Muscle Biopsy & Reaction Patterns

Muscle biopsy remains one of the most informative tests in neuromuscular medicine. While many diagnoses are now made non-invasively with genetics, antibodies, or EMG, muscle biopsy is essential when the clinical picture is ambiguous, when a specific dystrophy panel is negative, when inflammatory myopathy is suspected, or when a previously unrecognized entity needs characterization. The biopsy provides information about reaction patterns (denervation, dystrophy, inflammation, metabolic), specific structural abnormalities, and molecular alterations through immunohistochemistry. This page covers the techniques of muscle biopsy, normal histology, and the major reaction patterns.

Muscle Biopsy Techniques

Sample Acquisition

• Open biopsy: traditional; muscle wedge ~1 × 1 × 2 cm.
• Conchotome or percutaneous biopsy: less invasive, smaller sample.
• Select a muscle with mild-to-moderate weakness (severely affected muscles have end-stage changes only).
• Avoid recently EMG-tested muscles (artifacts).
• Common sites: quadriceps (vastus lateralis), biceps, deltoid.

Specimen Processing

• Frozen sections (most important for histochemistry and IHC): snap-frozen in isopentane cooled to -160°C (using liquid nitrogen).
• Fixed sections: formalin or glutaraldehyde for EM and standard histology.
• EM: small piece in glutaraldehyde for ultrastructure.

Standard Stains

• H&E: overall architecture, fiber size, inflammation, necrosis.
• Gomori (modified) trichrome: fiber type, connective tissue, ragged-red fibers.
• NADH-tetrazolium reductase (NADH-TR): mitochondrial pattern; target fibers; central cores.
• Cytochrome c oxidase (COX): mitochondrial; COX-negative fibers in mitochondrial disease.
• Succinate dehydrogenase (SDH): mitochondrial.
• ATPase reactions: at various pH; fiber type identification.
• PAS: glycogen.
• Oil Red O: lipid.
• Acid phosphatase: lysosomes.

Immunohistochemistry

• Dystrophin (rod domain, N-terminus, C-terminus): Duchenne/Becker dystrophy.
• Sarcoglycans (α, β, γ, δ): limb-girdle dystrophy subtypes.
• Dysferlin: dysferlinopathy.
• Caveolin-3: caveolinopathy.
• Merosin (laminin α2): congenital muscular dystrophy.
• Emerin: Emery-Dreifuss dystrophy.
• Calpain-3, dystroglycan, etc.
• MHC class I (sarcolemmal expression): inflammatory myopathy.
• CD3, CD8, CD4, CD20, CD68: inflammatory cells.
• p62, LC3, TDP-43: inclusion body myositis.

Normal Muscle Histology

• Fibers: polygonal in cross-section; relatively uniform size (40-80 μm diameter in adults).
• Nuclei: peripheral, beneath sarcolemma; central nuclei rare in adult.
• Endomysium: thin connective tissue between fibers.
• Perimysium: thicker connective tissue around fascicles.
• Epimysium: surrounding entire muscle.
• Fiber types:
  • Type 1 (slow, oxidative): rich in mitochondria; postural muscles.
  • Type 2A (fast, oxidative-glycolytic).
  • Type 2B (fast, glycolytic).
  • Identified by ATPase reactions or myosin heavy chain IHC.
  • Normally checkerboard pattern of fiber types.

Major Reaction Patterns

Neurogenic / Denervation

• Small angular atrophic fibers (often in clusters from a single denervated motor unit).
• Fiber type grouping: when reinnervation occurs, the new motor unit imposes its fiber type → groups of same-type fibers replace normal checkerboard.
• Target fibers (on NADH): central pale zone surrounded by dark rim; characteristic of denervation.
• Pyknotic nuclear clumps in severely atrophic fibers.
• Causes: ALS, SMA, peripheral neuropathy, radiculopathy.

Dystrophic Pattern

• Variability in fiber size: small atrophic + hypertrophic fibers.
• Increased endomysial fibrosis.
• Fatty infiltration: in advanced disease.
• Fiber necrosis (round, fragmented fibers with pale cytoplasm) and regeneration (small basophilic fibers with central nuclei).
• Increased central nuclei (regenerative + ongoing).
• Hyalinized “opaque” fibers: in Duchenne dystrophy especially.
• Causes: muscular dystrophies (Duchenne, Becker, LGMD, FSHD, OPMD, etc.).

Inflammatory Pattern

• Lymphocytic infiltration: perimysial, endomysial, perivascular, or perifascicular.
• Predominantly CD8+ T cells in polymyositis and inclusion body myositis.
• CD4+ T cells and macrophages in dermatomyositis.
• MHC class I sarcolemmal expression: characteristic of inflammatory myopathy (normally absent on mature fibers).
• Necrotic fibers and macrophage invasion of non-necrotic fibers (polymyositis).
• Perifascicular atrophy in dermatomyositis.
• Rimmed vacuoles + 15-18 nm filaments + p62/TDP-43 inclusions in inclusion body myositis.

