Educational scope notice: This is a study note for medical students, not medical advice, diagnosis, or treatment guidance. Clinical management should follow local protocols and current guidelines.
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 or from motor neuron loss, and those mimics follow different rules.
The companion notes divide the topic into families, assessment, and care. Four notes cover the inherited and acquired families, two cover the inflammatory diseases that present without a rash, and a companion cluster follows dermatomyositis in depth. Respiratory complications, which cut across all severe weakness, are framed here and detailed in their own place.
Myopathy and its closest mimics
A myopathy typically produces proximal, symmetric weakness with preserved sensation and normal or reduced reflexes. Getting up from a chair, climbing stairs, or lifting the arms overhead becomes difficult while hand dexterity and feeling remain intact early on.
Myasthenia gravis sits at the junction rather than in the fibre. Its signature is fatigable weakness: muscles weaken with repetition and recover with rest, often with ocular or bulbar symptoms (drooping eyelids, double vision, speech or swallowing that fades through a meal) while reflexes and sensation stay normal. Recovery after rest is the feature that points away from the muscle itself.
Lambert-Eaton myasthenic syndrome is the presynaptic counterpart. Strength may briefly improve with repeated effort rather than fade, autonomic symptoms such as dry mouth can accompany the weakness, and the electrical response grows with rapid stimulation instead of shrinking. An underlying small cell lung cancer is found in a substantial proportion of cases, so the diagnosis prompts a search for malignancy.
Motor neuron disease weakens through a different route again. Amyotrophic lateral sclerosis combines upper and lower motor neuron signs in the same body segment: a wasted, fasciculating limb with brisk reflexes, for example. Myopathy does not produce that combination, so reflex behaviour helps separate the two at the bedside.
When weakness is severe, breathing itself is at risk. The useful model is a pump. The lungs may still exchange gas while the diaphragm and expiratory muscles fail to move air, so ventilation and carbon dioxide clearance become the danger rather than oxygenation alone. Weak cough and secretion retention mark the patients who need monitoring before crisis, and nocturnal or supine testing often exposes diaphragm weakness that daytime saturation hides.
One emergency belongs at the front of the assessment. Guillain-Barre syndrome can present with rapidly ascending weakness, sometimes with tingling or pain, and may reach cranial and respiratory muscles. Rapid progression with areflexia points toward the peripheral nerve rather than the muscle and changes management entirely.
Choose a route through the topic
- Inflammatory Myopathies: the acquired immune-mediated family, its autoantibodies, biopsy patterns, and why treatment response differs between subtypes.
- 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.
A companion cluster on dermatomyositis covers the inflammatory disease with a rash in depth, from skin and muscle findings through diagnosis to treatment. Read the inflammatory overview first for the family map, then the family that fits the patient, with dermatomyositis alongside polymyositis and inclusion body myositis for comparison.
How the diagnosis is built
Creatine kinase (CK) is the most useful blood marker. It rises with active fibre injury and falls with successful treatment, so it both supports the diagnosis and tracks response. Three caveats keep it honest: CK rises transiently after exercise or intramuscular injections, so rest and repeat measurement matter; baseline levels vary between populations; and chronically very high values point toward dystrophinopathy while normal or mildly raised values in an older weak patient favour inclusion body myositis.
Because muscle also releases transaminases and lactate dehydrogenase, isolated “liver enzyme” elevation in a weak patient should prompt CK measurement before any liver workup.
Electromyography separates myopathic from neuropathic processes: short, small motor unit potentials with early recruitment suggest muscle disease, while large potentials with reduced recruitment suggest reinnervation after nerve loss. Fibrillation potentials and positive sharp waves mark active irritability. In myotonic disorders the discharge waxes and wanes with a characteristic diving sound.
Muscle biopsy identifies what is missing or misplaced: perifascicular atrophy in dermatomyositis, endomysial cytotoxic infiltrates in polymyositis, rimmed vacuoles in inclusion body myositis, ragged-red fibres in mitochondrial disease. Genetic testing has become the first confirmatory step when the phenotype points at a specific inherited defect, with biopsy reserved for unclear cases and treatment monitoring. Muscle MRI adds a noninvasive map, showing selective fatty replacement patterns that differ between dystrophies and can appear before weakness is obvious.
Treatment follows the family
Inflammatory disease with a rash or with exclusion-defined polymyositis responds to corticosteroids with steroid-sparing agents and immunoglobulin rescue, while inclusion body myositis does not and is managed supportively. Dystrophies are treated with steroids that prolong ambulation, emerging exon-skipping and gene-therapy approaches for specific mutations, and multidisciplinary care. Channelopathies respond to carbonic anhydrase inhibition and trigger avoidance; metabolic myopathies to trigger avoidance and activity planning. Toxic and endocrine myopathies improve when the cause is removed. The details sit in the family notes linked above.
Evidence anchors
- Dalakas MC. Inflammatory muscle diseases. N Engl J Med. 2015;372(18):1734-1747. doi:10.1056/NEJMra1402225
- Lundberg IE, Tjarnlund A, Bottai M, et al. 2017 European League Against Rheumatism/American College of Rheumatology classification criteria for adult and juvenile idiopathic inflammatory myopathies and their major subgroups. Arthritis Rheumatol. 2017;69(12):2271-2282. doi:10.1002/art.40322
- Birnkrant DJ, Bushby K, Bann CM, et al. Diagnosis and management of Duchenne muscular dystrophy, part 1: diagnosis, and neuromuscular, rehabilitation, endocrine, and gastrointestinal and nutritional management. Lancet Neurol. 2018;17(3):251-267. doi:10.1016/S1474-4422(18)30024-3