Multiple sclerosis, the most common primary disorder of myelin, is characterized by episodes of demyelinization scattered throughout the CNS and sparing the PNS. In this disease, myelin is most likely normally formed but breaks down later in life. MS is a disease of young adults; most initial symptoms occur between the ages of 15 and 40, but can occur as early as 10 or as late as 80. However, new attacks after age 55 are rare. Females tend to be more frequently affected than males.
MS is typified by the clinical presentation of neurologic symptoms that are separated by "space and time", i.e. neurologic symptoms arise from varying anatomic locations and occur at different points in time. For a diagnosis of MS, these six criteria are widely accepted:
1. the results of a neurological examination must be abnormal;
2. at least two separate parts of the nervous system must be involved;
3. disease of white matter (fiber tract damage) must predominate;
4. there must be two or more episodes of worsening symptoms separated by one month or more, or else slow or stepwise progression over at least six months;
5. age of onset is between 10 and 50 years;
6.   no other condition can explain the disease process (other lesions must be ruled out).
Although the typical course of MS is one of exacerbations and remissions, the disease may occasionally follow a steadily progressive course. The CSF is abnormal in many but not all cases. CSF protein electrophoresis shows an increase in oligoclonal bands; in addition, IgG antibodies directed against myelin basic protein may be found in the CSF of MS patients with active disease. Histopathological exam reveals diffuse dissemination of the demyelinated plaques throughout the neuraxis, but there are preferential areas of involvement. It is common to find areas of demyelination in a periventricular (especially periatrial region) distribution. The optic
pathways also tend to be involved, so visual symptoms are a common presentation.
MR is the procedure of choice for diagnosis of MS, as it is very sensitive for the evaluation of demyelinating disease and because CT scan may be normal in patients with MS. CT and MR are very useful in ruling out the presence of a mass lesion in a patient with MS.
There are common and less common abnormal CT and MR appearances of MS. Common appearances include enlarged ventricles; enlarged cortical sulci; combination of enlarged cortical sulci and ventricles (atrophy); periventricular plaques; periventricular areas of enhancement; and plaques in the corpus callosum. Less common appearances include plaques remote from the periventricular distribution; areas of cystic plaque formation remote from the ventricles; areas of parenchymal as well as periventricular enhancement secondary to acute demyelination; space-occupying areas of enhancement; atrophy of the corpus callosum; plaques in the middle cerebellar peduncle; single large area of increased signal intensity, nonspace-occupying; and plaques in the spinal cord.