Multiple sclerosis, or MS, is a disease of the central nervous system, the network of nerves found throughout your brain, spinal cord, and the rest of your body.

Your nervous system is made up of nerve fibers that transport electrical impulses.

These nerve fibers are wrapped in a fatty tissue called myelin, which helps transmit the impulses.

In multiple sclerosis, the myelin sheath that covers nerve fibers becomes inflamed and gradually is destroyed, leaving areas of patchy scar tissue (sclerosis, also called plaque or lesions) that disrupt the electrical impulses between the brain and other parts of the body.

In addition, the underlying nerve fibers can also be damaged or destroyed.

This nerve damage leads to a variety of MS symptoms, such as numbness or tingling, balance problems, weakness, pain, muscle spasms, and blurred vision.

MS Prevalence and Statistics

More than 2.3 million people are affected by multiple sclerosis worldwide, according to the National Multiple Sclerosis Society (NMSS), making it a relatively rare disease.

More than 10 percent of them, between 250,000 and 350,000, live in the United States.

MS is more common in women than men, although the sex difference is a lot smaller for some types of the disease.

Who Develops MS?

Anyone can develop MS, but many patients share the following characteristics:

Age: The majority experience their first symptoms between the ages of 20 and 40.

Race: Caucasians are more than twice as likely as other races to develop MS.

Gender: MS is 2 to 3 times more common in women as in men, according to the NMSS.

Climate: MS is five times more common in temperate climates — like the northern United States, Canada, and Europe — than in tropical climates.

Genes: People whose close relatives have MS are more susceptible to developing the disease, but there is no evidence the disease is directly inherited.

What Causes MS?

Scientists don’t know exactly what causes multiple sclerosis.

But there’s increasing evidence that the body’s immune system plays a prominent role in its development, and some scientists think MS is an autoimmune disease (like rheumatoid arthritis, psoriasis, and type 1 diabetes).

Your immune system, which usually protects the body by fighting foreign bodies such as bacteria, may instead attack the myelin in your central nervous system.

Some researchers suspect that these attacks are triggered by certain kinds of viral infections.

Researchers have also observed that some groups of people are more susceptible to MS than others, suggesting there’s a genetic component to the disease.

Types of Multiple Sclerosis

MS follows a few recognized patterns:

Relapsing-remitting MS (RRMS): In this type of MS, the patient experiences a series of actively symptomatic periods, called attacks, flares, or relapses.

These attacks are followed by quiet periods called remissions, during which symptoms become much less severe.

Remissions may last months or years before a relapse occurs.

RRMS is the most common type of MS — estimates vary, but between 80 and 90 percent of people with MS are diagnosed with this relapsing-remitting course of the disease, and most people with RRMS eventually develop secondary-progressive MS.

Secondary-progressive MS (SPMS): This pattern begins after a relapsing-remitting course. The disease advances progressively, punctuated by acute attacks.

More than half of patients who start out with RRMS will develop SPMS within 10 years, with the number rising to 90 percent within 25 years.

Primary-progressive MS (PPMS): This pattern is marked by a gradual worsening of symptoms. Patients generally do not experience acute exacerbations.

While there are no distinct remissions, patients with PPMS may have temporary plateaus during which symptoms lessen somewhat. About 10 to 15 percent of people with MS are diagnosed with PPMS, according to Johns Hopkins Medicine.

Progressive-relapsing MS (PRMS): In this pattern, patients experience gradual progression of disease that’s accompanied by acute exacerbations as well.

PRMS is a relatively rare form of the disease: Fewer than 10 percent of people with MS have PRMS.

In addition to these four types of MS, a small percentage of people with MS have a benign form of the disease, according to Johns Hopkins Medicine.

In the benign form of MS, after the initial attacks, symptoms progress very little over the course of a person’s lifetime.

There’s some controversy over how (or whether) to classify people with benign MS, since the progress of the disease can vary over a person’s lifetime, according to the NMSS.

And a small number of patients have malignant MS, which is marked by a rapid decline that leads to disability and possibly death.

MS Complications

Bladder problems are very common in people with MS, affecting at least 80 percent of them, according to the National MS Society.

Some people have trouble holding their urine (incontinence), while others can’t fully empty their bladder (retention).

This bladder dysfunction may also lead to kidney infections.

If microbes find their way into the bloodstream, the infection can cause sepsis, a whole-body inflammation that, in turn, may cause organ failure and death.

In fact, sepsis may be the biggest cause of MS-related deaths, according to a 2014 report in the journal PLoS One.

Sometimes, people with MS have trouble chewing and swallowing. This can allow foods and liquids to deposit in the lungs.

That may lead to a potentially fatal complication: aspiration pneumonia, which develops from inflammation and fluid accumulation in the lungs.

MS may also cause the respiratory muscles to become weakened, reducing airway clearance, which raises the risk of lung and other respiratory tract infections.

Lung infections were the second-biggest cause of MS-related deaths, PLoS One reported in 2014; other research gives it the top spot.

Many people with MS require canes or crutches, which increases the risk of physical trauma from falls and accidents.

MS and Depression

Depression is common among people who have MS, though scientists don’t fully understand the relationship between depression and MS.

On the one hand, depression may be a direct result of the immune system’s attack on the protective myelin sheaths that envelop nerve fibers, causing behavioral changes, including depression.

Multiple sclerosis can also change what is known as the body’s neuroendocrine system, which oversees hormone release, including hormones implicated in depression, such as serotonin.

On the other hand, depression may develop as a result of the stresses and challenges associated with having MS.

Additionally, the medications used to treat MS, such as interferon beta, can also cause depression.

MS Prognosis

Though MS can sometimes be a debilitating disease, the majority of people who have it don’t become severely disabled, according to the National MS Society.

About a third of people with MS completely lose their ability to walk, while many others can function using canes, crutches, or scooters or motorized wheelchairs for long distances.

People with MS who have the best prognosis are usually those who:

  • Are female
  • Were younger than 30 when the disease started
  • Have infrequent attacks
  • Have RRMS
  • Have few signs of disease in diagnostic scans

MS Life Expectancy

MS is rarely fatal, and many people with MS have a life expectancy that’s about as long as that of the general population.

Additionally, with better treatments now available, the life expectancy for people with MS has increased over the years.

According to a 2014 study published in the journal Multiple Sclerosis and Related Disorders, people in the United States with MS have a life expectancy that’s six years shorter than people without the disease,

MS is associated with various life-threatening complications, hence the lower life expectancy.

There’s no cure for MS, but there are numerous MS medications that may slow the progression of some forms of the disease and reduce the frequency and severity of relapses.

People with MS who take these medications have a better life expectancy than those who don’t, a 2012 research report in the journal Neurology showed.