Small-cell lung cancer: a case report on the manifestation of paraneoplastic motor neuron disease after effective tumor shrinkage during chemotherapy

A 72-year-old Japanese man with locally advanced small-cell lung cancer presented with gait disturbance, caused by weakness in both lower extremities; this occurred during chemotherapy that effectively reduced the size of his tumor. Muscle weakness was severe in the bilateral proximal upper extremities, and moderate in the bilateral distal upper extremities and the bilateral lower limbs. Numbness and mild hypoesthesia were also present in the right upper and lower extremities. Involvement of brain and spinal lesions were an unlikely cause of these symptoms. Findings from a nerve conduction study suggested a mild peripheral sensory neuropathy complication. The patient died of respiratory failure and CO 2 narcosis at 9 months after the diagnosis of malignancy and at 4 months after the first manifestation of muscular weakness. Anti-Hu antibody was reported to be positive after his death. Thus, paraneoplastic motor neuron disease (MND) was definitely confirmed. In conclusion, this was a rare case of paraneoplastic MND; the clinical course was uncommon in that the MND occurred when chemotherapy had successfully reduced the size of the tumor.