Myopathy

Myopathy is disease of skeletal muscle itself, as opposed to disease of the nerves that supply it. The hallmark presentation is symmetrical proximal weakness — difficulty rising from a chair, climbing stairs, or lifting the arms overhead — with sensation preserved, because the sensory nerves are not involved.

Definition

Myopathy is a disorder in which the primary functional or structural abnormality lies within skeletal muscle fibres, typically producing symmetrical proximal weakness with intact sensation and, in many forms, elevated serum creatine kinase.

Scope

This entry covers myopathy as a clinical category spanning inherited muscular dystrophies, inflammatory myopathies, metabolic and mitochondrial muscle disease, and toxic and endocrine causes. It describes the shared myopathic pattern of weakness, the role of muscle enzymes, electromyography and muscle biopsy in characterisation, and the broad mechanistic groupings. It is a reference overview, not diagnostic or treatment guidance.

Key concepts

• Symmetrical proximal weakness
• Preserved sensation
• Serum creatine kinase elevation
• Inflammatory myopathy (polymyositis, dermatomyositis, inclusion body myositis)
• Muscular dystrophy
• Metabolic and mitochondrial myopathy
• Electromyography and muscle biopsy

Mechanisms

Myopathies share a final common picture of muscle-fibre dysfunction, but their mechanisms differ widely. Inherited dystrophies arise from defects in structural or membrane proteins that leave fibres vulnerable to injury; inflammatory myopathies involve immune attack on muscle, with distinct mechanisms underlying dermatomyositis, polymyositis, and inclusion body myositis; and metabolic and mitochondrial myopathies impair the muscle's energy supply. Across these groups, fibre damage often releases creatine kinase into the blood, and electromyography together with muscle biopsy helps separate the underlying processes.

Clinical relevance

Distinguishing muscle disease from nerve and junction disease, and recognising treatable inflammatory and endocrine causes, is an important task in neurology and general medicine. This entry frames how myopathies are classified and reasoned about for educational reference and is not a basis for individual diagnosis or treatment.

Epidemiology

Myopathies are collectively uncommon but diverse. Among acquired forms, the inflammatory myopathies are an important group, with inclusion body myositis being the most common acquired myopathy in older adults; inherited dystrophies such as the dystrophinopathies are leading causes in younger patients. Reported frequencies vary by subtype, age, and population.

History

The classification of muscle disease was transformed by the combination of histochemistry, electron microscopy, and later molecular genetics, which moved diagnosis from purely clinical description toward defined structural and genetic categories. Comprehensive references such as Engel and Franzini-Armstrong's Myology codified this integrated approach, and successive reviews refined the boundaries between the inflammatory myopathies.

Seminal works

• dalakas-2015
• dalakas-hohlfeld-2003
• engel-1994

Frequently asked questions

How does myopathy differ from neuropathy?

Myopathy is disease of the muscle itself and typically causes symmetrical proximal weakness with normal sensation, whereas neuropathy is disease of the nerves and more often causes distal weakness with sensory loss and reduced reflexes.

Why is creatine kinase often measured in suspected myopathy?

Many myopathies damage muscle fibres in a way that releases the enzyme creatine kinase into the blood, so an elevated serum level can be a marker of ongoing muscle injury, although it is neither always present nor specific to a single cause.