GABA(B) receptor antibodies in limbic encephalitis and anti-GAD-associated neurologic disorders.

BACKGROUND: γ-Aminobutyric acid-B receptor antibodies (GABA(B)R-ab) were recently described in 15 patients with limbic encephalitis (LE), associated with small-cell lung cancer (SCLC) or with concurrent glutamic acid decarboxylase (GAD) antibodies. We analyzed the frequency of GABA(B)R-ab in 147 patients with LE or neurologic syndromes associated with GAD-ab.
METHODS: We examined the presence of GABA(B)R-ab in 70 patients with LE (33 paraneoplastic with onconeural antibodies, 18 paraneoplastic without onconeural antibodies [5 with Gad-ab], and 19 idiopathic with either GAD-ab [5 patients] or seronegative) and 77 patients with GAD-ab-associated neurologic syndromes other than LE (29 stiff-person syndrome, 28 cerebellar ataxia, 14 epilepsy, and 6 with diverse paraneoplastic neurologic syndromes). GABA(B)R-ab were analyzed in serum or CSF by indirect immunofluorescence on HEK293 cells transfected with GABA(B1) and GABA(B2) receptor subunits.
RESULTS: GABA(B)R-ab were detected in 10 of the 70 patients with LE (14%). Eight had SCLC and 2 were idiopathic. One of the 8 patients with LE with SCLC had an additional onconeural antibody (Hu) and 2 GAD-ab. GABA(B)R-ab were identified in 7 (70%) of the 10 patients with LE and SCLC without onconeural antibodies. GABA(B)R-ab antibodies were not found in patients with GAD-ab and stiff-person syndrome, idiopathic cerebellar ataxia, or epilepsy. However, one patient with GAD-ab, paraneoplastic cerebellar ataxia, and anaplastic carcinoid of the thymus also presented GABA(B)R-ab.
CONCLUSIONS: GABA(B)R-ab are the most common antibodies found in LE associated with SCLC previously considered "seronegative." In patients with GAD-ab, the frequency of GABA(B)R-ab is low and only observed in the context of cancer.