Amyotrophy is a medical term for muscle atrophy, a decrease in muscle size. It relates more specifically to the skeletal striated muscles, which are muscles under voluntary control.

The characteristics of amyotrophy are variable. Depending on the case, this muscle atrophy can be:

• localized or generalized, that is to say, it can affect a single muscle, all the muscles of a muscle group or all the muscles of the body;
• acute or chronic, with rapid or gradual development;
• congenital or acquired, that is, it can be caused by an abnormality present from birth or be the consequence of an acquired disorder.

Muscle atrophy can have different origins. It may be due to:

• a physical immobilization, i.e. prolonged immobilization of a muscle or muscle group;
• hereditary myopathy, an inherited disease that affects the muscles;
• acquired myopathy, a disease of the muscles whose cause is not hereditary;
• nervous system damage.

The case of physical immobilization

Physical immobilization can lead to atrophy due to lack of muscle activity. Muscle immobilization can, for example, be due to the placement of a cast during a fracture. This atrophy, sometimes called muscle wasting, is benign and reversible.

The case of hereditary myopathy

Myopathies of hereditary origin can be the cause of muscular atrophy. This is particularly the case in several muscular dystrophies, diseases characterized by degeneration of muscle fibers.

Some of the hereditary causes of muscular atrophy include:

• Duchenne muscular dystrophy, or Duchenne muscular dystrophy, which is a rare genetic disorder characterized by progressive and generalized muscle degeneration;
• Steinert’s disease, or Steinert’s myotonic dystrophy, which is a disease that can manifest as amyotrophy and myatonia (disorder of muscle tone);
• facio-scapulo-humeral myopathy which is a muscular dystrophy affecting the muscles of the face and those of the shoulder girdle (connecting the upper limbs to the trunk).

The case of acquired myopathy

Amyotrophy can also be the consequence of acquired myopathies. These non-hereditary muscle diseases can have several origins.

Acquired myopathies can be of inflammatory origin, in particular during:

• polymyosites which are characterized by inflammation of the muscles;
• dermatomyosites which are characterized by inflammation of the skin and muscles.

Acquired myopathies may also not present any inflammatory character. This is particularly the case with myopathies ofiatrogenic origin, that is, muscle disorders due to medical treatment. For example, in high doses and in the long term, cortisone and its derivatives can be the cause of atrophy.

Neurological causes of muscular atrophy

In some cases, atrophy may have a neurological origin. Muscle atrophy is caused by damage to the nervous system. This can have several explanations, including:

• la Charcot’s disease, or amyotrophic lateral sclerosis, which is a neurodegenerative disease affecting motor neurons (neurons involved in movement) and causing amyotrophy and then progressive paralysis of the muscles.
• the spinal amyotrophie, a rare genetic disorder that can affect the muscles of the root of the limbs (proximal spinal atrophy) or the muscles of the extremities of the limbs (distal spinal atrophy);
• la poliomyelitis, an infectious disease of viral origin (poliovirus) which can cause atrophies and paralysis;
• nerve damage, which can occur in one or more nerves.

The evolution of muscular atrophy depends on many parameters including the origin of the muscle atrophy, the patient’s condition and medical management. In some cases, muscular atrophy can increase and spread to other muscles in the body, or even to the whole body. In the most severe forms, muscle atrophy may be irreversible.

Treatment consists of treating the origin of the muscular atrophy. Drug treatment can for example be implemented during inflammatory myopathy. Physiotherapy sessions may be recommended in the event of prolonged physical immobilization.