Discussion Author: Reginald L. Baker
Multiple sclerosis(MS) is by far the most common of all demyelinating diseases - except for age-related vascular (small vessel) disease.
The precise etiology of MS remains unknown, although most investigators favor autoimmune-mediated demyelination in genetically susceptible individuals. An underlying virus may serve as an immunogen. Though MS most often affects young adults of Northern European ancestry and occurs most often in temperate climates, it has world-wide racial and geographic distribution. The onset of symptoms is usually between 20 and 40 years of age and female to male ratio in adults is 1.7-2:1.
More than 85% of MS patients have ovoid periventricular lesions that are oriented perpendicularly to the long axis of the brain and lateral ventricles. The next most common site is the corpus callosum, involved in 50-90% of patients with clinically definite MS. In adults the brainstem and cerebellum are comparatively less common sites. Approximately 10% of MS plaques in adults are infratentorial, whereas the posterior fossa is a frequent site of MS plaques in children and adolescents.
Computed tomography (CT) scans are often normal in early disease. Lesions are typically iso- or hypodense with brain on noncontrast enhanced CT (NECT). Enhancement after contrast is variable with some showing no change and others enhancing intensely. Both solid and ringlike patterns are observed.
Most MS plaques are iso- to hypointense on T1-weighted scans and hyperintense compared to brain on T2-weighted scans. Because there are many causes of white matter hyperintensities on T2W1, most authorities require the presence of three or more discrete lesions that are 5 mm or greater in size, as well as lesions that occur in a characteristic location and have a compatible clinical history, to establish the MR diagnosis of MS. Oblong lesions at the callososeptal interface are typical. Enhancement following contrast administration represents blood-brain barrier disruption. Enhancement is highly variable and typically transient, seen during the active demyelinating stage. Both solid and ringlike enhancement patterns are seen.
Cranial neuropathies, particularly optic neuritis, are common in patients with MS. Lesions in the brainstem tracts and nuclei are seen on T2W1, whereas contrast-enhanced T1-weighted studies may delineate enhancement in the nerves themselves. Fat-saturation and short inversion recovery scans are useful in separating optic nerve enhancement form high-signal orbital fat.
Other diseases that affect the CNS white matter may appear clinically and radiographically similar to MS. Considerable care must be taken to exclude these disease processes prior to making a diagnosis. These include CNS tumors (especially lymphomas and gliomas), spinal cord compression, vasculitides, Behcetâ€™s disease, neurosarcoidosis, post-infectious and post-vaccinal encephalomyelitis, human immunodeficiency virus (HIV) encephalopathy, Lyme disease, and vitamin B12 deficiency.
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