What are the different types of myopathies?

Types of Myopathies

Myopathies are classified into two major categories: hereditary (congenital) myopathies and acquired myopathies, with further subdivision based on the predominant tissue involvement—smooth muscle cells (myopathies), enteric neurons (neuropathies), or interstitial cells of Cajal (mesenchymopathies). 1

Primary Classification Framework

Hereditary/Congenital Myopathies

These are genetic disorders affecting muscle structure and function:

• Muscular dystrophies: Diseases of muscle membrane or supporting proteins characterized by ongoing muscle degeneration and regeneration, including limb-girdle muscular dystrophies, facioscapulohumeral muscular dystrophy, Duchenne and Becker muscular dystrophy 2, 3
• Congenital myopathies: Defined by specific morphological abnormalities including nemaline myopathies (rods), central core and multiminicore diseases (cores), centronuclear-myotubular myopathy (central nuclei), myosin storage myopathy (hyaline bodies), and congenital fiber type disproportion 1
• Myotonic dystrophies: Including DM1 and DM2, characterized by progressive muscle weakness, myotonia, and multisystem involvement with cardiac manifestations in approximately 80% of DM1 patients 1
• Metabolic myopathies: Glycogen storage diseases (such as McArdle disease) and lipid deposition myopathies causing lipid accumulation in skeletal muscle fibers 2

Acquired Myopathies

These develop secondary to external factors or systemic diseases:

Idiopathic Inflammatory Myopathies (IIM)

• Dermatomyositis: Characterized by skin manifestations and muscle inflammation 1, 4
• Polymyositis: Primary muscle inflammation without skin involvement 1, 4
• Inclusion body myositis (IBM): The most prevalent acquired myopathy above age 50, poorly treatable with distinctive vacuoles and aggregates 4, 2
• Immune myopathies with perimysial pathology (IMPP): Associated with antisynthetase antibodies 4
• Immune myopathies with endomysial pathology (IM-EP): Including brachio-cervical inflammatory myopathy 4

Secondary Myopathies from Systemic Diseases

• Connective tissue disorders: Systemic sclerosis (scleroderma) causing smooth muscle atrophy and gut wall fibrosis, systemic lupus erythematosus, rheumatoid arthritis, and Still's disease 1
• Infiltrative diseases: Amyloidosis (often associated with myeloma producing lambda chains), which can cause both myopathy and neuropathy 1
• Endocrine myopathies: Adrenal dysfunction (both overproduction and insufficiency), parathyroid and thyroid disorders, and pituitary gland disorders 2
• Sarcoidosis: Inflammatory disease manifesting as muscle weakness and myalgia 2

Drug-Induced Myopathies

These can be classified by presence or absence of pain and neuropathy:

• Painless myopathies without neuropathy: Corticosteroids 5
• Painless myopathies with neuropathy: Colchicine, chloroquine, hydroxychloroquine 5
• Myasthenic syndromes: D-penicillamine, antibiotics, beta-blockers 5
• Painful myopathies: Statins, clofibrate, cyclosporin, zidovudine, cimetidine 5, 2
• Toxic myopathies: Long-term alcohol use, SGLT2 inhibitors, antimalarials 2

Infectious Myopathies

Viral causes including HIV, dengue virus, influenza virus, hepatitis B and C viruses, and SARS-CoV-2 2

Radiation-Induced Myopathy

Chronic irradiation damage, typically after pelvic irradiation for gynecological or genitourinary cancers 1

Cardiomyopathies (Cardiac Muscle Involvement)

The American Heart Association classifies cardiomyopathies into distinct morphological and functional phenotypes 1:

• Dilated cardiomyopathy (DCM): Ventricular dilation and depressed myocardial performance
• Hypertrophic cardiomyopathy (HCM): Inappropriate ventricular hypertrophy
• Restrictive cardiomyopathy: Infiltrative diseases affecting cardiac muscle
• Arrhythmogenic right ventricular cardiomyopathy (ARVC): Arrhythmia of ventricular origin
• Inflammatory cardiomyopathy: Including viral cardiomyopathies

These are further subdivided into primary cardiomyopathies (genetic, nongenetic, and acquired, confined to heart muscle) and secondary cardiomyopathies (myocardial involvement as part of systemic disorders including amyloidosis, hemochromatosis, sarcoidosis, autoimmune/collagen vascular diseases, toxins, cancer therapy, and endocrine disorders) 1

Clinical Pitfalls

Important diagnostic consideration: Some conditions like systemic sclerosis, vasculitis, and amyloid can present as both myopathy and neuropathy, making classification challenging 1. The radiological, isotopic, manometric, and histological diagnoses may differ, requiring comprehensive evaluation 1. Primary myopathies are more common in children and often familial with multivisceral involvement and high mortality, while secondary myopathies predominate in adults 1.