Myopathies are a group of primary muscle disorders causing muscle weakness, occasionally stiffness, and rarely muscle pain. They can be classified into inherited or acquired myopathy. Acquired myopathy can be further divided into idiopathic (cause unknown), infectious, metabolic, inflammatory, endocrine, and drug induced based on the etiology. Depending on the involvement, the patient can present with difficulty with mobility, including difficulty with rising from a chair, stair negotiation, or walking, and impaired activities of daily living, such as dressing and personal care due to difficulty raising the arms. Sensation is not primarily affected although the pain is not uncommon.

Inherited Myopathy

Inherited myopathy has a genetic basis and is passed from parent to child. The muscular dystrophies are the most well-known form of inherited myopathy, with progressive weakness and degeneration of the muscles controlling movement. Duchenne muscular dystrophy is the most common form of muscular dystrophy, primarily affecting 1 boy in 3,300, and is caused by absence of dystrophin, a protein in the muscle cell membrane. Other muscular dystrophies include Becker muscular dystrophy, which is less severe than Duchenne MD; fascioscapulohumeral muscular dystrophy (FSH, FSHD), in which the muscles of the face, shoulder blades, and upper arms are most affected; and myotonic muscular dystrophy, characterized by progressive muscle wasting and weakness with prolonged muscle contractions (myotonia) and inability to relax certain muscles after use.

Acquired Myopathy

Inflammatory myopathy, the most common acquired myopathy, includes a group of myopathies with chronic muscle inflammation marked by weakness and, occasionally, muscle pain. It is reported to occur in 8.9 in 1,000,000 persons, with prevalence gradually increasing over the time due to improved diagnostic techniques.1 Other forms of acquired myopathy can be secondary to infection, drugs ortoxins, or systemic (typically endocrine) disorders. Polymyositis, dermatomyositis, and inclusion body myositis are the common inflammatory myopathies.  

Neuromuscular Junction Disorders

Myathenia gravis is a disorder affecting neuromuscular junctions, the contact points between the muscles and nerves.  It presents with fatigue, fluctuating weakness, typically affecting the muscles that control eye and eyelid movement, chewing, and talking, resulting in drooping of the eyelid (ptosis), difficulty swallowing (dysphagia) or speaking (dysarthria).
    

• 1. Furst DE, Amato AA, Iorga ŞR, et al. Epidemiology of adult idiopathic inflammatory myopathies in a U.S. managed care plan. Muscle Nerve 2012;45(5):676-683.