Anti-gamma-aminobutyric acid type B (GABA-B) receptor encephalitis is a rare autoimmune disorder caused by antibody-mediated disruption of inhibitory neurotransmission in limbic circuits. It typically affects middle-aged men, presenting with refractory seizures, cognitive decline, and psychiatric symptoms. Up to half of cases are paraneoplastic, most often linked to small-cell lung cancer (SCLC). Diagnosis is confirmed by detecting GABA-B receptor antibodies in serum or cerebrospinal fluid (CSF). First-line therapy includes corticosteroids, intravenous immunoglobulin (IVIG), or plasma exchange, with tumor-directed therapy in paraneoplastic cases. Prognosis is worse in patients with SCLC, severe symptoms, or ICU admission.

A 65-year-old man with hypertension, hyperlipidemia, transient ischemic attack, and a known left hilar lung mass presented with worsening dementia, behavioral changes, and refractory seizures. He was initially admitted to neurocritical care after a generalized tonic-clonic seizure requiring ventilatory support. Following discharge, his family noted transient improvement before sudden onset of agitation, noncooperation, inability to follow commands, rigidity, intermittent jerking movements, and rapid memory decline, prompting readmission. On examination, he exhibited startle myoclonus, disinhibited behavior, restlessness, nonverbal status, and bilateral lower extremity hyperreflexia. Brain MRI and MRA were unremarkable. Lumbar puncture revealed anti-GABA-B receptor antibodies in the CSF. He received five days of intravenous immunoglobulin and methylprednisolone, leading to neurological improvement; he became calmer, minimally verbal, and able to follow simple commands. Subsequent chest CT demonstrated a left lower lobe infrahilar opacity, and biopsy during a later admission confirmed SCLC. He remains on antiepileptic therapy and has initiated oncologic treatment.

This case demonstrates the importance of testing for autoimmune encephalitis in rapidly progressive cognitive decline, as timely immunotherapy can yield neurological improvement. It also highlights the need for oncologic evaluation given the paraneoplastic association with SCLC. Notably, startle myoclonus in this patient represents an atypical feature, making this case particularly rare.