Isolated seizures are a common early feature of paraneoplastic anti-GABA_B receptor encephalitis

To report the clinical features and long-term outcome of 22 newly diagnosed paraneoplastic patients with GABA_B receptor antibodies (GABA_BR-Abs).

Retrospective clinical study of CSF-confirmed cases of GABA_BR-Abs encephalitis.

We identified 22 patients (4 female) with GABA_BR-Abs, with a median age of 64 years (range 55-85). All were paraneoplastic: 20 small-cell lung cancer, one malignant thymoma, and one uncharacterized lung mass. The most frequent first symptom was the isolated recurrent seizures without cognitive inter-ictal impairment in 17 patients (77%). In the other, three presented the first behavioral disorders and two presented de novo status epilepticus (SE). After a median delay of 10 days (range 1-30), the recurrent seizures' phase was followed by an encephalitic phase characterized by confusion in 100% of cases and SE in 81% (n = 17), with 53% (n = 9) non-convulsive SE. Dysautonomic episodes were frequent (36%, n = 8, bradycardia and central apnea) and killed three patients. CSF study was abnormal in 95% of the cases (n = 21). At the encephalitic phase, MRI showed a temporal FLAIR hypersignal in 73% (n = 16) of the cases. First-line immunotherapy was initiated after a median delay of 26 days (range 6-65) from disease onset, and a partial response was observed in 10 out of 20 patients (50%). There was no complete response. Two years after onset, a massive anterograde amnesia affected all still alive patients. Nine patients died from cancer progression (median survival: 1.2 years).

Paraneoplastic GABA_BR-Abs encephalitis is characterized by a stereotype presentation with an epilepsy phase before an encephalitic phase with dysautonomia. The functional prognosis is poor.