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-Barré syndrome Guillain-Barré Syndrome (GBS) Guillain-Barré syndrome is an acute, usually rapidly progressive but self-limited inflammatory polyneuropathy characterized by muscular weakness and mild distal sensory loss. Cause is thought... read more , chronic inflammatory demyelinating polyneuropathy Chronic Inflammatory Demyelinating Polyneuropathy (CIDP) Chronic inflammatory demyelinating polyneuropathy is an immune-mediated polyneuropathy characterized by symmetric weakness of proximal and distal muscles and by progression continuing > 2 months... read more , some other peripheral nerve polyneuropathies Polyneuropathy A polyneuropathy is a diffuse peripheral nerve disorder that is not confined to the distribution of a single nerve or a single limb and typically is relatively symmetrical bilaterally. Electrodiagnostic... read more ) 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 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.
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