MUSCLE ATROPHY

MUSCLE ATROPHY

- Muscle atrophy is defined as a decrease in the mass of the muscle it can be a partial or complete wasting away of muscle, and is most commonly experienced when persons suffer temporary disabling circumstances such as being restricted in movement and/or confined to bed as when hospitalized. When a muscle atrophies, this leads to muscle weakness, since the ability to exert force is related to mass. Modern medicine's understanding of the quick onset of muscle atrophy is a major factor behind the practice of getting hospitalized patients out of bed and moving about as active as possible as soon as is feasible, despite sutures, wounds, broken bones and pain.

- Muscle atrophy results from a co-morbidity of several common diseases, including cancer, AIDS, congestive heart failure, COPD (chronic obstructive pulmonary disease), renal failure, and severe burns patients who have "cachexia" in these disease settings have a poor prognosis. Moreover, starvation eventually leads to muscle atrophy.

- Disuse of the muscles, such as when muscle tissue is immobilized for even a few days of unuse such as when the patient has a primary injury such as an immobilized broken bone (set in a cast or immobilized in traction) will also lead rapidly to disuse atrophy and minimizing such occurrences as soon as possible is a primary mission of occupational & physical therapists employed within hospitals working in co-ordination with orthopedic surgeons.

- The similar in effect neurogenic atrophy is muscle atrophy that results from damage to the nerve that stimulates the muscle causing a shriveling about otherwise healthy limbs. Also, time in a circa zero g environment without exercise will lead to atrophy. This is partially due to the smaller amount of exertion needed to move about, and that muscles are not used to maintain posture. In a similar effect, patients with a broken leg joint undergoing as little as three weeks of traction can lose enough back and buttocks muscle mass and strength as to have difficulty sitting without assistance and experience pain, stress and burning even after a very short ten minute exposure when such positioning is contrived during recovery.

CLINICAL SETTINGS

- There are many diseases and conditions which cause a decrease in muscle mass, known as atrophy, including inactivity, as seen when a cast is put on a limb, or upon extended bedrest (which can occur during a prolonged illness); cachexia - which is a syndrome that is a co-morbidity of cancer and congestive heart failure; chronic obstructive pulmonary disease; burns, liver failure, etc., and the wasting Dejerine Sottas syndrome (HSMN Type III). Other syndromes or conditions which can induce skeletal muscle atrophy are liver disease, and starvation.

QUALITY OF LIFE

- Muscular atrophy decreases qualities of life as the sufferer becomes unable to perform certain tasks or worsen the risks of accidents while performing those (like walking). Muscular atrophy increases the risks of falling in conditions such as IBM (inclusion body myositis). Muscular atrophy affects a high number of the elderly.

OTHER MUSCLE DISEASES, DISTINCT FROM ATROPHY

- During aging, there is a gradual decrease in the ability to maintain skeletal muscle function and mass. This condition is called "sarcopenia". The exact cause of sarcopenia is unknown, but it may be due to a combination of the gradual failure in the "satellite cells" which help to regenerate skeletal muscle fibers, and a decrease in sensitivity to or the availability of critical secreted growth factors which are necessary to maintain muscle mass and satellite cell survival.

In addition to the simple loss of muscle mass (atrophy), or the age-related decrease in muscle function (sarcopenia), there are other diseases which may be caused by structural defects in the muscle (muscular dystrophy), or by inflammatory reactions in the body directed against muscle (the myopathies).

PATHOPHYSIOLOGY

Muscle atrophy occurs by a change in the normal balance between protein synthesis and protein degradation. During atrophy, there is a down-regulation of protein synthesis pathways, and an activation protein degradation. The particular protein degradation pathway which seems to be responsible for much of the muscle loss seen in a muscle undergoing atrophy is the ATP-dependent ubiquitin/proteasome pathway. In this system, particular proteins are targeted for destruction by the ligation of at least four copies of a small peptide calledubiquitin onto a substrate protein. When a substrate is thus "poly-ubiquitinated", it is targeted for destruction by the proteasome. Particular enzymes in the ubiquitin/proteasome pathway allow ubiquitination to be directed to some proteins but not others - specificity is gained by coupling targeted proteins to an "E3 ubiquitin ligase". Each E3 ubiquitin ligase binds to a particular set of substrates, causing their ubiquitination.

POTENTIAL TREATMENT

- Muscle atrophy can be opposed by the signaling pathways which induce muscle hypertrophy, or an increase in muscle size. Therefore one way in which exercise induces an increase in muscle mass is to downregulate the pathways which have the opposite effect.

- One important rehabilitation tool for muscle atrophy includes the use of functional electrical stimulation to stimulate the muscles. This has seen a large amount of success in the rehabilitation of paraplegic patients. 

SOURCE:

- http://en.wikipedia.org/wiki/Muscle_atrophy