Coexistence of Lambert–Eaton myasthenic syndrome and autoimmune encephalitis with anti-CRMP5/CV2 and anti-GABAB receptor antibodies in small cell lung cancer: A case report

RATIONALE: Autoimmune encephalitis and Lambert–Eaton myasthenic syndrome are classic paraneoplastic neurological conditions common in patients with small cell lung cancer. PATIENT CONCERNS: The patient complained of tiredness, fluctuating recent memory loss, and inability to find his home. His family members reported a change in character, irritability, and paranoia. One month later, the patient had 1 grand mal seizure lasting 5 minutes. DIAGNOSIS: The patient was diagnosed with limbic encephalitis combined with Lambert–Eaton myasthenic syndrome. The gamma-aminobutyric acid B (GABA(B)) receptor and collapsin response mediator protein 5 (CRMP5, also called CV2) antibody test results were positive. Nine months after the onset of symptoms, the patient was diagnosed with small cell lung cancer. INTERVENTIONS: The patient was administered intravenous immunoglobulin for 5 days. He was then treated with 60 mg prednisone once per day. The prednisone dose was gradually reduced by 1 tablet every 2 weeks. After the diagnosis, the patient underwent 6 courses of chemotherapy with cisplatin combined with sequential chemoradiation therapy. OUTCOMES: The patient was able to take care of himself. Neurological examination revealed a lower limb proximal muscle strength level of 4 and a reduced limb tendon reflex. The patient had deficits in short-term memory, a Mini-Mental State Examination score of 26, Montreal Cognitive Assessment score of 24, Self-rating Depression Scale score of 54 (mild depression), and Self-Rating Anxiety Scale score of 42 (normal). LESSONS: Autoimmune diseases of the peripheral and central nervous systems can be observed at the same time in patients with small cell lung cancer, even when magnetic resonance imaging findings are negative and immune therapy is effective.