Page 615 - Concise Pathology for Exam Preparation ( PDFDrive )

600 SECTION II Diseases of Organ Systems

2. Neurogenic atrophy occurs when a motor neuron or its axon degenerates, leading to
atrophy of both Type I and Type II fibres.
3. Muscular dystrophy (MD) is an inherited progressive primary muscle disease that
most commonly presents in early childhood.
4. Congenital myopathies are rare, primary, nonprogressive muscle diseases that
present at birth with poor muscle tone (eg, central core disease, nemaline [rod]
myopathy).
5. Polymyositis and dermatomyositis are connective tissue disorders which involve
muscle.

Q. Classify and describe muscular dystrophies.

Ans. Muscular dystrophies include Duchenne and Becker dystrophy (X-linked inheri-
tance), limb girdle dystrophy (autosomal recessive inheritance), fascioscapulohumeral and
oculopharyngeal dystrophy (autosomal dominant inheritance). The most common types
have been described below:
(a) Duchenne muscular dystrophy (DMD)
• Most severe and most common type of dystrophy
• X-linked inheritance; females are carriers
• Presents with progressive muscle weakness and wasting, which manifests by
5 years
• Paralysis and death by second to third decade
• Weakness begins in the pelvic girdle muscles, and then shoulder girdle is
affected.
• Characterized by a positive Gower manoeuvre (requiring the assistance of upper
extremities to stand up) and pseudohypertrophy (enlargement of calf muscles with
weakness)
• May effect cardiac muscle resulting in arrhythmias and heart failure
• May be associated with intellectual impairment
• Death results from respiratory insufficiency, pulmonary infection and cardiac failure
Pathogenesis:
• Abnormalities in a gene encoding dystrophin located in the Xp21 region
• Deletion (most common), point mutation also seen
• Dystrophin and dystrophin-associated protein complex anchors actin to membrane
glycoprotein; absence of dystrophin causes transfer of the force of contraction to
connective tissue and myocyte degeneration
(b) Becker muscular dystrophy (BMD)
• Less common; less severe
• BMD patients have mutations in dystrophin gene resulting in decreased amount of
dystrophin, usually of abnormal molecular weight.

Morphology of Dystrophic Muscle

Most histopathological abnormalities are common to DMD and BMD and include
• Variation in fibre size (presence of both small and large fibres) with fibre splitting
• Increased number of internalized nuclei (normally less than 3% are internalized)
• Degeneration, necrosis and phagocytosis of muscle fibres
• Regeneration of muscle fibres and proliferation of endomysial connective tissue. DMD cases
also show enlarged, rounded hyaline fibres with loss of cross striations (hypercontracted
fibres); a finding rare in BMD.

Laboratory Findings

• Antenatal diagnosis of dystrophin defect using recombinant DNA technology
• Serum CK (creatinine kinase) levels decline as muscle tissue is progressively replaced
by fat and fibrous tissue.

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