Muscle Biopsy Pathology
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Direct answer
Histochemistry on snap-frozen cryostat sections — not routine paraffin — is what makes muscle biopsy informative: enzyme stains (myosin ATPase at varied pH, NADH-tetrazolium reductase, succinate dehydrogenase, cytochrome oxidase) map fibre types and mitochondrial function, while modified Gomori trichrome reveals ragged-red fibres. The first split is myopathic versus neurogenic: randomly scattered small round fibres with central nuclei, necrosis, regeneration and connective-tissue increase signal primary muscle disease, whereas angular atrophic fibres in groups, target fibres and fibre-type grouping signal denervation. Immunohistochemistry then names the dystrophinopathies and inflammatory myopathies — dystrophin absent in Duchenne, patchy in Becker and mosaic in carriers; perifascicular atrophy points to dermatomyositis; endomysial CD8-positive cells invading non-necrotic fibres to polymyositis.
What you must remember
- Fibre types: type 1 slow oxidative and type 2 fast glycolytic fibres identified on ATPase reactions at acid and alkaline pH; the normal checkerboard mosaic disappears when reinnervation groups fibres of one type.
- Myopathic checklist: variation in fibre size with small round fibres, internalised nuclei, segmental necrosis with basophilic regenerating fibres, and fat-fibrous replacement in advanced dystrophy.
- Neurogenic checklist: small angulated fibres in clusters, grouped atrophy, target fibres, and pyknotic nuclear clumps — chronic denervation fingerprints.
- Ragged-red fibres: subsarcolemmal red accumulations on Gomori trichrome, with COX-negative or SDH-bright fibres signalling mitochondrial disease.
- Rimmed vacuoles: basophilic-rimmed vacuoles with tubulofilamentous inclusions, characteristic of inclusion body myositis and some distal myopathies.
- Inflammatory patterns: dermatomyositis — perifascicular atrophy with perivascular inflammation and complement deposition on capillaries; polymyositis — endomysial CD8 cells invading otherwise healthy fibres.
- Dystrophin reading: absent in Duchenne muscular dystrophy, patchy and abnormal in Becker, and a mosaic in manifesting carriers — always confirmed genetically today.
Working through a boy with a waddling gait
A five-year-old with calf pseudohypertrophy, a positive Gower sign and a creatine kinase above 15,000 units per litre undergoes open biopsy of a mildly weak quadriceps — never the most wasted muscle, never a site recently needled for electromyography, because needle trauma reads as myopathy. Frozen sections show marked fibre-size variation, rounded fibres, scattered hyaline hypercontracted fibres and patchy necrosis with regeneration; on immunohistochemistry, dystrophin is absent with the rod-domain antibodies while beta-spectrin preserves membrane integrity as the internal control. That pattern is Duchenne muscular dystrophy, and genetic testing then identifies the frame-shifting deletion, closing the diagnostic loop for family counselling and future carrier detection. The same tissue, read for inflammation instead, would have gone down the myositis pathway — the stain you order is the diagnosis you get.
Where candidates slip
Transport dominates the errors: muscle must reach the laboratory fresh on ice within about an hour for snap-freezing in isopentane cooled by liquid nitrogen, because formalin fixation abolishes enzyme histochemistry and thereby the entire diagnostic superstructure. Type 2 fibre atrophy is misread as denervation; it is the non-specific signature of disuse, corticosteroid exposure and cachexia, and it is diffuse rather than grouped. Steroid myopathy coexisting with dermatomyositis fools the unwary — a treatment-related type 2-predominant atrophy superimposed on perifascicular disease. Finally, polymyositis is overdiagnosed histologically; many legacy cases were reclassified as inclusion body myositis or muscular dystrophy once rimmed vacuoles and protein-immunostains entered routine use.
Frequently asked questions
Why must muscle be snap-frozen rather than fixed in formalin?
Enzyme histochemistry — ATPase, NADH-TR, SDH, cytochrome oxidase — depends on labile enzyme activity that fixation destroys, and lipid and glycogen stains likewise demand frozen sections.
How does grouped atrophy differ from myopathic fibre-size variation?
Grouped atrophy clusters angulated small fibres of one type after reinnervation, whereas myopathic atrophy scatters small round fibres randomly among preserved ones.
What is a ragged-red fibre?
A fibre with subsarcolemmal red accumulation on modified Gomori trichrome, indicating surplus abnormal mitochondria, often COX-negative on enzyme histochemistry.
Which histological pattern favours dermatomyositis over polymyositis?
Perifascicular atrophy with perivascular inflammation and capillary complement deposition, rather than endomysial CD8-positive cells invading non-necrotic fibres.
What does dystrophin immunohistochemistry show in Duchenne versus Becker dystrophy?
Complete absence of sarcolemmal staining in Duchenne, and patchy, weak or variably interrupted staining in Becker, with manifesting carriers showing a mosaic pattern.
What are rimmed vacuoles and which myositis loves them?
Vacuoles with basophilic rims containing tubulofilamentous inclusions, the hallmark of inclusion body myositis, which also shows endomysial inflammation and COX-negative fibres.