Acquired Disorders of Skeletal Muscle

Acquired Disorders of Skeletal Muscle

Acquired muscle disorders are conditions that develop later in life and cause muscle weakness, pain, or cramps. They usually have an acute or subacute onset and are more common in adults.

Main Causes

• Inflammatory myopathies

• Toxic muscle injury

• Post-infectious rhabdomyolysis

• Muscle infarction (especially in diabetes)

Polymyositis

• An autoimmune disease mediated by CD8⁺ cytotoxic T cells.

• Immune cells directly attack muscle fibers.

• Histology shows muscle fiber necrosis and regeneration.

• Causes progressive, symmetric proximal muscle weakness.

Dermatomyositis

• Occurs in both children and adults.

• Most common in children

• In adults, it may be associated with paraneoplastic disorder

• Caused by immune-mediated damage to small blood vessels, leading to muscle ischemia.

• Characterized by:

• Skin manifestations such as rash on sun exposed skin(heliotrope rash)and Gottron papules on eyelids

• Systemic manifestations:interstitial lung disease

• Anti–Mi-2 antibodies are highly specific.

• Histology shows perifascicular atrophy and inflammation.

Inclusion Body Myositis

• Most common inflammatory myopathy in individuals over 50 years.

• Slowly progressive and often resistant to treatment.

Morphologic hallmark:

• Rimmed vacuoles in muscle fibers

• Accumulation of abnormal proteins (β-amyloid, tau, TDP-43)