Multiple sclerosis (MS) is thought to be an autoimmune, inflammatory disease affecting the central nervous system and peripheral nerves.

The cause remains unknown, but some studies indicate a link between the Epstein Barr Virus, while others indicate environmental factors, a lack of vitamin D, or parasites as a stimulus of the persistent immune response in the central nervous system. It can be unpredictable and, in some cases, disabling. But not all forms of MS are the same.

To help distinguish between the different types of the condition, the National Multiple Sclerosis Society (NMSS) identified four distinct categories.

To accurately define the different forms of MS, in 1996, the NMSS surveyed a group of scientists who specialized in MS patient care and research. After analyzing the scientists’ responses, the organization categorized the condition into four primary types.

These course definitions were updated in 2013 to reflect advances in research. They are:

  • clinically isolated syndrome (CIS)
  • relapsing-remitting MS (RRMS)
  • primary-progressive MS (PPMS)
  • secondary-progressive MS (SPMS)

The four categories defined by the NMSS are now relied upon by the medical community at large and create a common language for diagnosing and treating MS. The categories’ classifications are based on how far the disease has progressed in each patient.

Clinically isolated syndrome (CIS) is a single episode of neurologic symptoms that lasts 24 hours or more. Your symptoms cannot be tied to fever, infection, or other illness. They’re the result of inflammation or demyelination in the central nervous system.

You might have only one symptom (monofocal episode) or several (multifocal episode).

If you have CIS, you may never experience another episode. Or this episode could be your first MS attack.

If an MRI detects brain lesions similar to those found in people with MS, there’s a 60 to 80 percent chance you’ll have another episode and a diagnosis of MS within a few years.

At this time, you might have a diagnosis of MS if an MRI detects older lesions in a different part of your central nervous system. That would mean you’ve had a previous attack, even if you weren’t aware of it.

Your doctor might also diagnose MS if your cerebrospinal fluid contains oligoclonal bands.

The most common type is relapsing-remitting MS (RRMS). According to the NMSS, approximately 85 percent of people with MS have this type at the time of diagnosis.

When you have RRMS you may experience:

  • clearly defined relapses or flare-ups that result in episodes of intensive worsening of your neurologic function
  • partial or complete remissions or recovery periods after the relapses and between attacks when the disease stops progressing
  • mild to severe symptoms as well as relapses and remissions that last for days or months

While the vast majority of people with MS have the RRMS form, some are diagnosed with a progressive form of the disease: primary-progressive MS (PPMS) or secondary-progressive MS (SPMS).

Each of these types indicates that the disease continues to worsen without improvement.

This form of MS progresses slowly yet steadily from the time of its onset. Symptoms stay at the same level of intensity without decreasing, and there are no remission periods. In essence, patients with PPMS experience a fairly continuous worsening of their condition.

However, there can be variations in the rate of progression over the course of the disease — as well as the possibility of minor improvements (usually temporary) and occasional plateaus in symptom progression.

The NMSS estimates that approximately 15 percent of people with MS have PPMS at the onset of the condition.

SPMS is more of a mixed bag. Initially, it may involve a period of relapsing-remitting activity, with symptom flare-ups followed by recovery periods. Yet the disability of MS doesn’t disappear between cycles.

Instead, this period of fluctuation is followed by a steady worsening of the condition. People with SPMS may experience minor remissions or plateaus in their symptoms, but this isn’t always the case.

Without treatment, about half of people with RRMS go on to develop SPMS within a decade.

Early MS can be challenging for doctors to diagnose. As such, it can be helpful to understand the characteristics and symptoms of MS at the time of initial diagnosis — particularly since the vast majority of people with the disease exhibit characteristics of relapsing-remitting MS.

Although MS currently has no cure, it isn’t normally fatal. In fact, most people who have MS never become severely disabled, according to the NMSS.

Identifying MS early at the relapsing-remitting stage can help ensure prompt treatment to avoid developing more progressive forms of the illness.