Muscle and nerve biopsies are essential for diagnosing neuromuscular diseases. Enzyme histochemistry on frozen sections provides functional information about muscle fiber types, mitochondrial integrity, and lysosomal activity that cannot be obtained from routine histology or IHC alone.
Muscle Biopsy Processing
A muscle biopsy (typically vastus lateralis, deltoid, or biceps brachii) must be handled specially. The specimen is oriented under a dissecting microscope to identify the fiber direction, mounted on a cork disc in OCT compound, and snap-frozen in isopentane cooled to -160°C in liquid nitrogen. Frozen sections are cut at 8-10 µm in a cryostat at -25°C. A standard muscle biopsy panel includes: H&E, modified Gomori trichrome, ATPase at pH 9.4, 4.6, and 4.3, NADH-TR, COX, SDH, and acid phosphatase.
ATPase and Fiber Typing
Myosin ATPase histochemistry at different pH levels differentiates muscle fiber types based on the pH lability of myosin heavy chain isoforms. At pH 9.4, type I fibers are light, type II fibers are dark. At pH 4.6, type I fibers are dark, type IIA fibers are light, type IIB fibers are intermediate. At pH 4.3, type I fibers are dark, type II fibers are light.
Normal muscle shows a checkerboard pattern of type I and type II fibers with random distribution. Type I fiber predominance suggests congenital myopathy or chronic denervation. Type II fiber atrophy is the most common abnormality — seen in disuse, steroid myopathy, cachexia, and paraneoplastic syndromes. Fiber type grouping (groups of >15 fibers of the same type replacing the checkerboard pattern) indicates denervation-reinnervation (neurogenic disease). Congenital fiber type disproportion shows uniformly small type I fibers with normal type II fibers.
Mitochondrial Enzyme Histochemistry
NADH-TR demonstrates mitochondrial distribution and oxidative enzyme activity. Type I fibers are dark; type II fibers are light. Abnormal NADH-TR patterns include: central cores (central or eccentric rounded areas devoid of staining — central core disease); target/targetoid fibers (three zones: dark periphery, pale intermediate ring, dark center — denervation); ragged red fibers (subsarcolemmal mitochondrial accumulations appearing as dark peripheral aggregates — mitochondrial myopathy); moth-eaten fibers (irregular, patchy loss of staining — myopathy, denervation).
COX (cytochrome c oxidase) — absent or reduced activity in COX-negative fibers indicates mitochondrial DNA (mtDNA) mutation or depletion. COX-negative fibers increase with normal aging but excessive numbers indicate mitochondrial disease. SDH (succinate dehydrogenase) — encoded entirely by nuclear DNA; preserved SDH in COX-negative fibers (“COX-negative, SDH-positive”) is the diagnostic hallmark of mtDNA-related mitochondrial myopathy.
Lysosomal Enzyme Histochemistry
Acid phosphatase — increased lysosomal activity (bright red punctate staining) indicates muscle fiber necrosis and regeneration. Acid phosphatase-positive fibers are seen in inflammatory myopathies, muscular dystrophies, and toxic myopathies. Non-specific esterase — increased activity in small angular fibers in denervation. NSE also highlights motor endplates and can identify acetylcholinesterase at the neuromuscular junction.
Modified Gomori Trichrome
The modified Gomori trichrome stain is a standard component of the muscle biopsy panel. Ragged red fibers (RRF) appear as fibers with irregular peripheral red granular accumulations — representing subsarcolemmal mitochondrial proliferation and the morphological hallmark of mitochondrial myopathy. Nemaline rods appear as red, rod-like structures in the sarcoplasm (nemaline myopathy). Cytoplasmic bodies — red, spherical inclusions in myofibrillar myopathies. Rimmed vacuoles — vacuoles with basophilic granular rims in inclusion body myositis.
Nerve Biopsy
Nerve biopsy (typically sural nerve) is processed for paraffin sections (H&E, LFB-PAS, Congo Red), frozen sections (enzyme histochemistry), teased fiber preparations, and electron microscopy. Teased fiber preparations — individual nerve fibers separated and mounted on slides — allow assessment of axonal degeneration and segmental demyelination. Luxol Fast Blue stains myelin blue. Semithin sections (0.5-1.0 µm) from resin-embedded nerve provide superior resolution for assessing axonal loss, demyelination, and inflammatory infiltrates.
Ultrastructural Correlation
Many enzyme histochemical findings require correlation with electron microscopy. Ragged red fibers on Gomori trichrome correspond to mitochondrial aggregates with paracrystalline inclusions on EM. Central cores correlate with core regions lacking mitochondria and sarcomeres. Specific granule types (nemaline rods, cytoplasmic bodies, reducing bodies) are identified definitively by EM. The combination of enzyme histochemistry and EM provides comprehensive characterization of muscle pathology. Quality assurance programs for neuromuscular pathology include EQA for histochemical techniques and diagnostic concordance.
