Clinically isolated syndrome (CIS) refers to an episode of neurological symptoms that may indicate the start of multiple sclerosis (MS). It may involve one or more symptoms.

Some people with CIS will have only a single symptom, such as vision loss from inflammation of the optic nerve. However, others will experience multiple symptoms that stem from inflammation in various parts of the brain. In either case, the symptoms last at least 24 hours.

Not everyone with CIS will develop MS, but if an MRI scan indicates the involvement of several parts of the brain, the risk of progression to MS is high.

According to the National Multiple Sclerosis Society, 70% of people who have CIS are aged 20–40 years, and the condition is two to three times more common in females than males.

This article discusses the symptoms, causes, diagnosis, and treatment of CIS, as well as the outlook for people with the condition. It also examines the relationship between CIS and MS.

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CIS can be monofocal or multifocal. Monofocal means that a person has a single symptom, whereas multifocal refers to the presence of more than one symptom. Multiple symptoms indicate damage in more than one area of the brain. A person with CIS does not have an infection or fever, and after the episode, they experience either a full or partial recovery.

The symptoms vary, depending on where in the brain the damage occurs.

Optic neuritis

Optic neuritis, which is the inflammation of the optic nerve, may cause symptoms such as:

  • reduced vision in one eye
  • eye pain, especially with eye movement
  • swelling in the eye
  • the inability of both pupils to react in the same way to light

Brain stem and cerebellum

The brain stem lies at the base of the brain and controls involuntary functions, such as the heartbeat and breathing. The cerebellum, which is close to the brain stem, coordinates movements. If CIS involves this area, the symptoms may include:

  • an inability to move both eyes together when looking to the side
  • uncontrolled movements of the eyes
  • uncoordinated movements of body parts
  • dizziness
  • loss of sensation in the face
  • problems with the muscles that play a role in speech

Spinal cord

Damage to the column of nerve tissue that runs down the center of the back may cause symptoms that include:

  • an inability to control the bladder
  • problems with bowel control
  • weakness on one side of the body
  • numbness or tingling of body parts

CIS occurs due to inflammatory demyelination. This term refers to inflammation that leads to a loss of myelin, a fatty substance that covers the nerve cells in the brain and spinal cord.

The function of myelin is to protect and speed nerve transmission. In MS, the attack on myelin is due to an abnormal immune system response.

Both CIS and MS involve symptoms that result from inflammation and a loss of myelin leading to dysfunction in the brain and spinal cord. The difference is that CIS is a single episode that occurs within a limited time, whereas MS is a chronic disease.

In people with CIS, an MRI scan will generally reveal involvement only in the brain region responsible for the current symptoms. However, in those with MS, this imaging test may show evidence of demyelination in multiple areas of the brain, sometimes including areas that previous episodes have affected.

When making a diagnosis, a doctor will usually take a medical history and carry out a physical exam to try to rule out other conditions that may cause inflammation. To help them do this, they may order blood tests and tests that analyze cerebrospinal fluid, a watery substance that cushions the brain and spinal cord.

An MRI scan can also help them identify or rule out other causes of the symptoms.

Many CIS episodes are mild and do not require treatment. However, doctors will likely recommend treatment if a person experiences pain in the eye, severe vision loss, or both, or if they have pronounced incoordination, dizziness, and dysfunction in the ability to move body parts.

The use of MS medications for CIS varies among clinicians and national guidelines, but commonly used treatments include:

High doses of corticosteroids

Doctors prescribe corticosteroids when a person’s symptoms are disabling. The medications lower inflammation and immune system activity.

Beta-interferon or glatiramer acetate

The Food and Drug Administration (FDA) has approved medications called disease-modifying therapies (DMTs). Rather than taking these by mouth, a person receives them through intramuscular injections. Older 2012 research suggests that they may prevent CIS from developing into MS or delay this progression.

CIS may be the first manifestation of MS, but it does not necessarily mean that someone will later develop MS.

The Department of Veterans Affairs (DVA) explains that if an MRI scan shows that a person with CIS has brain lesions similar to those that occur in MS, they have a 70–80% risk of developing MS. In contrast, if their MRI scan does not show such brain lesions, they have a 20–30% risk.

According to the DVA, classifying CIS as low or high risk is important because in high risk cases, doctors encourage immediate treatment to delay the onset of MS. Early treatment can also minimize disability.

Another risk factor for progression to MS is the manifestation of several symptoms. A person with only one symptom during the episode is less likely to develop MS than someone with an array of symptoms.

The authors of a 2018 study explain that because of limited long-term studies, the progression pattern of CIS to MS is not clear. However, they reviewed published studies and used the data to estimate that 85% of people with CIS develop MS within 20 years. They also note that the decrease in life expectancy of individuals with CIS is marginal.

Older research from 2012 sheds additional light on the outlook of people whose CIS develops into MS. The authors state that after 15–20 years, one-third of people with CIS will experience minimal or no disability, and one-half will have a progressive form of MS with increasing disability.

CIS is a single episode of neurological symptoms that lasts at least 24 hours. The symptoms depend on the area of the brain where inflammation occurs, but they may include vision loss, uncoordinated movements, and loss of bladder control.

In about 85% of people, a CIS episode is the first sign of MS. If the symptoms are mild, a person may not need treatment. For disabling symptoms, doctors may prescribe corticosteroids to reduce inflammation or DMTs to delay the progression to MS.