demyelinating diseases

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 CNS differs chemically and immunologically from that formed by Schwann cells peripherally. Thus, some myelin disorders (eg, Guillain-Barré syndrome, chronic inflammatory demyelinating polyneuropathy, some other peripheral nerve polyneuropathies—see Peripheral Nervous System and Motor Unit Disorders: Polyneuropathy) tend to affect primarily the peripheral nerves, and others affect primarily the CNS (see Table 1: Demyelinating Disorders: Disorders That Can Cause CNS Demyelination). The most commonly affected areas in the CNS are the brain, spinal cord, and optic nerves. Demyelination is often secondary to an infectious, ischemic, metabolic, or hereditary disorder. 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 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. Table 1 Disorders That Can Cause CNS Demyelination Category Disorders Hereditary disorders Phenylketonuria and other aminoacidurias Tay-Sachs, Niemann-Pick, and Gaucher’s diseases Hurler’s syndrome Krabbe’s disease and other leukodystrophies* Adrenoleukodystrophies* Adrenomyeloneuropathy* Leber’s hereditary optic atrophy and related mitochondrial disorders Hypoxia and ischemia Carbon monoxide toxicity and other syndromes of delayed hypoxic cerebral demyelination Progressive subcortical ischemic demyelination Nutritional deficiencies Central pontine myelinolysis (may also be caused by Na fluxes) Demyelination of the corpus callosum (Marchiafava-Bignami disease) Vitamin B12 deficiency Direct viral invasion of CNS Progressive multifocal leukoencephalopathy Subacute sclerosing panencephalitis Tropical spastic paraparesis/HTLV-1–associated myelopathy Primary demyelinating disorders Recurrent, progressive disorders (multiple sclerosis and its variants) Monophasic disorders such as optic neuritis, acute transverse myelitis, acute disseminated encephalomyelitis, and acute hemorrhagic leukoencephalitis Neuromyelitis optica *Some subtypes may also cause peripheral demyelination. HTLV-1 = human T-lymphotropic virus 1.