David Lacomis, MD

Diseases of Muscle:

Histopathologic 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

Perimysialconnective tissue

Endomysialconnective 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 Spindleand 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 Type II Type IIB

Slow twitch, oxidative; stain dark with Gomori trichrome, NADH, SDH, and ATPase at acidic pH; more lipid than type II

Fast twitch, glycolytic; stain dark with ATPase at alkaline pH and with PAS stains, as well as phosphorylase

Intermediate staining intensity with ATPase pH4.6

NADH = nicotinamide adenine dinucleotideSDH = succinic dehydrogenaseATPase = adenosine triphosphatase

Type I fibers are light Type II fibers are dark (pattern reverses at ATPase pH 4.3)

Normal (ATPase pH 9.4)

Ultrastructure of a Sarcomere*

*Extends from Z-band to Z-band. A band includes overlap of actin and myosin. Note arrangement of thick and thin filaments.

Z ZM

H band

ActinMyosin I bandI band

A band

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

ACQUIRED INHERITED

Inflammatory Myopathies Dystrophies

Polymyositis (PM) Dystrophinopathies

Dermatomyositis (DM) Limb-Girdle

Inclusion body myositis (IBM) Myotonic

Granulomatous myositis Facioscapulohumeral (FSHD)

Infectious myositis Oculopharyngeal (OPD)

Toxic Distal

Endocrine Congenital

Metabolic

Mitochondrial

Glycogen & lipid storage

Muscle Biopsy

Often necessary for final diagnosis of myopathy Choose site based on clinical, electrodiagnostic, or

imaging features Avoid “end-stage” fatty muscle

Frozen sections most useful Routine stains Histochemistry Immunohistochemistry

Polymyositis(Longitudinal Paraffin-Embedded Section)

In all myopathies, degenerating fibers stain pale initially and then become digested by macrophages.

Mononuclear inflammatory cell infiltrates and many basophilic regenerating fibers (arrow)

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)

Invasion of a Non-necrotic Fiber by Inflammatory Cells

Seen in polymyositis, inclusion body myositis, and a few dystrophies.

Myophagocytosis(Esterase Stain)

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

Dermatomyositis

Perifascicular atrophy & Degeneration Perimysial nflammatory cells surround a blood vessel. Inflammatory cells tend to be B-cells. Vasculitis with bowel infarction and subcutaneous

calcifications sometimes occur in the childhood form.

Perifascicular Atrophy(NADH-Reacted Section)

MAC is the terminal component of the complement pathway. It is often deposited in capillaries in dermatomyositis.

Membrane Attack Complex (MAC)(Immunohistochemical Stain)

Features of chronic myopathy with endomysial inflammation and rimmed vacuoles are characteristic.

Inclusion Body Myositis (IBM)

Vacuole

Invaded fiber

Lymphocytic inflammation

“Rimmed vacuoles”

IBM: Vacuoles contain amyloid.

(Congo Red)

IBM Intracytoplasmic (within Vacuoles) or Intranuclear Filamentous Inclusions

Giant cell

Granulomas tend not to cause significant damage to adjacent myofibers.

Granulomatous Myositisin a Patient with Sarcoidosis

Characteristic of most endocrine myopathies and steroid myopathy

Endocrine Disturbance Type II Fiber Atrophy(ATPase pH9.4)

Inherited PolyneuropathyChronic 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)

Opaque or hyaline fibers (arrows) Increase in endomysial connective tissue

Frozen Section from a Patient withDuchenne Muscular Dystrophy

Group of basophilic regenerating fibers

Normal Immunohistochemical Stain for Dystrophin(Subsarcolemmal Staining)

Duchenne Muscular Dystrophy (Absent Staining for Dystrophin)

split fiber(non-specific chronic change)

Becker Muscular Dystrophy (Reduced but Present Staining)

Female Carrier of Duchenne Muscular Dystrophy

(A Mosaic Staining Pattern)

INHERITANCE GENETICABNORMALITY

DISORDER

X-linked DystrophinEmerin

Duchenne, Becker MDEmery-Dreifuss MD

AD MyotilinLamin A/CCaveolin – 3PABP2-crystallin/Desmin

Limb-Girdle MD (LGMD 1A)LGMD 1BLGMD 1COculopharyngealMyofibrillar Myopathy

AR Calpain – 3Dysferlin Sarcoglycana Sarcoglycan SarcoglycanΔ SarcoglycanTelethonin

