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

Updated: Nov 05, 2018
  • Author: Ashish S Ranade, MBBS, MS, MRCS; Chief Editor: Jeffrey A Goldstein, MD  more...
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Answer

Two subtypes of SMA deserve special mention with regard to their typical histologic appearance. The first, Werdnig-Hoffmann disease (type I SMA), is typically diagnosed in patients from birth to age 6 months. Its histologic pattern is usually one of extremely small and reasonably uniform small muscle fibers (see the images below).

Spinal muscle atrophy, Werdnig-Hoffman disease. Sm Spinal muscle atrophy, Werdnig-Hoffman disease. Small muscle fibers within separate muscle fascicles.
Spinal muscle atrophy, Werdnig-Hoffman disease. Ma Spinal muscle atrophy, Werdnig-Hoffman disease. Marked variation in muscle fiber size, as well as relative increase in associated connective tissue.

The second subtype, Kugelberg-Welander disease (type III SMA), is usually diagnosed in patients aged 2-15 ears. The same tendency toward small muscle fiber diameter is seen but with much less uniformity (see the images below). Substantial variation, with intermixing of larger and smaller muscle fibers, may be observed.

Spinal muscle atrophy, Kugelberg-Welander disease. Spinal muscle atrophy, Kugelberg-Welander disease. Marked variation in muscle fiber size, along with increased perimysial connective tissue.
Spinal muscle atrophy, Kugelberg-Welander disease. Spinal muscle atrophy, Kugelberg-Welander disease. Muscle-fiber variation with some demonstrating internal nuclei.

In both forms of SMA, substantial increases in muscular connective tissue lead to both characteristic histologic findings and clinical findings such as increased muscle firmness. Centrally migrated or otherwise internalized nuclei are considered pathologic if they are present in more than about 3% of muscle fibers. Such nuclear findings are common in a variety of muscle diseases, including SMA.


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