Muscular dystrophies md pathology 10 marks question give beautiful answer to replicate
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Duchenne muscular dystrophy muscle biopsy histology
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Muscular Dystrophies - Pathology (10 Marks)
Definition
Muscular dystrophies are a group of inherited (genetically determined) disorders of skeletal muscle characterized by progressive degeneration and weakness of muscle fibers, usually becoming clinically evident after infancy. They result from mutations in genes coding for proteins of the dystrophin-glycoprotein complex, which links the muscle cytoskeleton to the extracellular matrix and maintains sarcolemmal integrity during contraction.
Classification
| Type | Inheritance | Gene/Protein Defect |
|---|---|---|
| Duchenne muscular dystrophy (DMD) | X-linked recessive | Dystrophin (absent) |
| Becker muscular dystrophy (BMD) | X-linked recessive | Dystrophin (reduced/abnormal) |
| Facioscapulohumeral dystrophy | Autosomal dominant | DUX4 derepression (4q35) |
| Myotonic dystrophy | Autosomal dominant | DMPK CTG repeat expansion |
| Limb-girdle dystrophies | AD/AR | Sarcoglycans, calpain, dysferlin, etc. |
| Congenital muscular dystrophies | Autosomal recessive | Laminin-α2 (merosin), others |
(Duchenne and Becker are the prototype dystrophinopathies and are the usual focus of a pathology exam answer.)
Etiopathogenesis of Duchenne/Becker Muscular Dystrophy
- Gene: DMD gene on Xp21, one of the largest human genes (2.3 million base pairs, 79 exons), encoding dystrophin.
- Duchenne MD: Frameshift/nonsense mutations → complete absence of dystrophin.
- Becker MD: In-frame deletions → reduced amount or abnormal (truncated) dystrophin, giving a milder phenotype.
- Normal function of dystrophin: Anchors the subsarcolemmal cytoskeleton (actin) to the dystrophin-associated glycoprotein complex in the sarcolemma, which in turn binds laminin in the extracellular matrix. This complex stabilizes the sarcolemma against the mechanical stress of repeated contraction-relaxation cycles.
- Consequence of deficiency: Sarcolemmal fragility → recurrent cycles of muscle fiber necrosis and regeneration → progressive fibrous and fatty replacement of muscle → weakness.
- Deficiency of dystrophin also destabilizes associated proteins (e.g., neuronal nitric oxide synthase, sarcoglycans), amplifying membrane instability and calcium influx-mediated fiber injury.
Morphology (Muscle Biopsy Findings)
- Variation in fiber size - abnormally large, rounded, hypercontracted fibers alongside small atrophic fibers.
- Segmental fiber necrosis with myophagocytosis (macrophages removing necrotic debris).
- Regenerating fibers - basophilic, with large nuclei and prominent nucleoli.
- Endomysial and perimysial fibrosis - progressive replacement of muscle by connective tissue.
- Fatty infiltration of muscle, most marked in advanced disease (produces the clinically apparent "pseudohypertrophy" of the calves).
- Immunohistochemistry: absent dystrophin staining on the sarcolemma in Duchenne MD; patchy/reduced staining in Becker MD.
- Late stage: "end-stage muscle" showing near-total replacement by fat and fibrous tissue with only scattered residual fibers.
Clinical Correlation
- Duchenne MD: Onset by age 1-5 years, progressive proximal (pelvic and shoulder girdle) weakness, calf pseudohypertrophy, Gower's sign, markedly elevated serum creatine kinase (CK), wheelchair-bound by early teens, death by 20s-30s from respiratory failure or dilated cardiomyopathy (cardiac muscle also expresses dystrophin).
- Becker MD: Later onset, slower progression, longer survival, milder cardiac involvement.
- Female carriers may show mild weakness or subclinical elevation of CK due to skewed X-inactivation.
Diagnosis
- Elevated serum CK (markedly high in Duchenne, less so in Becker).
- Genetic testing for DMD mutations (deletion/duplication analysis, sequencing).
- Muscle biopsy with dystrophin immunostaining/immunoblot when genetic testing is inconclusive.
- Electromyography shows a myopathic pattern (distinguishing from neurogenic disorders).
Summary Diagram (conceptual flow for answer)
Dystrophin gene mutation → Loss/reduction of dystrophin → Sarcolemmal instability → Repeated contraction-induced membrane damage → Fiber necrosis + attempted regeneration → Progressive fibrofatty replacement → Muscle weakness, pseudohypertrophy, cardiomyopathy.
Here is a biopsy image showing the classic Duchenne dystrophy findings for illustration in your answer sheet diagram:
(Note: I was unable to retrieve a verified image link from the library for this specific figure - for your written exam, simply sketch a labeled diagram showing: variable fiber size, necrotic fiber with macrophages, regenerating basophilic fiber, endomysial fibrosis, and fat replacement.)
Source: Robbins, Cotran & Kumar - Pathologic Basis of Disease (Pathogenesis of Dystrophinopathies), and Goldman-Cecil Medicine (Clinical Manifestations and Diagnosis, Dystrophinopathies section).
Quick evidence check: This is a well-established classical pathology topic with no significant recent evidence updates expected to change the core pathophysiology or histology described above, so no PubMed cross-check was necessary for this conceptual answer.
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