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Myopathy
Specialty	Rheumatology

In medicine, a myopathy is a muscular disease in which the muscle fibers do not function for any one of many reasons, resulting in muscular weakness. "Myopathy" simply means muscle disease (myo- Greek μυο "muscle" + pathos -pathy Greek "suffering"). This meaning implies that the primary defect is within the muscle, as opposed to the nerves ("neuropathies" or "neurogenic" disorders) or elsewhere (e.g., the brain etc.). Muscle cramps, stiffness, and spasm can also be associated with myopathy.

Muscular disease can be classified as neuromuscular or musculoskeletal in nature. Some conditions, such as myositis, can be considered both neuromuscular and musculoskeletal.

Classes

There are many types of myopathy. ICD-10 codes are provided here where available.

Congenital

• (G71.0) Dystrophies (or muscular dystrophies) are a subgroup of myopathies characterized by muscle degeneration and regeneration. Clinically, muscular dystrophies are typically progressive, because the muscles' ability to regenerate is eventually lost, leading to progressive weakness, often leading to use of a wheelchair, and eventually death, usually related to respiratory weakness.
• (G71.1) Myotonia
  • Neuromyotonia
• (G71.2) The congenital myopathies do not show evidence for either a progressive dystrophic process (i.e., muscle death) or inflammation, but instead characteristic microscopic changes are seen in association with reduced contractile ability of the muscles. Congenital myopathies include, but are not limited to:
  • (G71.2) nemaline myopathy (characterized by presence of "nemaline rods" in the muscle),
  • (G71.2) multi/minicore myopathy (characterized by multiple small "cores" or areas of disruption in the muscle fibers),
  • (G71.2) centronuclear myopathy (or myotubular myopathy) (in which the nuclei are abnormally found in the center of the muscle fibers), a rare muscle wasting disorder
• (G71.3) Mitochondrial myopathies, which are due to defects in mitochondria, which provide a critical source of energy for muscle
• (G72.3) Familial periodic paralysis
• (G72.4) Inflammatory myopathies, which are caused by problems with the immune system attacking components of the muscle, leading to signs of inflammation in the muscle
• (G73.6) Metabolic myopathies, which result from defects in biochemical metabolism that primarily affect muscle
  • (G73.6/E74.0) Glycogen storage diseases, which may affect muscle
  • (G73.6/E75) Lipid storage disorder

Acquired

• (M33.0-M33.1)
  • Dermatomyositis produces muscle weakness and skin changes. The skin rash is reddish and most commonly occurs on the face, especially around the eyes, and over the knuckles and elbows. Ragged nail folds with visible capillaries can be present. It can often be treated by drugs like corticosteroids or immunosuppressants. (M33.2)
  • Polymyositis produces muscle weaknesss. It can often be treated by drugs like corticosteroids or immunosuppressants.
  • Inclusion body myositis is a slowly progressive disease that produces weakness of hand grip and straightening of the knees. No effective treatment is known.
• (M61) Myositis ossificans
• (M62.89) Rhabdomyolysis and (R82.1) myoglobinurias

Glucocorticoid induced myopathy: Glucocorticoids, whether as endogenous as seen in Cushing's disease or exogenously during prolonged glucocorticoid therapy with drugs such as hydrocortrisone, prednisone, methylprednisolone, triamcinolone or dexamethsone, cause proximal myopathy. Fluorinated glucocorticoids (triamcinolone and prednisone) are more likely to induce myopathy. Glucocorticoids affect fast twitch muscle fibers, a group of muscles with the ability of short term anaerobic function. Glucocorticoids affect both synthesis and degradation of muscle fibers. In rats, fast twitch type II are affected the most. The degradation is due to increase catabolism of myofibrilar content of the muscle. Glucocorticoids mediate their catabolic effect through expression of myostatin(1). Myostatin is a member of the growth and differentiation family of proteins. Glucocorticoids increase both the mRNA and the protein expression rate of myostatin independently. After expression of myostatin induced by glucocorticoids in the muscle cells (myocytes), it acts both by endocrine (secretion from myocytes to the blood) and paracrine (secrete from myocytes locally and acts on itself) mechanisms. Myostatin, in turn, activates a set of factors in the muscle leading to activation of ubiquitin-proteasome pathway and eventually to the decline in myosin heavy chain type II. The ubiquitin-proteasome pathway is the protein degradation machinery inside of the cells, and degradation of muscle ensues. In fact, "knockout mice," mice with a deletion of the myostatin gene, do not develop glucocorticoid myopathy(2).

Some manifestations of glucocorticoid myopathy are muscle weakness, difficulty standing up without using one's hands, and difficulty combing one's hair. After a prolonged period of exposure to glucocorticoids (endogenous or exogenous), severe muscle weakness develops. Histologically, angulation of muscle fibers, muscle fiber atrophy, and in prolonged cases, replacement of muscle fiber by fat can be seen. The absence of inflammatory cells paired with the presence of muscle atrophy is the hallmark of the condition.

There are drugs undergoing limited therapeutic trials that could counteract the effects of glucocorticoids on muscle. Exercise, IGF1, testosterone and glutamine (3) have been shown in animal to decrease the deleterious effect of glucocorticoids, but so far no randomised trial has been done in humans.

1)Ma K, Mallidis C, Bhasin S, Mahabadi V, Artaza J, Gonzalez-Cadavid N, Arias J, Salehian B.Glucocorticoid-induced skeletal muscle atrophy is associated with upregulation of myostatin gene expression.Am J Physiol Endocrinol Metab. 2003 Aug;285 3)Salehian B, Mahabadi V, Bilas J, Taylor WE, Ma K. The effect of glutamine on prevention of glucocorticoid-induced skeletal muscle atrophy is associated with myostatin suppression. Metabolism. 2006 Sep;55(9):1239-47.

Symptoms

• Common muscle weakness. Also (R25.2) cramps (M25.6) stiffness, and (R29.0) tetany

Treatments

Because different types of myopathies are caused by many different pathways, there is no single treatment for myopathy. Treatments range from treatment of the symptoms to very specific cause-targeting treatments. Drug therapy, physical therapy, bracing for support, surgery, massage, and even acupuncture are all current treatments for a variety of myopathies.

References

1. "Myopathy - Definition from the Merriam-Webster Online Dictionary".

External links

• GeneReviews/NCBI/NIH/UW entry on Myopathy with Deficiency of ISCU
• See http://neuromuscular.wustl.edu/ for medical descriptions.

Template:Medical conditions

• Disorders of muscles