Mitochondrial Pattern

• Ragged-red fibers: on Gomori trichrome; subsarcolemmal red accumulations of mitochondria.
• COX-negative fibers: absence of cytochrome c oxidase activity.
• “Blue ragged fibers”: COX-negative + SDH-positive blue staining.
• Abnormal mitochondria on EM: paracrystalline inclusions, hyperplasia.
• Causes: mitochondrial DNA mutations, nuclear DNA mutations affecting mitochondria, mtDNA depletion syndromes.

Metabolic Storage

• Glycogen accumulation (PAS-positive): glycogenoses (Pompe, McArdle, etc.).
• Lipid accumulation (Oil Red O): carnitine deficiency, multiple acyl-CoA dehydrogenase deficiency.
• Acid maltase deficiency (Pompe): vacuolar myopathy with PAS-positive content; lysosomal storage.

Congenital Myopathy Patterns

• Central cores: areas devoid of mitochondria on NADH; central core disease.
• Minicores: smaller versions; multi-minicore disease.
• Nemaline bodies (rods): protein aggregates; nemaline myopathy.
• Centronuclear pattern: central nuclei; centronuclear myopathies.
• Fiber type disproportion: type 1 fibers small.

Toxic / Drug-Induced Patterns

• Statin myopathy: necrosis + regeneration; sometimes autoimmune (anti-HMGCR antibody).
• Steroid myopathy: type 2 atrophy.
• Chloroquine / hydroxychloroquine: vacuolar myopathy.
• Colchicine: vacuolar myopathy with autophagic features.
• Alcohol: acute necrotizing + chronic atrophic.

Summary of Pattern Recognition

🔍 Did You Know?

The ragged-red fiber on modified Gomori trichrome stain is one of the most recognizable histologic features in muscle pathology and one of the most clinically informative. The “ragged” appearance reflects the irregular subsarcolemmal accumulation of structurally abnormal mitochondria — the cell’s response to mitochondrial dysfunction by attempting to proliferate functional mitochondria. The red color comes from the trichrome staining of these mitochondria. The recognition of ragged-red fibers usually points to mitochondrial myopathy and prompts further evaluation: combined COX/SDH histochemistry (looking for COX-negative fibers), electron microscopy (looking for paracrystalline inclusions and other ultrastructural abnormalities), and mitochondrial DNA sequencing or genome-wide sequencing. The clinical syndromes that produce ragged-red fibers include MELAS (encephalopathy + stroke-like episodes), MERRF (myoclonic epilepsy with ragged-red fibers), Kearns-Sayre syndrome (chronic progressive external ophthalmoplegia + pigmentary retinopathy + cardiomyopathy), and others. The lesson: a single histologic finding can transform the diagnostic approach to a patient with muscle weakness, identifying mitochondrial disease that would otherwise remain undiagnosed. The story illustrates the enduring value of careful muscle biopsy in the molecular era — sometimes the pattern is the most efficient way to reach the diagnosis.

Pitfalls and Pearls

• Select a moderately affected muscle: severely affected end-stage muscle is uninformative.
• Avoid recently EMG-tested muscle: artifacts.
• Frozen sections for histochemistry and IHC: essential.
• Neurogenic pattern: angular atrophic + fiber type grouping + target fibers.
• Dystrophic pattern: variable size + necrosis + regeneration + fibrosis.
• Inflammatory pattern: lymphocytic infiltrate + MHC-I sarcolemmal expression.
• Ragged-red fibers: mitochondrial myopathy.
• COX-negative fibers + SDH-positive (blue ragged): mitochondrial DNA mutations.
• Central cores on NADH: central core disease.
• Nemaline rods: nemaline myopathy.
• Rimmed vacuoles + p62/TDP-43: inclusion body myositis.
• PAS-positive glycogen: glycogenoses (Pompe, McArdle).
• Oil Red O lipid: lipid storage myopathies.
• Dystrophin IHC: Duchenne (absent) vs Becker (reduced/abnormal).
• Statin myopathy: necrosis + regeneration; consider anti-HMGCR antibody.
• Steroid myopathy: type 2 atrophy without inflammation.

References

1. Dubowitz V, Sewry CA, Oldfors A. Muscle Biopsy: A Practical Approach. 5th ed. Elsevier; 2020.
2. Engel AG, Franzini-Armstrong C, eds. Myology. 3rd ed. McGraw-Hill; 2004.
3. Karpati G, Hilton-Jones D, Bushby K, Griggs RC, eds. Disorders of Voluntary Muscle. 8th ed. Cambridge University Press; 2010.
4. Bohlega S, Tan KL, Salih MA. Recent advances in the molecular genetics of pure motor neuropathies. Curr Opin Neurol. 2013;26(5):514-520.
5. Love S, Budka H, Ironside JW, Perry A, eds. Greenfield’s Neuropathology. 9th ed. CRC Press; 2015.