Wikipedia

Myopathy

In medicine, myopathy is a disease of the muscle[1] in which the muscle fibers do not function properly. Myopathy means muscle disease (Greek : myo- muscle + patheia -pathy : suffering). This meaning implies that the primary defect is within the muscle, as opposed to the nerves ("neuropathies" or "neurogenic" disorders) or elsewhere (e.g., the brain).

Myopathy
Other namesMuscle disease
SpecialtyRheumatology, Neuromuscular medicine

This muscular defect typically results in myalgia (muscle pain), muscle weakness (reduced muscle force), or premature muscle fatigue (initially normal, but declining muscle force). Muscle cramps, stiffness, spasm, and contracture can also be associated with myopathy. Myopathy experienced over a long period (chronic) may result in the muscle becoming an abnormal size, such as muscle atrophy (abnormally small) or a pseudoathletic appearance (abnormally large).

Capture myopathy can occur in wild or captive animals, such as deer and kangaroos, and leads to morbidity and mortality.[2] It usually occurs as a result of stress and physical exertion during capture and restraint.

Muscular disease can be classified as neuromuscular or musculoskeletal in nature. Different myopathies may be inherited, infectious, non-communicable, or idiopathic (cause unknown). The disease may be isolated to affecting only muscle (pure myopathy), or may be part of a systemic disease as is typical in mitochondrial myopathies.

Signs and symptoms

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Common symptoms include muscle weakness, cramps, stiffness, and tetany.[3]

Diagnosis

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The evaluation of a suspected myopathy combines clinical assessment with laboratory, electrophysiological, imaging, histological and genetic testing, since no single test is diagnostic for all forms.[3]

Laboratory tests

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Serum creatine kinase (CK) is the most commonly used biomarker of muscle injury, and is often supplemented by aldolase, lactate dehydrogenase and the aminotransferases. However, elevated CK is not specific to myopathy, and isolated raised CK ("hyperCKemia") has a poor diagnostic yield on its own.[3][4] Where an idiopathic inflammatory myopathy is suspected, testing for myositis-specific autoantibodies (MSAs) is recommended, ideally before starting immunosuppression, as these antibodies define distinct clinical subtypes and patterns of organ involvement.[5]

Electrodiagnostic testing

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Electromyography (EMG) and nerve conduction studies help distinguish myopathic from neurogenic causes of weakness and may show characteristic myopathic motor unit potentials or myotonic discharges. A myopathic EMG combined with proximal weakness and hyperCKemia substantially increases the likelihood of a positive muscle biopsy, although a normal EMG does not exclude myopathy.[4]

Imaging

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Magnetic resonance imaging of muscle can assess deep muscles not readily sampled by EMG, identify subclinical involvement, characterise patterns of muscle edema and fatty replacement, and guide the selection of a biopsy site.[3]

Muscle biopsy

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Muscle biopsy with histological, histochemical and immunohistochemical analysis remains a cornerstone for confirming inflammatory and structural myopathies and for excluding non-inflammatory causes.[5] Its diagnostic value depends on patient selection, being highest when hyperCKemia, proximal weakness and a myopathic EMG are all present.[4]

Genetic testing

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For inherited myopathies and muscular dystrophies, molecular genetic testing of peripheral blood, increasingly via next-generation sequencing panels, can confirm the diagnosis and often removes the need for muscle biopsy.[3] Genetic testing is now recommended as a first-tier investigation in selected presentations such as unprovoked rhabdomyolysis, where it frequently identifies an underlying myopathy even when EMG and biopsy are unremarkable.[6]

Pathophysiology

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Myopathies share a final common feature of impaired skeletal muscle fibre function, but arise through diverse mechanisms that underpin their classification.[3] In the inherited myopathies, pathogenic variants disrupt structural proteins of the muscle membrane and cytoskeleton (as in the muscular dystrophies), ion channels (the channelopathies and myotonias), or the enzymes of muscle energy metabolism.[3] Contemporary classification frameworks increasingly integrate the underlying genotype and pathomechanism alongside the clinical phenotype.[7]

In metabolic myopathies, defects affecting glycogen, lipid or mitochondrial metabolism impair the production of adenosine triphosphate (ATP) within the muscle cell, so that symptoms are often dynamic and precipitated by exertion rather than static.[3] In the idiopathic inflammatory myopathies, immune-mediated injury produces endomysial inflammation, and the predominant immunopathology differs between subtypes, supporting a clinico-sero-pathological classification into dermatomyositis, polymyositis, immune-mediated necrotising myopathy, anti-synthetase/overlap myositis and inclusion-body myositis.[8] Acquired myopathies of systemic disease similarly act through distinct routes, including endocrine dysregulation, drug and toxin exposure, critical illness and paraneoplastic mechanisms.[3]

Systemic diseases

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Myopathies in systemic disease results from several different disease processes including endocrine, inflammatory, paraneoplastic, infectious, drug- and toxin-induced, critical illness myopathy, metabolic, collagen-related,[9] and myopathies with other systemic disorders. Patients with systemic myopathies often present acutely or subacutely. On the other hand, familial myopathies or dystrophies generally present in a chronic fashion with exceptions of metabolic myopathies, in which symptoms on occasion can be precipitated acutely. Metabolic myopathies, which affect the production of ATP within the muscle cell, typically present with dynamic (exercise-induced) rather than static symptoms.[10] Most of the inflammatory myopathies can have a chance association with malignant lesion; the incidence appears to be specifically increased only in patients with dermatomyositis.[11]

There are many types of myopathy. ICD-10 codes are provided here where available.

