[Case of antiamphiphysin antibody-positive stiff-person syndrome with breast cancer diagnosed by FDG-PET].

We report a case of antiamphiphysin antiboddy-positive stiff-person syndrome associated with breast cancer, which was detected only by FDG-PET. A 46-year-old woman was admitted to our hospital because of painful muscle cramp and stiffness of both legs. Laboratory results were negative for anti-GAD antibody, but highly positive for antiamphiphysin antibody (1: 61,440). She had been diagnosed as having paraneoplastic stiff-person syndrome. However, mammogram, thoracic CT, breast MRI and ultrasonic echography showed no abnormal findings. A whole-body fluorodeoxyglucose (FDG) PET was performed, showing an increased uptake in the left axillary lymph nodes. Then, the left axillary lymph nodes were resected and immunohistochemically diagnosed as breast adenocarcinoma. Treatment of stiff-person syndrome was initiated with corticosteroids followed by chemotherapy against breast cancer, which led to a remarkable improvement of her neurological symptoms. If there is possibility of paraneoplastic syndromes like stiff-person syndrome, FDG-PET is very useful for detecting the occult carcinoma.