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Disease Profile

Monomelic amyotrophy

Prevalence estimates on Rare Medical Network websites are calculated based on data available from numerous sources, including US and European government statistics, the NIH, Orphanet, and published epidemiologic studies. Rare disease population data is recognized to be highly variable, and based on a wide variety of source data and methodologies, so the prevalence data on this site should be assumed to be estimated and cannot be considered to be absolutely correct.


US Estimated

Europe Estimated

Age of onset





Autosomal dominant A pathogenic variant in only one gene copy in each cell is sufficient to cause an autosomal dominant disease.


Autosomal recessive Pathogenic variants in both copies of each gene of the chromosome are needed to cause an autosomal recessive disease and observe the mutant phenotype.


dominant X-linked dominant inheritance, sometimes referred to as X-linked dominance, is a mode of genetic inheritance by which a dominant gene is carried on the X chromosome.


recessive Pathogenic variants in both copies of a gene on the X chromosome cause an X-linked recessive disorder.


Mitochondrial or multigenic Mitochondrial genetic disorders can be caused by changes (mutations) in either the mitochondrial DNA or nuclear DNA that lead to dysfunction of the mitochondria and inadequate production of energy.


Multigenic or multifactor Inheritance involving many factors, of which at least one is genetic but none is of overwhelming importance, as in the causation of a disease by multiple genetic and environmental factors.


Not applicable


Other names (AKA)

Hirayama disease; Spinal muscular atrophy juvenile nonprogressive; Benign focal amyotrophy;


Musculoskeletal Diseases


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.[1]

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.[1]


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.[1][2]

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.[1]

This table lists symptoms that people with this disease may have. For most diseases, symptoms will vary from person to person. People with the same disease may not have all the symptoms listed. This information comes from a database called the Human Phenotype Ontology (HPO) . The HPO collects information on symptoms that have been described in medical resources. The HPO is updated regularly. Use the HPO ID to access more in-depth information about a symptom.

Medical Terms Other Names
Learn More:
80%-99% of people have these symptoms
Distal upper limb amyotrophy
EMG abnormality
Muscle weakness
Muscular weakness
30%-79% of people have these symptoms
Abnormality of peripheral nerve conduction
Degeneration of anterior horn cells
5%-29% of people have these symptoms
Abnormality of the immune system
Immunological abnormality
Muscle twitch
Percent of people who have these symptoms is not available through HPO
Cervical spinal cord atrophy
Cold paresis
EMG: neuropathic changes
Insidious onset
Gradual onset
Interosseus muscle atrophy
No previous family history
Upper limb muscle weakness
Decreased arm strength
Weak arm

[ more ]



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.[1] 


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.[1]

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.[4] 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.[1]


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.[1] Surgery for the treatment of MMA is debated, as there are benefits and risks associated with surgery.[5] People who are diagnosed with MMA will likely be recommended to see a neuromuscular specialist who can watch for progression of the disease.[2]

Learn more

These resources provide more information about this condition or associated symptoms. The in-depth resources contain medical and scientific language that may be hard to understand. You may want to review these resources with a medical professional.

Where to Start

In-Depth Information

  • The Monarch Initiative brings together data about this condition from humans and other species to help physicians and biomedical researchers. Monarch’s tools are designed to make it easier to compare the signs and symptoms (phenotypes) of different diseases and discover common features. This initiative is a collaboration between several academic institutions across the world and is funded by the National Institutes of Health. Visit the website to explore the biology of this condition.
  • Online Mendelian Inheritance in Man (OMIM) is a catalog of human genes and genetic disorders. Each entry has a summary of related medical articles. It is meant for health care professionals and researchers. OMIM is maintained by Johns Hopkins University School of Medicine. 
  • Orphanet is a European reference portal for information on rare diseases and orphan drugs. Access to this database is free of charge.
  • PubMed is a searchable database of medical literature and lists journal articles that discuss Monomelic amyotrophy. Click on the link to view a sample search on this topic.


  1. Kuwabara S. Monomelic amyotrophy. Orphanet. October 2012; https://www.orpha.net/consor/cgi-bin/OC_Exp.php?lng=en&Expert=65684.
  2. Monomelic Amyotrophy Information Page. National Institute of Neurological Disorders and Stroke. May 24, 2017; https://www.ninds.nih.gov/Disorders/All-Disorders/Monomelic-Amyotrophy-Information-Page.
  3. Amyotrophy, Monomelic. Online Mendelian Inheritance in Man. October 19, 2017; https://www.omim.org/entry/602440.
  4. Boruah DK, Sanyal S, Prakash A, Achar S, Dhingani DD, and Sarma B. Bimelic symmetric Hirayama disease: Spectrum of magnetic resonance imaging findings and comparative evaluation with classical monomelic amyotrophy and other motor neuron disease. Iranian Journal of Neurology. July 6, 2017; 16(3):136-145. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5673986/.
  5. Harrop JS and Ghobrial GM. Hirayama Disease (Monomelic Amyotrophy): An Underreported Lower Motor Neuron Disease in the North American Adolescent Population. Congress of Neurological Surgeons. Fall 2014; https://www.cns.org/publications/congress-quarterly/congress-quarterly-fall-2014/hirayama-disease-monomelic-amyotrophy.

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