JPC SYSTEMIC PATHOLOGY

MUSCULOSKELETAL SYSTEM

March 2025

M-M06 

 

Signalment (JPC #2984049): Two-year-old male Weimaraner, Canis familiaris 

 

HISTORY: This dog was thin with atrophy of all muscles except those of the neck and tongue. At necropsy, neck muscles were thick, giving a “buffalo hump” appearance. The diaphragmatic muscle surrounding the central tendon was pale and 1.5 cm thick. The body muscle mass was reduced and muscles were diffusely pale.

 

HISTOPATHOLOGIC DESCRIPTION: Skeletal muscle, diaphragm: The diaphragm is diffusely thickened up to five times normal (compared with the control tissue). Myofibers are diffusely disorganized, shortened, and vary significantly in size and orientation. Diffusely, myocytes undergo one of the following changes: degeneration, necrosis, rare regeneration, frequent hypertrophy, loss with replacement by fibrous connective tissue and occasional adipose tissue, and atrophy. Degenerate myocytes have swollen, vacuolated sarcoplasm with loss of cross striations; necrotic myocytes have shrunken, angular, hypereosinophilic, fragmented sarcoplasm with contraction bands, pyknotic nuclei, multifocal mineralization, and scattered infiltration by moderate numbers of macrophages; regenerative myocytes contain small amounts of basophilic sarcoplasm and numerous linearly arranged, internalized nuclei ("rowing") with prominent satellite cell nuclei at the cell margins; hypertrophic myocytes are up to five times wider than normal with abundant sarcoplasm and several internalized nuclei; and atrophic myofibers are shrunken and often surrounded by fibrous connective tissue. Multifocally the endomysial and perimysial connective tissue is expanded by moderate amounts of collagenous connective tissue and fibroblasts (fibrosis) admixed with few lymphocytes and plasma cells. There is multifocal infiltration of adipocytes.

 

Skeletal muscle, diaphragm, unaffected age-matched control: No significant lesions.

 

MORPHOLOGIC DIAGNOSIS: Skeletal muscle, diaphragm: Myocyte degeneration, necrosis, regeneration, hypertrophy, and loss, diffuse, severe, with fibrosis, mineralization, and fibrofatty replacement, weimaraner, (Canis familiaris), canine.

 

ETIOLOGIC DIAGNOSIS: Congenital muscular dystrophy 

 

CAUSE: X-linked dystrophin gene deficiency

 

GENERAL DISCUSSION:

• Muscular dystrophies are inherited progressive myopathies characterized histologically by ongoing muscle fiber necrosis and regeneration
• True muscular dystrophies occur in the dog, cat, and mouse; are similar to Duchenne and Becker muscular dystrophies in humans; animal models have similar protein and genetic defects to the diseases in humans, creating similar pathology, but the clinical course often differs
• Progressive muscular weakness can lead to respiratory and/or cardiac failure
• In late disease, muscle fibers are replaced by fibrosis and fat

 

PATHOGENESIS:

• Hereditary muscular dystrophy (X-linked recessive trait)
  • Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (milder form) are the most common forms of muscular dystrophy in humans and are associated with defects (absence or genetic mutations) in the dystrophin gene for the cytoskeletal protein dystrophin 
  • mdx mouse (C57BL/10ScSn mouse) and xmd dog are animal models for Duchenne muscular dystrophy
  • Dystrophin anchors cytoskeletal actin to extracellular laminin and is believed to maintain myocyte membrane integrity during contraction
  • Absence of dystrophin allows gaps to develop in the myocyte membrane; resulting in calcium influx and hypercontraction, degeneration, and necrosis

 

TYPICAL CLINICAL FINDINGS:

• mdx mice: Minimal muscle weakness; tremors and mild incoordination; high serum creatine kinase (CK)
• Canine X-linked muscular dystrophy (xmd) Golden Retrievers: Rapid progression between 3-6 months of age and disease may stabilize at 8-12 months; limb weakness; stiff, shuffling gait and muscular atrophy; inability to open jaws; death from cardiac or respiratory failure; clinical signs similar to DMD in man; markedly elevated serum creatine kinase (CK) and asparate aminotransferase (AST)
• Hypertrophic feline muscular dystrophy (HFMD): Dystrophin deficiency in cats manifests as muscle hypertrophy, tongue protrudes due to glossal hypertrophy, bunny-hopping in rear limbs, reduced activity; markedly elevated CK

