predominate. Identifying autoimmune triggers in cases of rapidly progressive hypersomnia is thus critical for guiding appropriate treatment. CASE REPORT A 60-year-old man developed subacute severe daytime sleepiness, cataplexy, episodic limb weakness, and cognitive decline. Polysomnography indicated mild sleep-disordered breathing. The MSLT revealed a mean sleep latency of 6.2 min with 3 SOREMPs. CSF hypocretin-1 was markedly low (22.836 pg/mL). Anti-Ma2 antibodies were positive in both serum and CSF. Neuroimaging showed periventricular white matter changes on MRI and medial temporal hypermetabolism on PET-CT. Immunotherapy with corticosteroids and IVIG resulted in partial neurological improvement and significant reduction in sleepiness. CONCLUSIONS Anti-Ma2 encephalitis is a treatable cause of secondary NT1. Immunotherapy can substantially improve hypersomnolence, although adjunctive symptomatic treatment may be necessary for residual symptoms. Autoimmune evaluation - including antibody panels and hypocretin testing - is crucial in cases of acute/subacute hypersomnia with atypical features to enable timely diagnosis and treatment.