Thymoma-associated limbic encephalitis: remission after thymectomy and rituximab

We report a 59-year-old patient presenting with disorientation, memory loss and simple partial seizures with olfactory hallucinations and parasthesia in the right arm. On admission, neurological examination was normal except for marked neuropsychological impairment, mainly involving short and long-term memory.

Bilateral temporal epileptic discharges were seen in the EEG. High intensity signals in the medial and anterior temporal lobes were seen on T2-weighted and FLAIR MRI images, suggestive of a limbic encephalitis. Chest-CT scan showed a tumorous mass infiltrating the pleura and the inferior lobe of the right lung. Laboratory findings including autoreactive antibodies (Hu, Yo, Ri, Ma1, Ma2, CV2, amphiphysin, voltage-gated potassium channel) were normal, as were CSF findings.

Three weeks after the onset of neurological symptoms, thymectomy was performed. Histological examination revealed a WHO type B1 thymoma with a share of 5% of a type B2 (stage IV A of the classification of Masaoka).

After thymectomy, the patient was treated with a single dose of 700mg of the CD20 antibody rituximab and steroid pulse therapy for five days. Memory greatly improved within one month; repeat neuropsychological examination was normal. No further seizures occurred under oxcarbazepine and levetiracetam. A second MRI two months later showed marked resolution of the signal alterations; EEG demonstrated a focal bilateral temporal slowing without epileptic discharges.

We report the rare condition of limbic encephalitis associated with thymoma. While its pathogenesis is uncertain, and causative antibodies are rarely identified, it is postulated that the encephalitis is due to an autoimmune reaction involving antibodies that cross-react to thymic and limbic antigens. Besides thymectomy, treatment of choice remains unclear. Our patient demonstrated a complete remission under combined therapy with rituximab and steroids.