What is monomelic amyotrophy?

Updated: Feb 20, 2018
  • Author: Sridharan Ramaratnam, MD, MBBS; Chief Editor: Helmi L Lutsep, MD  more...
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Answer

Monomelic amyotrophy

  • This disorder is defined in several reports as a benign disorder characterized by wasting that is confined to a single limb or part of a limb. [24]

  • Wasting of right forearm and both hand muscles in Wasting of right forearm and both hand muscles in a patient with Hirayama Disease. Note the oblique atrophy of right forearm.
  • Wasting of small muscles of the hands in a patient Wasting of small muscles of the hands in a patient with Hirayama Disease.
  • In India, Korea, and Japan, such occurrences are termed benign focal amyotrophy, Hirayama disease, wasted leg syndrome, or monomelic amyotrophy. Isolated reports of this disorder also have come from Brazil, France, Germany, Italy, Spain, Turkey, Poland, and Canada. [10, 25, 26, 11, 27, 28, 29]

  • The etiology is unknown. Degenerative, infectious, and ischemic mechanisms have been postulated. [30] A flexion myelopathy has been postulated on the basis of MRI studies, where the dura and spinal cord may be compressed repeatedly during neck flexion, possibly inducing an ischemic myelopathy. [31, 32] Venous congestion in the spinal canal as demonstrated in phase contrast magnetic resonance (MR) angiography may also have a role in promoting anterior horn damage. [33]

  • T2-weighted cervical spine MRI of a patient with H T2-weighted cervical spine MRI of a patient with Hirayama disease showing focal cord hyperintensity at C5-C6 level.
  • T2-weighted cervical spine MRI of the same patient T2-weighted cervical spine MRI of the same patient during neck flexion showing anterior displacement of the posterior dural wall with flattening and compression of the cord against the bodies of the vertebrae with prominent dorsal epidural flow voids.
  • Although some authors consider this disorder as a variant of SMA with a focal emphasis and a benign course, no deletions of the survival motor neuron gene (as are found in proximal SMAs) have been reported. [34] Monomelic amyotrophy was associated with the 7472 insC mutation in the mtDNA tRNA (Ser(UCN)) gene in one family with a correlation between mutation load and clinical severity. [35]

  • HyperIgEaemia is often associated with Hirayama disease, although a causal relationship has not been established. [36]

  • The term monomelic amyotrophy should be reserved only for cases of focal muscle wasting that is confined to a single limb without secondary causes.

  • The disorder is generally sporadic, involving young men aged 18-50 years.

  • Gradual in onset, the course may be nonprogressive or feature initial progression for 1-5 years followed by a plateau state. Spread to the other limb or limbs has occasionally been documented.

  • Recurrent forms and familial occurrences have been reported. [37]


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