Myopathy: Muscle Disease Types and Classifications

Myopathy Definition

• Myopathy is a muscle disease unrelated to innervation or neuromuscular junction disorders, with various etiologies.

General Characteristics

• Most types of myopathy result in weakening and atrophy of skeletal muscles, specifically proximal muscles (thigh and shoulder muscles).

Hereditary Myopathies

• Hereditary myopathies are inherited disorders primarily affecting skeletal muscle tissue, caused by mutations in genes encoding proteins crucial for muscle structure and function.

Muscular Dystrophies

• Muscular dystrophies are a heterogeneous group of hereditary illnesses affecting both children and adults, with at least 30 different genes responsible for the disease development.
• The most common muscular dystrophies are Duchenne and Becker muscular dystrophy, resulting from a genetic defect on the X chromosome.
• These diseases are characterized by muscle wasting and weakness, with elevated levels of creatine phosphokinase (CPK).

Congenital Myopathies

• Congenital myopathies are present at birth, with no necrotic or degenerative changes, and CPK levels are often normal.
• This group includes conditions such as nemaline myopathy, central core disease, X-linked myotubular myopathy, and centronuclear myopathy.

Metabolic Myopathies

• Metabolic myopathies comprise a diverse group of disorders arising from defects in cellular energy metabolism, including the breakdown of fatty acids and carbohydrates to generate ATP.
• Reduction of ATP is a key feature of these disorders.

Mitochondrial Myopathies

• Mitochondrial myopathies result from primary dysfunction of the mitochondrial respiratory chain, causing muscle disease.
• These disorders have genetic etiologies and can frequently present with multi-system dysfunction.

Genetic Defects

• Genetic defects in calcium, sodium, potassium, and chloride channels in skeletal muscles result in periodic paralyses and nondystrophic myotonias.
• This group of diseases includes myotonia congenita, paramyotonia congenita, hyper and hypokalemic periodic paralysis, and potassium-aggravated myotonia.

Idiopathic Inflammatory Myopathies

• Polymyositis (PM) causes muscle aches, cramping, and tenderness, with severe muscle weakness that may fluctuate over weeks to months.
• Dermatomyositis (DM) is characterized by a skin rash, in addition to muscle symptoms of PM, and often affects children between the ages of 5 and 14 years.