Inherited forms

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Acquired

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The Food and Drug Administration is recommending that physicians restrict prescribing high-dose simvastatin (Zocor, Merck) to patients, given an increased risk of muscle damage. The FDA drug safety communication stated that physicians should limit using the 80-mg dose unless the patient has already been taking the drug for 12 months and there is no evidence of myopathy. "Simvastatin 80 mg should not be started in new patients, including patients already taking lower doses of the drug," the agency states.

Myocardium / cardiomyopathy

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Differential diagnosis

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At birth

  • None as systemic causes; mainly hereditary

Onset in childhood

  • Inflammatory myopathies: dermatomyositis, polymyositis (rarely)
  • Infectious myopathies
  • Endocrine and metabolic disorders: hypokalemia, hypocalcemia, hypercalcemia

Onset in adulthood[11]

  • Inflammatory myopathies: polymyositis, dermatomyositis, inclusion body myositis, viral (HIV)
  • Infectious myopathies
  • Endocrine myopathies: thyroid, parathyroid, adrenal, pituitary disorders
  • Toxic myopathies: alcohol, corticosteroids, narcotics, colchicines, chloroquine
  • Critical illness myopathy
  • Metabolic myopathies
  • Paraneoplastic myopathy

Treatments

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Because different types of myopathies are caused by many different pathways, there is no single treatment for myopathy. Treatments range from treatment of the symptoms to very specific cause-targeting treatments. Drug therapy, physical therapy, bracing for support, surgery, and massage are all current treatments for a variety of myopathies.[citation needed]

References

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  1. ↑ "Myopathy - Definition from the Merriam-Webster Online Dictionary".
  2. ↑ Green-Barber JM, Stannard HJ, Old JM (2018). "A suspected case of myopathy in a free-ranging eastern grey kangaroo (Macropus giganteus)". Australian Mammalogy. 40: 122–126. doi:10.1071/AM16054.
  3. 1 2 3 4 5 6 7 8 9 "Diagnostic Approach". Diagnostic Approach to Myopathy. StatPearls/NCBI Bookshelf. 2021.
  4. 1 2 3 Lai, C.H. (2015). "Diagnostic outcome of muscle biopsy". Muscle & Nerve. PMID 25187298.
  5. 1 2 Lundberg, I.E. (2016). "Diagnosis and classification of idiopathic inflammatory myopathies". Journal of Internal Medicine. doi:10.1111/joim.12524.
  6. ↑ "The utility of electrodiagnostic testing in unprovoked rhabdomyolysis". Muscle & Nerve. 2024. PMID 38533679.
  7. ↑ "A Viewpoint on Reframing Genetic Myopathy Classification". PMC. 2025.
  8. ↑ Dalakas, M.C. (2020). "Inflammatory myopathies: update on diagnosis, pathogenesis and therapies". PMC.
  9. ↑ Voermans NC, van Alfen N, Pillen S, Lammens M, Schalkwijk J, Zwarts MJ, van Rooij IA, Hamel BC, van Engelen BG (June 2009). "Neuromuscular involvement in various types of Ehlers-Danlos syndrome". Ann. Neurol. 65 (6): 687–97. doi:10.1002/ana.21643. PMID 19557868. S2CID 22600065.
  10. ↑ Darras, Basil T.; Friedman, Neil R. (February 2000). "Metabolic myopathies: a clinical approach; part I". Pediatric Neurology. 22 (2): 87–97. doi:10.1016/S0887-8994(99)00133-2. PMID 10738913.
  11. 1 2 Chawla J (2011). "Stepwise approach to myopathy in systemic disease". Front Neurol. 2: 49. doi:10.3389/fneur.2011.00049. PMC 3153853. PMID 21886637.
  12. ↑ Seene T (July 1994). "Turnover of skeletal muscle contractile proteins in glucocorticoid myopathy". J. Steroid Biochem. Mol. Biol. 50 (1–2): 1–4. doi:10.1016/0960-0760(94)90165-1. PMID 8049126. S2CID 27814895.
  13. ↑ "Information On Sycamore Poisoning". Rainbow Equine Hospital. Retrieved 16 May 2017.
  14. ↑ "Equine Atypical Myopathy toxin and biochemical tests and tree sample testing available at the RVC". Royal Veterinary college - University of London. 13 February 2017. Retrieved 16 May 2017.
  15. ↑ "2019 ICD-10-CM Diagnosis Code I42.9: Cardiomyopathy, unspecified". The Web's Free 2019 ICD-10-CM/PCS Medical Coding Reference. 1 October 2018. Retrieved 5 February 2019.
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