Monomelic amyotrophy (MMA) is a rare disease of the nerves that control voluntary movements of the limbs.
One of the motor neuron diseases (MND), degenerative conditions that involve the nerves of the upper or lower parts of the body, MMA is generally a benign disease associated with minimal disability. Onset of MMA primarily occurs between the ages of 15 and 25. The main features of the disease are wasting and weakness of a single upper or lower limb. Generally, MMA progresses slowly overa period of 2–4 years, and then reaches a stationary phase during which the disease remains stable for years.
Monomelic amyotrophy may also be known as benign focal amyotrophy, single limb atrophy, Hirayama syndrome or Sobue disease. Descriptive terms such as brachial monomelic amyotrophy (MMA confined to an arm) or monomelic amyotrophy of the lower limb (MMMA of a leg) may be used to specify the type of limb affected. O'Sullivan-McLeod syndrome, a variant of MMA, is a slowly progressive form of the disease that causes weakness and wasting of the small muscles of the hand and forearm.
Monomelic amyotrophy occurs worldwide and is most prevalent in Asia, and especially in Japan and India. According to a report in 1984, MMA of the lower limb occurs in about four in a million people in India. There is a preponderance of males with MMA; estimates of the male to female ratio range from 5:1 to 13:1.
Causes and symptoms
As of 2004, the underlying cause or causes for MMA remain unresolved. Most cases are sporadic and occur in an individual without a family history of MMA. Numerous factors—such as viral infection, vascular insufficiency (inadequate blood supply) of the spinal cord, heavy physical activity, radiation injury, traumatic injury, and atrophy of the spinal cord—have been suggested as possible causes of MMA. There are a few reports of familial cases of MMA.
Symptoms of MMA appear slowly and steadily over a period of time. The main features of MMA are muscle weakness and atrophy (wasting) in a portion of one limb. The weakness and wasting progresses slowly and may spread to the corresponding limb on the opposite side of the body. Symptoms can develop elsewhere in the affected limb or another limb at the same time or later in the course of the disease. Patients may notice worsening of symptoms on exposure to the cold. Other symptoms of MMA include tremor, fasciculations, cramps, mild loss of sensation, excessive sweating, and an abnormal sympathetic skin response. It is rare that individuals with MMA experience significant functional impairment.
Diagnosis of MMA is based on physical exam and medical history. Physical findings include reduced muscle girth (width around the arm or leg) and decreased strength in the affected limb. Tendon reflexes tend to be normal or sluggish. Cranial nerves, pyramidal tracts, sensory, cerebellar or extrapyramidal systems are not affected. Patients may report or display symptoms described above. They may also indicate difficulty carrying out activities of daily living such as writing, lifting, getting dressed, or walking.
Tests that may aid in diagnosis of MMA include electromyography (EMG), imaging studies such as magnetic resonance imaging (MRI) and computed tomography (CT ) scans, and muscle biopsy . EMG shows chronic loss of nerve cells confined to specific areas of the affected limb. MRI has been reported to be a useful means of determining which muscles are affected in a given patient. Muscle biopsy shows evidence of atrophy of the neurons. EMG, muscle biopsy, or isometric strength testing may also reveal significant findings in seemingly normal muscles of the affected and the contralateral limb.
There is no cure for MMA. The goal of treatment, which is largely supportive, is to help patients optimize function and manage any disability associated with the disorder. Treatment primarily consists of rehabilitation measures such as physical therapy and occupational therapy. Severe muscle weakness (present in a minority of cases) may require orthopedic intervention such as splinting.
In addition to routine health care through their primary care practitioners, individuals with MMA generally see specialists in neurology and rehabilitation. Some patients with MMA may receive comprehensive services through a muscular dystrophy association (MDA) clinic or another type of neuromuscular clinic. Given the rarity of MMA, the potential for rehabilitation in this disorder is unknown.
Recovery and rehabilitation
Rehabilitation for MMA consists of physical and occupational therapy. The goal of these therapies is to make full use of the patient's existing functions. Physical therapy can help a patient with MMA to strengthen muscles in a weak arm or leg. Occupational therapy can teach patients to use adaptive techniques and devices that may help compensate for difficulty with everyday tasks such as writing, buttoning, or tying shoes. Depending upon the degree of weakness in the affected limb, a person with MMA may need to use the unaffected limb for activities previously performed by the now atrophied limb.
As of 2004, there were no clinical trials for patients with MMA. As more is learned about how MMA or related motor neuron diseases develop, it is hoped that novel therapies may be developed in the future.
MMA is generally a benign condition. Disability associated with MMA is typically mild. In the majority of cases, the disorder usually ceases to progress within five years of onset. People with MMA can expect to have a normal life span.
Initially, symptoms of MMA can be similar to early signs of other, more serious neurological disorders such as amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease) and spinal muscular atrophy . For this reason, periodic neurological evaluation may be recommended to be sure that no symptoms of these or other motor neuron diseases develop.
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Dawn J. Cardeiro, MS, CGC