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Multiple sclerosis (MS) is a condition that disrupts the information pathways that connect your brain and your body. From time to time, the symptoms of MS can unexpectedly worsen. When symptoms suddenly flare up, it’s called a relapse or an exacerbation.

Relapses are periods of new or worsening MS symptoms that last 24 hours or longer. Most of the time, a relapse happens after a gap of 30 days of longer without a flare-up.

The most common kind of MS, called relapsing-remitting multiple sclerosis (RRMS), adheres to this pattern: An exacerbation followed by a stretch of time when symptoms either decrease or go away completely.

About 85 percent of people with MS are diagnosed with RRMS, which means that most people with MS can expect to experience relapses throughout their lives.

It’s important to note that, for some people, the changes brought on by a relapse may not go away. They become the new normal.

A relapse can be mild, or it can be so severe that it’s debilitating. During a relapse, you could notice any or all of the symptoms discussed below.

Fatigue

Most people feel tired from time to time, but the fatigue caused by an MS relapse is different. It is not just feeling tired or sleepy. It’s an actual decrease in your ability to perform tasks because of physical, psychological, or neurological changes.

As many as 92 percent of people with MS have fatigue. It can not only affect your quality of life but also be disabling.

Numbness

When MS affects your spine, one of the results can be tingling or numbness in your arms, legs, face, or other body parts. You could also experience weakness or extra sensitivity, especially to heat or touch.

Vision problems

If your optic nerve is being affected by MS, your symptoms could include changes in your ability to see clearly. In fact, common early MS symptoms can include:

  • blurred vision
  • double vision
  • loss of colored vision
  • pain with eye movement

It’s important to note that total vision loss is rare with MS.

Spasticity or stiffness

Research published in 2020 shows that 86 percent of those with MS experience muscle stiffness (spasticity) and more than half report muscle spasms (tremors). These intense contractions of your muscles can be painful and can interfere with your ability to move around and to sleep comfortably.

Bowel or bladder problems

During a relapse, you may experience new or worsening bladder issues, including:

  • having to urinate more often
  • feeling more urgency when you have to urinate
  • leaking urine

MS relapses can also cause constipation or an inability to control bowel movements. Researchers report that between 39 and 73 percent of those with MS have trouble with bowel movements.

Problems with cognition

MS can cause cognitive fog, or cog fog, which affects your ability to:

  • remember
  • pay attention
  • make decisions
  • complete a process
  • find the right word
  • think clearly and efficiently

However, in a 2018 study, researchers found that the only thinking skill that worsened during an MS relapse is information processing speed. In other words, while MS affects many of your cognitive functions, it’s likely that the only skill to worsen during a relapse is the speed at which you process information.

What if I have a fever?

Health experts note that if you have a fever, what you’re experiencing is probably not a relapse of MS. A fever indicates another health issue may be at the root of the symptoms you’re having.

Research has shown that an MS relapse happens when there’s a breach in your blood-brain barrier, allowing immune cells to penetrate and attack your nervous system. The resulting inflammation destroys myelin, a protein-rich sheath that protects neurons that are vital to our ability to think, feel, move, and function.

During a relapse, new or larger lesions (damaged areas) form in your brain, spinal cord, or optic nerves. The symptoms you experience depends on the extent of the damage and the areas involved.

Although researchers are still working to determine exactly what prompts a relapse, these factors appear to raise your risk:

Vitamin D

A 2019 journal article notes that people with MS who don’t have enough vitamin D in their bodies have a higher risk of experiencing a relapse. The journal article notesa study involving 7 million members of the U.S. military found that increasing vitamin D levels led to a 42 percent lower risk of MS and MS relapses.

Hormones

Hormones, especially reproductive hormones, can affect the likelihood of a relapse or the types of symptoms you experience during a relapse.

In clinical and animal studies, high levels of the hormone estrogen were shown to lower the risk of MS relapse. The National Multiple Sclerosis Society notes that women typically experience fewer MS relapses during pregnancy, due to bodily changes — such as hormone production.

Genetics

Researchers have pinpointed several genetic factors that could lead to more frequent relapses. One 2018 study found that a variation in LRP2, a nucleotide that is one of the building blocks of DNA, causes a significant increase in the risk of relapse. Other genes have been implicated in this process as well.

Age

Adults who are older than age 50 have a higher risk of relapse than younger adults. It’s possible that age-related inflammation causes this higher risk. It also may be due to an increasing number or severity of other health problems that cause your MS symptoms to worsen.

Infections

Both bacterial and viral infections can trigger an MS relapse. Researchers estimate that 27 percent of relapses are related to an infection in the weeks before a flare-up.

The frequency of relapses varies from person to person. It can depend on each person’s overall health and risk factors (described above).

In a 2018 study that evaluated relapse patterns among 5,311 people with MS, roughly 35 percent had one to two relapses in a year. Just over 20 percent had more than two relapses in a year. Symptoms generally go away or decrease in severity, but some can last for a month or longer.

If you’re having a relapse, one of the most common treatments is a short but intense course of corticosteroids to reduce the inflammation in your body. Your treatment could look something like this:

  • 3 to 5 days of high-dose corticosteroids given orally or by IV
  • 3 to 4 weeks of oral steroids, given in gradually smaller doses

Some people don’t respond well to corticosteroids. In those cases, treatment might involve therapeutic plasma exchange or immunoabsorption. These are in-hospital treatments that remove antibodies from your blood and replace them with filtered or “purified” plasma.

If you have MS, your symptoms may unexpectedly flare up from time to time, especially if you have RRMS. You could experience greater fatigue, numbness, difficulty with your eyesight, muscle spasms, stiffness, or bowel and bladder problems.

A relapse can last anywhere from 24 hours to a month or more, but treatment could lessen the severity or bring about a remission more quickly. Your doctor may prescribe corticosteroids or a plasma exchange to reduce the inflammation causing the flare-up.

Getting through a relapse isn’t easy. It’s important to work with healthcare professionals and seek the support of people you trust so you can cope with the difficulties until you’re feeling better.