Myofiber Injury

Clinical History

The 50-year-old patient presented with proximal upper and lower extremity muscle weakness of several months duration. They also related shortness of breath and arthralgias/joint pain. Laboratory studies showed elevated CPK (11000 range), aldolase (~75) and ESR, negative/normal CRP, dsDNA, Smith, Scl-70, Jo-1, CCP, SSA and SSB, U1-RNP, RNP70, Ro52, Ro60, RNA Pol III, centromere B, HMGCR and myositis specific autoantibody panel.  The patient was treated with prednisone prior to muscle biopsy and reported some improvement in their symptoms.  The patient was not being treated with a statin medication.

The patient’s muscle biopsy showed marked myopathic changes without significant associated chronic lymphoid inflammation. Which fiber in Figure #1 matches the myofiber indicated in Figure #2?

A.1

B.2

C.3

D. None

Correct answer: C. 3

The necrotic myofiber marked “3” which containing multiple macrophages matches the muscle fiber in brackets in Figure #2.

Figure 1: hematoxylin and eosin stain frozen section 400x original magnification

This high magnification images shows several normal appearing myofibers (for example “1”), two basophilic regenerating myofibers (for example “2”) and a necrotic muscle fiber (number “3”) which contains multiple macrophages (so-called macrophagocytosis).

Figure 2: Toluidine blue stained thick section (600x original magnification)

This high magnification images shows several normal appearing myofibers and a necrotic muscle fiber which contains multiple macrophages (two examples indicated by arrows).

Note the smudgy appearance of the sarcoplasm in the necrotic myofiber and how it contrasts with the intact sarcomeres seen in the adjacent myofiber (*) which give skeletal muscle its “striated” appearance.