 

TYPICAL GROSS FINDINGS:

• Dystrophin deficiency results in muscle atrophy in most dog breeds, but causes hypertrophy in the Rat Terrier breed and in mice and cats
• mdx mice: Externally normal, with no apparent muscle wasting
• Canine X-linked muscular dystrophy (xmd): Generalized muscular wasting; firm, thick, contracted muscles with fibrosis; splayed limbs; contracted joints; animals with fulminant neonatal disease have severe degeneration of diaphragm and other strap muscles (trapezius and sartorius muscles) with pale white streaks
• Feline X-linked muscular dystrophy: Thickening of the muscular wall of esophagus, diaphragm, tongue and skeletal muscle (neck and proximal limbs). 

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• DMD and other muscular dystrophies: Myofiber degeneration, regeneration, endomysial fibrosis; mineralization; internalized nuclei; variation in fiber diameter with marked hypertrophy of type-1 fibers (red, slow oxidative fibers); fiber splitting; phagocytosis of muscle fibers; hyalinized fibers with loss of cross striations (hypercontracted)
• Canine X-linked muscular dystrophy (xmd): 
  • Severely affected, dying pups have massive acute muscle fiber necrosis and mineralization in affected muscles including tongue
  • Numerous swollen and dark staining fibers (large dark fibers) is considered the earliest stage of muscle fiber degeneration; clusters of necrotic fibers (multifocal, polyphasic necrosis), regenerating fibers, mineralization
  • Older affected dogs have variation in muscle fiber size with internal nuclei, endomysial fibrosis, and fat infiltration
  • Cardiac muscle may have subepicardial necrosis, mineralization and fibrosis; left ventricular free wall and interventricular septum are most severely affected; may eventually result in congestive heart failure in surviving dogs
  • Immunohistochemistry for dystrophin (frozen sections) is usually negative in skeletal and cardiac muscle
• Feline X-linked muscular dystrophy: 
  • Marked variation in fiber size with numerous hypertrophied fibers often containing multiple internal nuclei
  • Scattered clusters of necrotic and regenerating myofibers with minimal endomysial fibrosis
  • Similar cardiac changes as dogs
• mdx mice: Balance of myofiber necrosis and regeneration (regeneration compensates for necrosis); lack of interstitial fibrosis; regenerated fibers with central nuclei

 

ULTRASTRUCTURAL FINDINGS:

• DMD and other muscular dystrophies: Mitochondrial and myelin body fibers in subsarcolemmal blebs; myofilament structures in disoriented arrays or swirls; swollen mitochondria and endoplasmic reticulum
• mdx mice: Disorganized Z bands, scattered hypercontracted fibers, empty sarcolemmal tubes

 

DIFFERENTIAL DIAGNOSIS:

• Dogs:
• Centronuclear myopathy (Labrador retrievers)
• X-linked myotubular myopathy (Labrador retrievers)
• Inherited myopathy (Great Danes)
• Myopathy of Bouvier des Flandres dogs
• Canine dermatomyositis (Shetland sheepdog, collie, Pembroke Welsh corgi)
• Juvenile-onset distal myopathy (Rottweiler)
• Exercise-induced collapse (Labrador retriever)
• Cats:
• α-dystroglycan deficiency (Sphynx and Devon Rex cats)
• Autosomal recessive 
• Neuromuscular dysfunctions from early age (3-23 weeks of age)
• Ventroflexion of neck, exercise intolerance, megaesophagus, and hypermetric forelimb gait are characteristic findings
• Laminin deficiency-associated muscular dystrophy
• Nemaline myopathy
• Polymyositis

 

COMPARATIVE PATHOLOGY:

