are diseases of skeletal muscle which are not caused by nerve
disorders. Myopathies are characterized by skeletal or voluntary
muscles to become weak or wasted. Voluntary muscles are the muscles
whose actions are normally controlled by the individual.
There are many different types of myopathies. Some inherit the disease while others get them with the use of steroids, or what is known as steroid myopathy.
Steroid myopathy causes weakness primarily to the proximal muscles of the upper and lower limbs and neck flexors. Steroid myopathy can arise due to excessive corticosteroid levels in the body. Increased level of corticosteroid can either be due to excessive endogenous corticosteroid production that may be caused by adrenal tumors. Steroid myopathy can also be due to steroid intake .
It has been observed that steroid myopathy tends to arise when an individual uses fluorinated steroids ( dexamethasone, fluoxymesterone ) as well as nonfluorinated steroids (cortisone, prednisone) . The exact mechanism why steroid myopathy occurs is not yet exactly determined but the generally accepted theory is it may be brought about by unbalanced metabolic pathways, including decreased protein synthesis, increased protein degradation, and carbohydrate metabolism.
Discontinuance or decreased of corticosteroid intake usually resolves the weakness observed with steroid myopathy; however, the length of the recovery period varies. In some individuals recovery only takes a couple of weeks while in others recovery takes several months.
Every year between and people per million receive a diagnosis of PM or DM.  Around children per million per year are diagnosed with DM (termed juvenile dermatomyositis ), with an average age of onset of seven years. Diagnosis of adult DM commonly occurs between 30 and 50 years of age. PM is an adult disease, usually emerging after the age of twenty. PM and DM are more common in females, more common in Caucasians, and least common in Asians. At any given time, about people per million have IBM; it emerges after the age of 30 (usually after 50), and may be more common in males.