What is the definition of Monomelic Amyotrophy?
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. Symptoms of the disease typically begin between the ages of 14-25 years-old. 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. It is possible that the disease is caused by movement of the sac that surrounds the spinal cord due to repeated downward movement (flexion) of the neck. 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. Treatment options may include muscle strengthening exercises and neck bracing.
What are the alternative names for Monomelic Amyotrophy?
- Hirayama disease
- Spinal muscular atrophy juvenile nonprogressive
- Benign focal amyotrophy
- Juvenile muscular atrophy of distal upper extremity (JMADUE)
- Juvenile muscular atrophy of distal upper limb
What are the causes for Monomelic Amyotrophy?
The exact cause of monomelic amyotrophy is not well-understood. It is thought that the disease may occur when the material that surrounds the spinal cord (thecal sac or dural sac) shifts in position. This may be caused by repeated downward movement (flexion) of the neck. Changing position of the dural sac can cause pressure to be placed on the spinal cord. This can impact the ability of signals to be sent from the brain to the muscles of the arm. This could cause the signs and symptoms of monomelic amyotrophy. However, it has not been confirmed that pressure on the spinal cord explains why some people develop monomelic amyotrophy. Other possible causes include immune system dysfunction or an infection.
What are the symptoms for Monomelic Amyotrophy?
Signs and symptoms of monomelic amyotrophy include muscle weakness of the hands and fingers. Symptoms of the disease begin between 14-25 years-old, and the symptoms typically begin in the hand or forearm of one side of the body. The muscle-wasting and weakness may progress in the affected limb over 3-9 years, but symptoms typically do not progress to the other side of the body. After a few years of worsening muscle weakness, symptoms of the disease typically stop progressing. The muscle weakness associated with the disease is not accompanied by pain.
In rare cases, symptoms of monomelic amyotrophy may spread to the opposite arm or may start worsening again after age 40-years-old. Other rare symptoms of the disease may include worsening weakness in cold temperatures (cold paresis), muscle cramps, having cold hands, and tremors.
What are the current treatments for Monomelic Amyotrophy?
Unfortunately, there is no cure for monomelic amyotrophy (MMA). However, there are treatment options that can help manage symptoms of the disease and slow the progression of muscle weakness. If the doctors think that MMA is caused by compression of the spinal cord, they may recommend use of a brace that can be worn around the neck to prevent downward movement (flexion) of the neck. Other treatment options include muscle strengthening exercises and therapies to improve hand coordination. Surgery for the treatment of MMA is debated, as there are benefits and risks associated with surgery. People who are diagnosed with MMA will likely be recommended to see a neuromuscular specialist who can watch for progression of the disease.
What is the outlook (prognosis) for Monomelic Amyotrophy?
The long-term outlook for people with monomelic amyotrophy (MMA) is generally good. Although symptoms of the disease may progress for a few years after the initial muscle weakness begins, the symptoms generally stabilize. The muscle weakness typically affects one arm and is not associated with pain or other symptoms.
Some people with MMA have loss of function of one hand. This can cause difficulties in caring for oneself, work, and social situations. Sessions with occupational therapists or social workers may help to overcome difficulties associated with muscle weakness.
How is Monomelic Amyotrophy diagnosed?
Monomelic amyotrophy (MMA) is suspected when a doctor observes signs and symptoms of the disease such as muscle weakness in one arm only that begins during adolescence or early adulthood. The diagnosis can be confirmed with imaging studies and laboratory tests.
Imaging studies that may help confirm a diagnosis of MMA include MRI or CT scans. These imaging studies may show signs of compression of portions of the spinal cord. Laboratory tests may include an electromyograph (EMG) that shows reduced response in the nerves that carry signals to the arm muscles. Other possible causes of muscle weakness such as trauma or injury must be ruled out to confirm the diagnosis of MMA.
Is Monomelic Amyotrophy an inherited disorder?
Monomelic amyotrophy (MMA) is not thought to be caused by changes in a specific gene. Most people who have MMA are the only people with the disease in the family. In some cases, people with MMA reported having other family members with the disease. In one case, it has been reported that two identical twins both developed the disease. This makes researchers think that there may be genetic factors that predispose people to develop MMA. However, not everyone who has these genetic factors would necessarily develop MMA. Instead, it is probably a combination of genetic and environmental factors that cause people to develop MMA.