LGMD 2ALGMD 2BLGMD 2CLGMD 2DLGMD 2ELGMD 2FLGMD 2G

Mutations in “Limb-Girdle” and Other Dystrophies

Sarcolemma

nucleus

Lamin A/C(emerin)

sarcoglycans

Dystroglycancomplex

Laminin-2

Extracellular Matrix

Dysferlin

Caveolin 3

Actin

Dystrophin

Locations of Affected Proteinsin Muscular Dystrophies

Emery-Dreifuss Muscular Dystrophy(Gomori Trichrome-Stained Frozen Section)

Necrotic fiber

Variation in fiber size with many hypertrophic fibers Increase in endomysial connective tissue Nonspecific so-called dystrophic changes seen in many of

the muscular dystrophies. Can also be seen in any chronic myopathic disorder. This disorder is due to loss of the protein emerin.

Myotonic Dystrophy

Chronic changes Marked excess in internalized nuclei Variation in fiber sizes Nuclear clumps (not shown)

(H & E, Paraffin)

The excess of internalized nuclei can lead to nuclear chains.

Myotonic Dystrophy(NADH-Reacted Section)

Ring fibers in which myofilaments are organized in different directions

Fascioscapulohumeral Dystrophy (FSHD)

The majority of dystrophies do not have a specific histopathologic appearance.

Clinical features are also very important. For example, winging of the scapula is

characteristic of FSHD.

FSH Dystrophy

Variable non-specific changes Range from scattered atrophy to

“dystrophic” features. Inflammation can be present (arrow).

Central areas of absent staining in the dark type I fibers Mitochondria absent

Congenital Myopathies: Central Core Myopathy(NADH)

Congenital Myopathies: Central Core Myopathy(NADH)

The core consists of disorganized myofibrils and the area is devoid of mitochondria.

Congenital Fiber Type Disproportion(H&E)

Bimodal size population

Smaller fibers are type I More numerous Stain lightly

Larger or normal fibers are type II

Congenital Fiber Type Disproportion(ATPase pH 4.3)

Eosinophilic inclusions present.

Nemaline Myopathy

Eosinophilic inclusions stain darkly.

Nemaline Myopathy(Gomori Trichrome)

Named for thread-like appearance Inclusions extend from Z-band to Z-band

Nemaline Myopathy(Electron Microscopy)

Muscle Biopsy from an Infant

Internalized nuclei predominant. Consistent with centronuclear myopathy. Can be seen in other disorders such as

myotonic dystrophy with congenital onset.

Muscle Biopsy from an Infant:Centronuclear Myopathy

Central position of the nucleus resembling an embryonic myotube

Metabolic: Inherited – Mitochondrial Myopathy

Ragged red fiber present (Gomori trichrome) Due to proliferation of abnormal mitochondria

SDH-rich fibers are seen with mitochondrial proliferation. SDH is a respiratory chain enzyme encoded by nuclear DNA.

Mitochondrial Myopathy(Succinic Dehydrogenase Reaction)

Cytochrome Oxidase (COX) Respiratory Chain EnzymeNormal Fibers

Many COX-Negative Fibers

COX-negative fibers are usually seen with mtDNA mutations.

Aggregates of mitochondria containing paracrystalline inclusions are frequent.

Non-specific

Mitochondrial Disorders(Electron Microscopy)

Mitochondrial Disorders(Electron Microscopy)

Higher power view of paracrystalline inclusion

Increased lipid storage Seen in carnitine deficiency states (primary or secondary) Sometimes as a consequence of certain toxins Focal increases can be non-specific.

(Oil-Red-O Stain)

Lipid Storage Myopathy(Electron Microscopy)

Some glycogen storage myopathies, such as myophosphorylase deficiency (McArdle’s Disease), cause subsarcolemmal blebs.

PAS-positive due to the presence of glycogen. Only with acid maltase deficiency is glycogen

deposited in lysomsomes.

Glycogen Storage Myopathies

Subsarcolemmal collection of glycogen is shown.

McArdle’s Disease(Electron Microscopy)

Acid Maltase Deficiency(Acid Phosphatase)

Due to the intralysosomal activity of this enzyme

Prominent staining with acid phosphatase in vacuoles

Vacuolar myopathy noted.

Normal Glycogen(PAS Stain) Control

Increased Glycogen

Acid maltase deficiency Increased glycogen (diffusely and in vacuoles)