A systemic autoimmune disease characterized by symmetric progressive muscular weakness of the shoulders and neck muscles, elevated serum levels of creatine kinase, aldolases, LDH and transaminases, altered electromyography, myofiber the generation and regeneration, chronic mononuclear cell infiltration, or perifasicular atrophy on muscle biopsy, dermatomyositis, skin rashes including heliotrope rash or Gottron papules (Bohan A, Peter JB).
Altered electrical activity on EMG consists of polyphasic, short, and small motor unit potentials, fibrillations, positive sharp waves, increased irritability, and high frequency repetitive discharges.
Differential diagnosis includes: muscular dystrophies, metabolic and mitochondrial myopathies, endocrine myopathies, and drug induced myopathies.
Differential diagnosis for skin lesions include, psoriasis, eczema, and v2241ucae vulgaris.
The most common causes of acquired muscle disease in adults.
Annual incidence estimated to be 5-10 cases per million in adults and 1-5 cases per million in children.
Overall estimated prevalence is 50-100 cases per million.
Polymyositis and dermatomyositis peak in prevalence throughout childhood and midlife, with a mean age of seven years of age and the peak age 30-50 years.
Inclusion body myositis peaks after at age 50.
These disorders are increasing in frequency.
Dermatomyositis is the predominant form of myositis in children accounting for 80-85% of cases, and in adults 35-50% areof cases.