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Clinically Isolated Syndrome (CIS)

Learn what it means to have a CIS, how a CIS can evolve into MS, and what treatment options are available to delay an MS diagnosis.

The Diagnostic Criteria for MS

In order to make a diagnosis of MS, the physician must:

  • Find evidence of damage in at least two separate areas of the central nervous system (CNS) AND
  • Find evidence that the damage occurred at least one month apart AND
  • Rule out all other possible diagnoses

Diagnosing MS

Diagnosing MS


The term “clinically isolated syndrome” (CIS) is used to describe a first episode of neurologic symptoms  that lasts at least 24 hours and is caused by inflammation and demyelination in one or more sites in the central nervous system (CNS). CIS can be either monofocal or multifocal:

  • Monofocal episode: The person experiences a single neurologic sign or symptom — for example, an attack of optic neuritis — that’s caused by a single lesion.
  • Multifocal episode: The person experiences more than one sign or symptom — for example, an attack of optic neuritis accompanied by weakness on one side — caused by lesions in more than one place.

Individuals who experience a CIS may or may not go on to develop MS. In diagnosing CIS, the physician faces two challenges: first, to determine whether the person is experiencing a neurologic episode caused by damage in the CNS, and second, to determine the likelihood that a person experiencing this type of demyelinating event is going to go on to develop MS.

  • High risk of developing MS:  When CIS is accompanied by MRI-detected brain lesions that are similar to those seen in MS, the person has a high likelihood of a second neurologic event and diagnosis of MS within several years. 
  • Lower risk of developing MS:  When CIS is not accompanied by MRI-detected lesions, the person has a lower likelihood of developing MS than the high-risk individual over the same time period.

An accurate diagnosis at this time is important because people with a high risk of developing MS are encouraged to begin treatment with a disease-modifying medication in order to delay or prevent a second neurologic episode and, therefore, the onset of MS. In addition, early treatment may minimize future disability caused by further inflammation and damage to nerve cells, which are sometimes silent (occurring even if no symptoms can be observed). Several medications are now approved by the U.S. Food and Drug Administration (FDA) for CIS: Avonex®, Betaseron®, Extavia® and Copaxone®.

Read more about CIS.