Myopathy means disease of the muscle fibre itself. In adults these disorders form a broad family of acquired and inherited conditions, and the first clinical task is to confirm that the muscle is actually where the problem sits. Weakness that looks muscular can also come from the neuromuscular junction (where a nerve signal is passed to the muscle), from motor neuron loss, or from an acute peripheral neuropathy.
Once the muscle is confirmed as the site, the cause divides the family in two. Inherited disease usually comes from a defective structural protein (the muscular dystrophies), an ion channel (the channelopathies and periodic paralysis), or an energy-pathway enzyme (the metabolic myopathies). Acquired disease comes from immune attack (the inflammatory myopathies), or from a drug, a toxin, or a hormone disturbance (the toxic and endocrine myopathies).
Choose a route through the topic
Start with the clinical pattern and the diagnostic route, then read the family that fits the patient. The inflammatory group is the most treatable, so it deserves early consideration in any subacute weakness.
- Recognising Myopathy and Its Mimics: the pattern of weakness, reflexes, and sensation that places the problem in the muscle, and how myasthenia gravis, Lambert-Eaton syndrome, motor neuron disease, and Guillain-Barre syndrome differ from it.
- Diagnosing Myopathy: what creatine kinase, electromyography, muscle biopsy, genetic testing, and imaging each contribute.
- Inflammatory Myopathies: the acquired immune-mediated family, its autoantibodies, biopsy patterns, and why treatment response differs between subtypes.
- Toxic and Endocrine Myopathies: weakness from drugs, alcohol, or hormone disturbance, and why it usually reverses when the cause is removed.
- Muscular Dystrophies: inherited structural protein defects, from dystrophinopathies through limb-girdle patterns to myotonic dystrophy.
- Metabolic Myopathies: enzyme defects in energy pathways, exercise-triggered symptoms, and the second-wind and mitochondrial patterns.
- Channelopathies and Periodic Paralysis: episodic ion-channel disorders, potassium-linked attacks, myotonias, and their treatment.
- Polymyositis: the rash-free inflammatory disease defined by exclusion, and the protocol it shares with dermatomyositis.
- Inclusion Body Myositis: the late-onset disease with finger-flexor and quadriceps weakness that does not respond to immunosuppression.
Dermatomyositis follows the inflammatory disease with a rash from presentation through diagnosis to treatment, and is best read alongside polymyositis and inclusion body myositis for comparison. The breathing failure that severe weakness can cause is the subject of Respiratory Problems in Neurological Disorders.