MMA occurs primarily in males between the ages of 15 and 25. Onset is insidious and progression is slow. In most cases, the cause is unknown, although there have been a few published reports linking MMA to traumatic or radiation injury. Diagnosis is made by physical exam and medical history. People believed to have MMA should be followed by a neuromuscular disease specialist for a number of months to make certain that no signs of other motor neuron diseases develop.
Treatment consists of muscle strengthening exercises and training in hand coordination.
The symptoms of MMA usually progress slowly for one to two years before reaching a plateau, then remain stable for many years. Disability is generally slight. Occasionally, the weakness progresses to the opposite limb. There is also a slowly progressive variant of MMA known as O'Sullivan-McLeod syndrome, which affects the small muscles of the hand and forearm and has a slowly progressive course. The goals of these studies are to increase understanding of these disorders and to find ways to treat, prevent, and ultimately cure them.