Overview of demyelinating disorders

[Sonia]

Most nerve fibers inside and outside the brain are wrapped with many layers of tissue composed of a fat (lipoprotein) called myelin. These layers form the myelin sheath. Much like the insulation around an electrical wire, the myelin sheath enables nerve signals (electrical impulses) to be conducted along the nerve fiber with speed and accuracy. When the myelin sheath is damaged, nerves do not conduct electrical impulses normally. Sometimes the nerve fibers are also damaged.
If the sheath is able to repair and regenerate itself, normal nerve function may return. However, if the sheath is severely damaged, the underlying nerve fiber can die. Nerve fibers in the central nervous system (brain and spinal cord) cannot fully regenerate themselves. Thus, these nerve cells are permanently damaged.
Some disorders that cause demyelination affect mainly the central nervous system. Others, such as chronic inflammatory demyelinating polyneuropathy, affect mainly nerves in other parts of the body.

[Vania]

Myelin sheaths cover many nerve fibers in the central and peripheral nervous system; they accelerate axonal transmission of neural impulses. Disorders that affect myelin interrupt nerve transmission; symptoms may reflect deficits in any part of the nervous system.
Myelin formed by oligodendroglia in the central nervous system (CNS) differs chemically and immunologically from that formed by Schwann cells peripherally. Thus, some myelin disorders (eg, Guillain-Barr#233; syndrome, chronic inflammatory demyelinating polyneuropathy, some other peripheral nerve polyneuropathies) tend to affect primarily the peripheral nerves, and others affect primarily the CNS (see table Disorders That Can Cause Demyelination of the Central Nervous System). The most commonly affected areas in the CNS are the brain, spinal cord, and optic nerves.
Demyelination is often secondary to an infectious, an ischemic, a metabolic, or a hereditary disorder or to a toxin (eg, alcohol, ethambutol ). In primary demyelinating disorders, cause is unknown, but an autoimmune mechanism is suspected because the disorder sometimes follows a viral infection or viral vaccination.
Demyelination tends to be segmental or patchy, affecting multiple areas simultaneously or sequentially. Remyelination often occurs, with repair, regeneration, and complete recovery of neural function. However, extensive myelin loss is usually followed by axonal degeneration and often cell body degeneration; both may be irreversible.
Demyelination should be considered in any patient with unexplained neurologic deficits. Primary demyelinating disorders are suggested by the following:

Diffuse or multifocal deficits
Sudden or subacute onset, particularly in young adults
Onset within weeks of an infection or vaccination
Deficits that wax and wane
Symptoms suggesting a specific demyelinating disorder (eg, unexplained optic neuritis or internuclear ophthalmoplegia suggesting multiple sclerosis)

Specific tests and treatment depend on the specific disorder.