[A case of small cell bronchogenic carcinoma accompanied by polyneuropathy and SIADH]

A 77-year-old man with small cell lung cancer (SCLC) accompanied by polyneuropathy and SIADH is reported. The patient first developed peripheral hyperesthesia and muscle weakness 6 months before admission. He was diagnosed as having SCLC by sputum cytology and supraclavicular lymph node biopsy. On admission, chest radiography and CT scan revealed a mediastinal mass, and the conduction velocity in peripheral nerves was delayed. He was also found to have SIADH, on the basis of an extremely low level of serum Na (114 mEq/l) and osmotic pressure (251 mOsm/kg) but with normal urinary Na and osmotic pressure. In association with complete disappearance of the mediastinal mass after intensive chemotherapy, SIADH and polyneuropathy also improved remarkably. When SCLC relapsed in the abdominal lymph nodes 11 months after complete disappearance of the mediastinal mass, polyneuropathy and SIADH also reappeared. These results indicate that both SIADH and polyneuropathy comprised a paraneoplastic syndrome.