David Lacomis, MD Acquired Diseases of Muscle: Histologic Features

Organization of Skeletal Muscle Including Connective Tissue (CT) Compartments EPIMYSIUM Loose CT Blood vessels PERIMYSIUM Septa Nerve branches Muscle spindles Fat Blood vessels ENDOMYSIUM Muscle fibers Capillaries Small nerve fibers

Perimysial connective tissue Endomysial connective tissue Normal H&E-stained frozen cross-section of skeletal muscle  Note uniform sizes, polygonal shapes, and eccentric nuclei.

Normal H&E-stained longitudinal paraffin section  Note the banding pattern.  Nuclei are eccentrically placed.

Spindle Nerve Twig Normal Structures: Muscle Spindle and Associated Nerve Fibers Gomori trichrome

 Can be identified by the esterase reaction due to the presence of acetylcholinesterase. Neuromuscular Junctions

Neuromuscular Junction Electron Microscopy postsynaptic presynaptic

Histochemical Staining Intensity Based on Fiber Types Type I Slow twitch, oxidative; stain dark with Gomori trichrome, NADH, SDH, and ATPase at acidic pH; more lipid than type II NADH= nicotinamide adenine dehydrogenase SDH= succinic dehydrogenase ATPase= adenosine triphosphatase Type IIB Intermediate staining intensity with ATPase pH4.6 Type II Fast twitch, glycolytic; stain dark with ATPase at alkaline pH and with PAS stains, as well as phosphorylase

 Type I fibers are light  Type II fibers are dark Normal ATPase pH 9.4

Ultrastructure of a Sarcomere  Extends from Z-band to Z-band.  Note arrangement of thick and thin filaments. ZZ M H band Actin Myosin I band A band  A band includes overlap of actin & myosin.

 Dark A- bands  Light I- bands  Z-band is present in the middle of the light band  Thin filaments are attached at the Z- band Normal electron microscopy

Classification of Myopathies ACQUIREDINHERITED Inflammatory Myopathies Dystrophies  Polymositis (PM)  Dystrophinopathies  Dermatomyositis (DM)  Limb-Girdle  Inclusion body myositis (IBM)  Myotonic  Granulomatous myositis  Facioscapulohumeral (FSHD)  Infectious myositis  Oculopharyngeal (OPD) Toxic  Distal EndocrineCongenital Metabolic  Mitochondrial  Glycogen & lipid storage

Polymyositis Longitudinal paraffin-embedded section  Mononuclear inflammatory cell infiltrates and many basophilic regenerating fibers

Polymyositis Longitudinal paraffin-embedded section (higher power)  Regenerating fiber (non-specific)  Fiber is basophilic due to presence of increased RNA and DNA.  Activated plump nuclei and prominent nucleoli

 As regeneration advances, a myotube “bridge” is formed. Polymyositis Longitudinal paraffin-embedded section (higher power)

Myophagocytosis Esterase stain  Macrophages are ingesting the remnants of a degenerating fiber. This is a non-specific myopathic finding.

Invasion of a Non-necrotic Fiber by Inflammatory Cells

 Mononuclear cells surround a non-necrotic fiber that abnormally expresses MHC-1.  Seen in polymyositis and inclusion body myositis as well as dystrophies (rarely). MHC-1

CD8  Inflammatory infiltrate in polymyositis is endomysial predominantly of the cytotoxic T-cell type.

Dermatomyositis  Perifascicular atrophy  Degeneration  Inflammatory cells in the perimysium surrounding a blood vessel  Inflammatory cells tend to be B-cells.

DermatomyositisATPase  Perifasicular atrophy and patchy staining ?? # of ATPase ??

 The perifascicular fibers may have an abnormal purplish appearance with Gomori trichrome.

Perifascicular Atrophy NADH-reacted section

Dermatomyositis

B-cell

Dermatomyositis CD4

Dermatomyositis CD8

Dermatomyositis Inflammatory Infiltrate in Skin

 MAC is the terminal component of the complement pathway.  It is often deposited in capillaries in dermatomyositis. Membrane Attack Complex (MAC) Immunohistochemical stain

 Increased staining in capillaries in patients with dermatomyositis  Degenerating fibers may also stain.

Dermatomyositis Electron microscopy  Tubuloreticular inclusion in a capillary endothelial cell

Invaded fiber  Features of chronic myopathy with endomysial inflammation and rimmed vacuoles are characteristic. Inclusion Body Myositis (IBM)

Lymphocytic inflammation “Rimmed vacuoles”

 Rimmed vacuoles may be “slit-like”

 IBM: Vacuoles contain amyloid. Congo Red

IBM: Vacuoles

 Vacuoles are difficult to identify in paraffin sections, but they may be highlighted by immunohistochemistry against the heat shock protein Ubiquitin.

IBM Eosinophilic Inclusion (Cytoid Body) Electron microscopy

IBM Intracytoplasmic (Within Vacuoles) or Intranuclear Filamentous Inclusions

Pyomyositis Gram Positive Cocci

Granulomatous Myositis in a Patient with Sarciodosis  Granulomas tend not to cause significant damage to adjacent myofibers. Giant cell See picture Granuloma 1

Parasites: Trichinella spiralis

 Characteristic of most Endocrine Disturbance Type II Fiber Atrophy ATPase pH9.4

Inherited Polyneuropathy Chronic Neurogenic Atrophy  Groups of angulated atrophic fibers  Marked variation in myofiber size

Acute Denervation NADH reaction  Manifested by small, darkly staining angulated fibers

 Denervated fibers also stain darkly with non-specific esterase. Denervation Esterase Stain

 Target fibers noted.  Light center surrounded by a darker rim.  Generally only seen in type I fibers. Chronic Neurogenic Processes NADH reaction

 Fiber type grouping Chronic Neurogenic Atrophy ATPase reaction

Werdnig-Hoffman Disease (Spinal Muscular Atrophy Type I)

 Denervated fibers are atrophic but round.  Interspersed hypertrophic round fibers are usually noted. Werdnig-Hoffman Disease (Spinal Muscular Atrophy Type I)