• Mouse:
• A strain and SJL mouse:
  • Dysferlin gene mutation
  • Progressive degenerative changes in proximal skeletal muscle groups
  • >70% of A strain mice develop rhabdomyosarcomas over 20 months of age
  • Similar to a form of limb-girdle muscular dystrophy in humans
• Merosin (laminin alpha 2)-deficient congenital muscular dystrophy
  • Autosomal recessive inheritance
  • Mouse homozygous recessive model dko
  • Absence of merosin, one of three laminin subunits fount in basement membrane of muscle fibers and Schwann cells
  • Unique feature in the mouse model is abnormal myelination of ventral spinal nerve root fibers, aggregates of unmyelinated axons, and lack of Schwann cells
• Other mouse models have been developed, but the above are naturally occurring 
• Rats: Dystrophin deficient strains have been developed that develop cardiomyopathy similar to human DMD cardiomyopathy
• Sheep: Muscular dystrophy of Merino sheep 
• Autosomal recessive disorder; skeletal muscle has normal dystrophin expression; only affects type 1 fibers
  • Equally affects both sexes; clinical signs begin around 3-4 weeks of age with variable progression – most have easily detectable hindlimb gait abnormalities by 6-12 months and are severely affected by 2-3 years of age; most die between 6-18 months of age
  • Abnormal growth and reduced flexion of the hindlimbs, affected muscles are pale to grey, hard, and atonic; vastus intermedius, soleus, anconeus, and medial head of the triceps are most affected
  • Formation of characteristic pale amphophilic central or peripheral sarcoplasmic masses, no muscle regeneration
  • Differential diagnoses include: 
    • Nutritional myopathy (white muscle disease, M-M11) due to vitamin E/selenium deficiency; nutritional myopathies are uncommon in carnivores
  • Monensin toxicity (M-T01)
  • Toxic plant myopathy (Cassia – coffee senna, M-T02)
  • Motor endplate disease: local myocyte degeneration only; recessive disease; abnormalities in nerve fibers and motor endplates; death at 20-25 days
• Cattle: MD reported in Meuse-Rhine-Yssel breed in the Netherlands; diaphragm and intercostal muscles affected; large elongated segments of sarcoplasm free of myofibrils (central cores)
• Chickens:
  • Hereditary muscular dystrophy in chickens 
  • Autosomal dominant defect in ubiquitin ligase gene (WWP1) results in progressive loss of type-2 (fast twitch) white muscle fiber function

• Superficial pectoralis (large breast muscle) most severely affected, White streaks in muscle bundles; tough or rubbery muscle

• Necrosis, hypertrophy, and regeneration in type-2 (white) muscle fibers; ringed fibers (ringbinden); interstitial fibrosis; numerous intracytoplasmic vacuoles, possibly from proliferation of transverse tubular system
• Differential diagnoses: Deep pectoral myopathy (AKA wooden breast)

• Japanese quail: Autosomal dominant trait of muscle weakness, stiffness, myotonia; lenticular cataracts and testicular degeneration; similar to human myotonic dystrophy
• Mink: Autosomal recessive trait; affects especially large proximal muscles and temporal muscles
• Humans:
  • X-linked: 
    • Most common forms of muscular dystrophy; caused by lack of dystrophin
    • Duchenne (DMD): Most severe/most common; death from respiratory compromise/cardiac decompensation 
    • Becker: Less severe
• Autosomal Muscular Dystrophies:
  • Limb girdle muscular dystrophies 
• Pig: 
• Recent reports characterizing Becker muscular dystrophy in pigs with an enlarged, firm, pale tongue that microscopically had severe fibrosis and fatty infiltration with myofiber degeneration, necrosis, and regeneration (Aihara, Vet Pathol. 2022)
• One study identified 14 missense mutations that resulted in shortened transcripts for dystrophin, most similar to Becker MD (Kamiya, Vet Pathol. 2024)

 

REFERENCES:

1. Aihara N, Kuroki S, Inamuro R, Kamiya Y, Shiga T, Kikuchihara Y, Ohmori E, Noguchi M, Kamiie J. Macroglossia in a pig diagnosed as Becker muscular dystrophy due to dystrophin pseudoexon insertion derived from intron 26. Vet Pathol. 2022;59(3):455-458.
2. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. Ames, IA: John Wiley & Sons, Inc.; 2016: 105,115.
3. Cooper BJ, Valentine BA. Muscle and tendon. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016:167,173-200.
4. Kamiya Y, Aihara N, Shiga T, Horiuchi N, Kamiie J. Diversity of mutations in the dystrophin gene and details of muscular lesions in porcine dystrophinopathies. Vet Pathol. 2024 May;61(3):432-441.
5. Ramos-Vara JA, Gelain ME. Chapter 18: Advanced Diagnostic Techniques. In: Raskin RE, Meyer DJ, & Boes KM eds. Canine and Feline Cytopathology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2022:651-652.
6. Valentine BA. Skeletal Muscle. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1007-1008.

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