muscle wasting A wasted muscle is one that has become thinner. It is a sign that all is not well with the motor nerve that innervates it, because a muscle depends on its motor nerve for survival and will die when it is permanently separated from it. A wasted muscle will be weak, and if separation from the motor nerve is complete, so will be the paralysis.

A muscle is composed of a large number of individual muscle fibres and its motor nerve contains a large (but smaller) number of individual nerve fibres. This smaller number is because an individual motor nerve fibre branches many times within the muscle, each muscle fibre receiving only one branch at its neuromuscular junction. Each individual nerve fibre is the axon of a motor nerve cell in the anterior horn of grey matter in the spinal cord, known as a lower motor neuron or anterior horn cell. The anterior horn cell, its axon (nerve fibre), and all the muscle fibres it innervates (there can be 100 or more) constitute a motor unit.

Causes of muscle wasting can be focal or localized, when only one motor nerve is affected, or generalized, when a disease process affects many motor nerves or lower motor neurons. One of the commonest causes of focal muscle wasting is trauma to a motor nerve. For example, a laceration at the wrist can sever the ulnar nerve, which innervates many of the small muscles in the hand, leading to pronounced wasting (and also to sensory loss, because the ulnar nerve contains sensory nerves as well as motor nerves). If the nerve is not surgically reconnected, the muscle fibres will die, muscle wasting will be permanent, and the muscles it supplies will be totally paralysed. A similar pattern of wasting can occur when the ulnar nerve is damaged at the elbow, perhaps because of a fracture there. Other types of mechanical trauma have more insidious effects, by the slow compression of nerves, such as in the carpal tunnel on the front of the wrist, or with protrusion of an intervertebral disc, when wasting may accompany pain, due to simultaneous damage to both motor and sensory nerve fibres. Another example of focal wasting occurs with a ‘Bell's palsy’ (an inflammatory paralysis of the facial nerve on one side). This results in inability to wrinkle the forehead muscles of that side, to close the eye and mouth, or to smile normally. This syndrome also illustrates the capacity of motor nerves to recover and to reinnervate the denervated face muscles. The new nerve fibres slowly extend down the nerve, although not always reaching the same muscle that they originally innervated. Thus a motor neuron that is active during blinking may come to innervate the muscles around the mouth, so that the mouth contracts with each attempted blink. This process of re-innervation rescues the muscle fibres from death.

Muscle wasting will also occur when a disease process affects motor nerves generally, a condition known as a peripheral neuropathy. Long nerves usually show signs of dysfunction before shorter nerves; thus wasting and weakness is usually first evident in the lower leg and in the hands. Among the many causes of peripheral neuropathies are toxic substances (organophosphates, for example); metabolic conditions such as diabetes; malnutrition leading to vitamin deficiencies; acute inflammatory neuropathies such as the Guillain–Barré syndrome, in which the motor nerves are attacked by cells of the immune system; and genetic disorders in which mutations lead to structural defects in the nerve. Some of these conditions are irreversible, but others, such as the Guillain–Barré syndrome, can partly or completely recover.

Finally, wasting can result from diseases that affect the anterior horn cells themselves. The commonest cause worldwide is the poliomyelitis virus, which is now partly controlled by immunization programmes. Those who survive the initial paralysis are left with varying degrees of muscle wasting, and in children this can interfere with growth, leading to limb shortening and spinal deformities. Motor neuron disease is a relatively rare cause of muscle wasting that is often accompanied by spontaneous muscle twitching (fasciculations). It usually affects individuals in middle or later life, but sometimes, especially in the very rare familial form of the disorder, may affect young people. Spinal muscular atrophy is a genetic disorder in which anterior horn cells fail to develop normally, leading to wasting and paralysis which in some forms are associated with death in infancy.

J. Newsom-Davis

See also central nervous system; motor neurons; muscle.