Prevalence and outcome of late-onset seizures due to autoimmune etiology: A prospective observational population-based cohort study.

Objective: The increasing incidence of new-onset seizures with age is well known. Often, the etiology cannot be clarified. In the present study, patients with unprovoked late-onset seizures and without known neoplasm, who might have had paraneoplastic encephalitis, were investigated for a potentially underlying autoimmunity. Methods: Sixty-six consecutive patients (36 women; aged >= 55 years) after having at least one seizure or seizures for <= 6 months were prospectively identified over a period of 4.75 years. All patients were tested for serum and cerebrospinal fluid (CSF) antibodies (Abs) to both neural cell-surface and intracellular antigens. Forty-five (68%) underwent brain magnetic resonance imaging (MRI). Follow-up in Ab-positive cases was >= 6 months. Results: Two patients had high titers of anti-CASPR2 (contactin-associated protein-like 2) Abs in serum and CSF and fulfilled the diagnostic criteria of definite limbic encephalitis. Another two patients had bilateral encephalitic temporal MRI abnormalities. They also satisfied the criteria of definite limbic encephalitis, even though they had no Abs in serum or CSF. All four were in the age range of 55-70 years. They received immunotherapy and/or antiepileptic drug treatment and became seizurefree. Significance: Our findings suggest that autoimmunity should be considered an important etiology in patients with late-onset seizures. Testing for neural antibodies and brain MRI may be worthwhile in this patient group.