Multiple sclerosis (MS) is caused by inflammation that damages myelin, the fatty coating around nerve cells, leading to disease symptoms. Clinically isolated syndrome, or CIS, is the first episode of neurological symptoms that last at least 24 hours and are suggestive of MS.
To be diagnosed with overt MS, a person must experience multiple episodes of neurologic symptoms caused by this kind of brain inflammation — that’s what the “multiple” refers to in “multiple sclerosis.”
CIS most commonly occurs in people between the ages of 20 and 40, and it is two to three times more common in women than in men.
Symptoms of CIS are generally similar to what can occur in a disease relapse in true MS. Some people with CIS have monofocal disease, meaning they just experience one type of symptom — for example, vision problems due to inflammation of the nerves that connect the eye to the brain, termed optic neuritis — due to a single lesion.
Others will have multifocal disease, where multiple symptoms and signs are present simultaneously, due to the presence of lesions in multiple areas of the central nervous system, which is comprised of the brain and spinal cord.
Some people with CIS will go on to have further episodes of symptoms, at which point they may be diagnosed with true MS. It also is possible for brain imaging to reveal signs of previous relapses in someone initially suspected of having CIS, so a diagnosis of MS may be made right away.
However, not all people with CIS will go on to develop MS, though the reasons for this are incompletely understood. Generally, people with CIS who have more MS-like brain lesions on MRI scans are at higher risk of later developing MS.
Other clinical, biological, and epidemiological factors — such as smoking, vitamin D levels, and age — also may influence this risk.
Certain MS treatments are approved for use in people with CIS, and may help to reduce the risk of conversion to true MS. The use of these medicines needs to be weighed against their risks, and should be discussed in detail between patients and their healthcare providers.
Last updated: Sept. 28, 2021
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