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Monomelic amyotrophy (MA) is a rare benign lower motor neuron disorder characterized by muscular weakness and wasting in the distal upper extremities during adolescence followed by a spontaneous halt in progression and a stabilization of symptoms.
ORPHA:65684Classification level: Disorder
- Benign focal amyotrophy
- Hirayama disease
- Juvenile muscular atrophy of distal upper extremity
- Juvenile muscular atrophy of the distal upper limb
- Prevalence: Unknown
- Inheritance: Unknown
- Age of onset: Adolescent, Adult
- ICD-10: G12.8
- OMIM: 602440
- UMLS: C1865384
- MeSH: C538253
- GARD: 9697
- MedDRA: 10069681
The prevalence is unknown. It is seen mainly in Asian countries (particularly in Japan and India) with only a very few cases reported in Europe and the United States. A nation-wide survey conducted from 1996 to 1998 in Japan found 333 identified cases with the estimated prevalence being approximately 1/33, 300.
MA is seen more frequently in males (male to female ratio of 20:1) with an age of onset in the second to third decade of life (ages 14-25). Muscle weakness and wasting begins in the hand or forearm on one side. There is no pain associated with this muscular atrophy, nor sensory changes. Progression is slow, usually over a 3-9 year time span, followed by a halt and stabilization of symptoms. Occasionally it can destabilize and continue progression after the age of 40. In rare cases it can progress to the opposite limb. Additional rare manifestations include worsening of symptoms with exposure to cold (cold paresis), muscle cramps, cold hands, irregular coarse tremor and/or contraction fasciculations. O'Sullivan-McLeod syndrome (see this term), considered to be a variant of MA, presents with weakness in the intrinsic hand muscles. It begins unilaterally but eventually spreads to the opposite limb with an asymmetrical distribution.
The exact etiology is unknown. One theory is that MA is caused by the anterior shift of the cervical dural sac, due to repeated neck flexion, leading to the compression of the anterior aspect of the spinal cord against the posterior margin of the vertebral body. Autoimmunity, toxins and infections have been proposed as possible causes, but more evidence is needed. Since there have been several familial cases of MA reported (<1% of MA patients), genetic susceptibility could also be a factor.
Diagnosis is based on clinical imaging and electromyography findings. Magnetic resonance imaging (MRI) shows distinctive images of the anterior horn being compressed when a patient is in the maximum cervical anteflexion position (snake eyes sign). Atrophy in the lower cervical spinal cord is also visible. Nerve conduction tests show reduced compound muscle action potential of the median and ulnar nerves in the affected limb and F-wave studies show reduced frequency and prolonged minimum latency in these muscles.
Poliomyelitis, multifocal motor neuropathy with conduction block, syringomyelia (see these terms), anterior interosseous or deep ulnar neuropathy, cervical vertebral abnormalities, spinal cord tumors, brachial plexopathy and trauma must be excluded.
Management and treatment
There is no cure for MA. Treatment is conservative and with early detection aims to slow the progression of muscle wasting. A cervical collar worn in the early stages of disease has been shown to halt progress of the disease in some cases, as it prevents neck flexion. Muscle strengthening exercises and hand coordination training can also be helpful.
MA is not life threatening but can cause a social disability in those with a complete loss of hand function. Early intervention can minimize progression of disease.