Which histologic findings are characteristic of spinal muscular atrophy (SMA)?

Updated: May 29, 2019
  • Author: Jeffrey Rosenfeld, MD, PhD, FAAN; Chief Editor: Stephen L Nelson, Jr, MD, PhD, FAACPDM, FAAN, FAAP  more...
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Histologic findings depend on the stage and progression of disease. Initial changes include atrophy of muscle fibers with compensatory hypertrophy. This results in groups of large and small fibers (fiber-type grouping).

During the first 6-8 weeks of life, differentiating congenital fiber type disproportion and SMA may be difficult. In the chronic forms of SMA, secondary myopathic changes may be seen in addition to type grouping and may histologically resemble the muscular dystrophies. [43, 44]

Classic histologic findings include the following:

  • Degeneration and loss of spinal motor neurons with a neurogenic pattern of muscle morphology

  • Occasional neuronal chromatolysis with loss of myelinated axons in both anterior and posterior roots

  • A disproportionate loss of myelin in the thoracic and lumbar segments (especially in the corticospinal tracts) with relative sparing of the cervical cord

  • Motor neurons in the brainstem, notably in the hypoglossal nucleus. (Reactive gliosis and secondary degeneration in roots and nerves are seen. However, these findings are not necessarily pathognomic for the SMAs.)

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