Onset of narcolepsy type 1 in a paraneoplastic encephalitis associated with a thymic seminoma.

Narcolepsy type 1 (NT1) results from probable autoimmune disruption of hypothalamic hypocretinergic neurons. Secondary narcolepsy can occur as a result of other conditions affecting the central nervous system, including limbic paraneoplastic encephalitis (PE). We report the case of a 19-year-old patient presenting with acute-onset diurnal hypersomnolence, hyperphagia, sexual dysfunction and psychiatric disturbances. Further investigations revealed a limbic PE associated with mediastinal thymic seminoma. Tumour removal and immunosuppressive treatment resulted in a partial benefit on psychiatric disturbances but did not improve daytime sleepiness. A comprehensive sleep assessment led to the diagnosis of secondary NT1 with reduced CSF hypocretin-1 levels and revealed the presence of the HLA DQB1*0602 allele, typically associated with idiopathic narcolepsy, for which we hypothesize a possible immunopathogenic role. Sodium oxybate was successfully administered. Narcolepsy is often overlooked in patients with limbic PE. A prompt assessment and an adequate symptomatic treatment can improve the disease burden.