This 66-year-old patient presented with complaints of asymmetric distal lower extremity weakness. Their past medical history was significant for atrial fibrillation, coronary artery disease, hyperlipidemia, hypertension, ischemic cardiomyopathy, prior myocardial infarction, and type 2 diabetes. Laboratory studies showed mild elevation of CPK. On physical examination they were noted to have loss of sensation in a stocking-like distribution.
Based on figures #1 – #5 what is your diagnosis?
A. Polymyositis
B. Type 2 atrophy
C. Vasculitis
D. Denervation
These low and higher magnification images show groups of atrophic muscle fibers and groups of relatively preserved to mildly hypertrophic muscle fibers (i.e. grouped atrophy and hypertrophy).
There is a mild increase in the number of muscle fibers with internalized nuclei. Notice the few markedly atrophic muscle fibers (arrows) reduced to clumps of nuclei (so-called “bag fibers” or “nuclear bags”).
Many of the atrophic muscle fibers show diffuse moderate to marked increase in sarcoplasmic staining for esterase enzyme histochemical stain (i.e. the staining is no longer restricted to the neuromuscular junction).
Loss of the normal checkerboard pattern of Type 1 and Type 2 fiber distribution can be appreciated on Nicotinamide adenine dinucleotide dehydrogenase‐tetrazolium reductase (NADH-TR). However fiber typing is best evaluated with ATPase enzyme histochemical stains (pHs 9.4, 4.6, and 4.3) or immunohistochemical stains for myosin heavy chain isoforms (fast and slow). See next figure.
These lower magnification images show groups of Type 2 (dark on MHCf) and Type 1 (dark on MHCs) muscle fibers. A “group” is described by a single myofiber completely surrounded by like-staining myofibers. Fiber type grouping is called when groups of both Type 1 and Type 2 fibers are present. Fiber type grouping indicates prior denervation and subsequent successful reinnervation. It is the innervating motor neuron that determines the fiber type. Grouping is caused by repetitive cycles of denervation and reinnervation by a different motor neuron (i.e. motor neurons take over abandoned territory).
Answer: Denervation
The morphologic features seen in the figures are those of denervation (neurogenic atrophy). Denervation may occur in the setting of peripheral neuropathy, radiculopathy, and motor neuron disease.
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