Monomelic amyotrophy (MMA) is a rare disease that causes muscle weakness in the upper extremities. MMA affects the lower motor neurons. Lower motor neurons are cells that help communicate information from the brain to the muscles that are involved in movement (skeletal muscles). Specifically, Monomelic amyotrophy causes weakness and loss of muscle mass in the arms and fingers. The disease is most common in Asia, especially in Japan and India. Males are more likely to develop the disease than females. The exact cause of Monomelic amyotrophy is unknown. Monomelic amyotrophy can sometimes run in families, but it is not thought to be caused by a specific genetic change. Diagnosis of the disease is based on imaging studies and electromyography (EMG) consistent with the disease.