Elevated LGI1-IgG CSF index predicts worse neurological outcome.

To determine whether CSF leucine-rich glioma-inactivated 1(LGI1)-IgG titer, index or IgG subclass has prognostic significance, we tested serum and CSF specimens collected concomitantly from 39 seropositive patients. LGI1-IgG index was elevated (>1) in 21 patients (54%), suggesting intrathecal synthesis. Patients with worse outcome at last follow-up (modified Rankin Scale >2) had significantly higher index (median 6.57 vs. 0.5, P = 0.048) compared to those with better outcome. Higher CSF LGI1-IgG4 subclass-specific titer and index correlated with worse outcome (P < 0.005 for both). These data suggest that evidence of intrathecal LGI1-IgG synthesis may correlate with neuronal injury and warrant consideration of aggressive immunotherapy.