Bone metastasis in patients with para neoplastic myasthenic syndrome - Possible indication for bone scintigraphy

Full text: Myasthenia gravis (MG) is a neuromuscular disorder caused by a decrease in the number of acetylcholine receptors at neuromuscular junctions and consequently characterized by weakness and fatigue. Paraneoplastic myasthenic syndrome (PMS) is a neurological disorder often difficult to diagnose in clinical practice, due to the lack, in most cases, of any sign of malignancy at the time when neurological impairment occurs. The connection between MG and pathological alterations of the thymus as well as between the presynaptic membrane alteration (Lambert-Eaton myasthenic syndrome) and the small-cell lung cancer is often demonstrated. Most researchers agree that myasthenic syndrome noticed in aged persons should be investigated as a possible paraneoplastic disorder. The aim of our study was to find if suspected PMS could be an indication to perform a bone scan, in presence of parameters suggesting malignancy (such as elevated serum levels of alkaline phosphatase, elevated tumor markers, unexplained bone pain etc.). Another question is whether bone metastases occur more frequently in malignancies associated with PMS than in the same diseases without neurological involvement, taking into account that neurological disorders are not produced by metastatic or direct invasion of the nervous system by the cancer. Our observations included 28 patients (13 men and 15 women), aged 42-80 years with myasthenic syndrome, who were referred by the neurology department for suspicion of bone metastasis. All patients had elevated serum levels of alkaline phosphatase, 18 patients had therapy-resistant bone and joints pain. Conventional imaging procedures (abdominal ultrasound, chest X-ray and computer tomography) were performed in all patients. Only in 6 patients the primary malignancy was diagnosed prior to bone scan (5 cases with thymoma and 1 case of digestive neoplasm). Bone scan was performed on a Diacam Siemens gamma camera and consisted of whole-body examination after injecting 750 MBq of technetium-99m labelled methylenediphosphonate. Static views were additionally performed when considered necessary. Results showed focal uptake consistent with bone metastasis in 13 cases, uncertain lesions (possible degenerative) in 6 cases and normal pattern in 9 cases. Most focal uptakes were suggestive of secondary lesions involving sternum and spine. Further investigations revealed the primary malignancy in 23 cases (11 thymoma, 3 lung cancer, 4 colonic cancer, 2 gastric cancer, 1 prostate carcinoma, 1 neoplasm of the cavum, 1 thyroid cancer). Based on our preliminary observations, we conclude that bone scan might be useful to detect secondary bone lesions in middle-aged and old aged patients with MG, under circumstances of clinical and biological suspicion of malignancy, even before confirmation of primary disease. Because of the usually small size of the primary lesions at the time of the PMS occurrence (for example, small thymoma), they are sometimes very difficult to diagnose by conventional procedures. Though metabolic imaging as PET scan is useful but in centres where this modality is not available, bone scintigraphy might be justified for selected patients and a positive yield in these patients can orient further investigation directed towards detecting the malignancy. (author)