From the Guidelines
Myopathies are disorders affecting skeletal muscle tissue that can arise from various etiologies, including genetic factors, inflammatory conditions, metabolic disorders, toxic exposures, endocrine abnormalities, and infectious agents. The main causes of myopathy can be broadly categorized into several groups.
- Genetic myopathies encompass conditions like Duchenne and Becker muscular dystrophies (caused by dystrophin gene mutations), myotonic dystrophy, and congenital myopathies, as seen in myotonic dystrophy type 1 (DM1) and type 2 (DM2), which are caused by expansion of an unstable CTG trinucleotide repeat in the 3′ untranslated region of the DMPK gene and expansion of a CCTG repeat in intron 1 of CNBP, respectively 1.
- Inflammatory myopathies include polymyositis, dermatomyositis, and inclusion body myositis, which involve immune-mediated muscle damage, as described in the idiopathic inflammatory myopathies (IIMs) 1.
- Metabolic myopathies result from disorders in energy metabolism pathways, such as glycogen storage diseases (McArdle's disease), lipid metabolism disorders, and mitochondrial myopathies.
- Toxic myopathies can develop from medications like statins, colchicine, corticosteroids, and alcohol abuse.
- Endocrine disorders affecting muscle function include hypothyroidism, hyperthyroidism, Cushing's syndrome, and diabetes.
- Infectious causes include viral myositis (influenza, HIV), bacterial infections, and parasitic infections like trichinosis. Proper diagnosis requires comprehensive evaluation including clinical history, physical examination, laboratory tests (CK levels, inflammatory markers), electromyography, muscle biopsy, and genetic testing when appropriate, as the treatment approaches vary significantly depending on the underlying cause of the myopathy.
- A comprehensive diagnostic approach is essential to determine the specific etiology of myopathy, as treatment strategies differ significantly depending on the underlying cause.
- Understanding the specific etiology is crucial for appropriate management, as treatment approaches vary significantly depending on the underlying cause of the myopathy, and this is supported by the most recent and highest quality study on myotonic dystrophy 1.
From the Research
Etiology of Myopathy
The etiology of myopathy is complex and multifaceted. Some studies suggest that myopathies can be caused by:
- Inflammatory processes, such as those seen in dermatomyositis, polymyositis, and inclusion body myositis 2, 3, 4
- Drug-induced mechanisms, including the use of statins, glucocorticoids, immunologic drugs, and antimicrobials 5
- Autoimmune disorders, such as those seen in idiopathic inflammatory myopathies (IIMs) 4, 6
- Genetic predisposition, which can increase the risk of developing certain types of myopathies 5
Types of Myopathies
There are several types of myopathies, including:
- Dermatomyositis, which is characterized by skin rash and muscle weakness 2, 3, 4
- Polymyositis, which is characterized by muscle weakness and inflammation 2, 3, 4
- Inclusion body myositis, which is characterized by muscle weakness and the presence of inclusion bodies in muscle tissue 2, 3
- Immune-mediated necrotizing myopathy, which is characterized by muscle necrosis and inflammation 4, 6
- Drug-induced myopathies, which can be caused by a variety of medications 5
Risk Factors
Several risk factors have been identified for the development of myopathies, including: