Multiple sclerosis (MS) is a neurological disorder characterized by the loss of the myelin sheath, the insulating cover around nerve fibers, as the result of an erroneous inflammatory attack by the immune system.
The loss of myelin leads to impaired nerve function in the brain and/or spinal cord, ultimately resulting in the disease’s symptom. Specific symptoms vary from person to person, depending on which specific parts of the nervous system are most affected.
MS is categorized into four main types by the National MS Society Advisory Committee on Clinical Trials in MS. These types are based on the phase and severity of disease progression. They are: clinically isolated syndrome (CIS); relapsing-remitting MS (RRMS); secondary progressive MS (SPMS); and primary progressive MS (PPMS). There also are rarer types of MS and MS-like disease.
Relapsing-remitting MS (RRMS)
The most common form of MS is RRMS, which is estimated to account for about 85% of all cases.
This form of the disease is characterized by relapses (also called exacerbations), which are defined by the appearance of new symptoms or the return of old symptoms for 24 hours or more, without a change in the body temperature or infection.
These relapses are followed by remissions, which are periods of partial or complete recovery from symptoms. During remissions, all of the symptoms may disappear or some may continue and become permanent. However, no apparent progression of the disease occurs during this time.
Relapsing-remitting multiple sclerosis can be characterized as active (with relapses or evidence of new activity seen on an MRI), or not active, and worsening or not worsening — meaning there may be or not be an increase in disability over a specific period of time following the relapse.
Biologically, RRMS differs from progressive types of MS because in RRMS the relapses represent new inflammatory attacks on the brain and/or spinal cord. By contrast, much less inflammation is present in the progressive forms of the disease.
Clinically isolated syndrome (CIS)
CIS is the first clinical presentation of disease characterized by inflammatory demyelination. Conceptually, CIS can be thought of as a “single relapse” of RRMS. People with CIS may never have another episode of symptoms, or they could progress to full MS at a later date.
The 2017 diagnostic criteria state that a diagnosis of MS requires two nervous system attacks at different locations and at two different times. That’s why it is called “multiple” sclerosis. As such, patients generally are considered to have CIS until physicians find more evidence for MS.
If CIS is accompanied by MRI findings that show a past episode had occurred, an MS diagnosis can be made. People with CIS and brain lesions have a 60–80% chance of progressing to MS and may be treated with disease-modifying medicines. Early treatment could delay the onset of MS or prevent a second neurologic episode.
CIS can be monofocal or multifocal. Monofocal CIS is a single neurologic sign or symptom caused by a single lesion. Multifocal CIS is the experience of one or more symptoms caused by more than one lesion.
Secondary progressive MS (SPMS)
SPMS is a stage of MS that follows RRMS. With this type of MS, a person’s symptoms steadily worsen, even if the individual experiences no relapses.
Symptom flareups still may occur in SPMS, but symptom changes are generally much less drastic than in the RRMS stage and symptoms do not disappear even in the remission phases. A diagnosis of SPMS usually comes after reviewing the progression of the disease for six months, though there is no single test that defines when MS changes from RRMS to SPMS.
Most people with RRMS eventually will develop SPMS. However, because of advancements in disease-modifying treatments, fewer people today develop SPMS than ever before, and the transition to SPMS tends to occur later in life.
The reasons that RRMS develops into SPMS are not fully understood. The median time from the onset of RRMS to progression to SPMS is about two decades. The progression tends to go faster in people who were older at the onset of RRMS, and in those who experience an incomplete recovery from their first relapse.
SPMS can be classified as active or non-active, based on MRI evidence of increasing nervous system damage and whether an individual experiences occasional relapses. SPMS also can be classified as with or without progression, based on how symptoms change over time.
In RRMS, symptoms are thought to be driven by active inflammation that causes damage in the brain. By contrast, SPMS is driven mainly by neurodegeneration — nerve damage that continually worsens over time — mostly without active inflammation.
Primary progressive MS (PPMS)
Approximately 15% of people with MS are diagnosed with primary progressive multiple sclerosis (PPMS).
Like SPMS, PPMS is characterized by symptoms that become worse over time, without the periods of relapse and remission that characterize RRMS. However, disease progression starts right from disease onset — hence the term “primary” progressive.
PPMS can be classified as either active or non-active, depending on whether a patient experiences occasional relapses or there is evidence of new lesions on an MRI. It also can be defined as with or without progression, based on symptom changes over time.
An accurate diagnosis of PPMS requires at least three characteristics. The patient must have:
- one year of disease progression (worsening of neurological function without remission), and two additional characteristics that could include a brain lesion that is recognized as typical of MS;
- two or more lesions of a similar pattern in the spinal cord;
- evidence of immune system activity in the central nervous system (such as the presence of certain antibodies in the fluid around the CNS).
People with PPMS typically have fewer brain lesions and inflammation than people with RRMS, but usually have more spinal cord lesions.
Other forms of MS
Other terms are sometimes used to describe rare versions of MS and MS-like diseases.
Malignant MS is a form of MS in which the disease progresses very rapidly with severe relapses. Patients with this form of the disease typically require an aid to walk 100 meters (328 feet) within five years of disease onset. Other names for this form are fulminant/fulminate, Marburg, aggressive, or advanced MS.
Radiologically isolated syndrome (RIS)
Sometimes, a person will have evidence of brain lesions that look like MS, as visible via MRI, but will not have overt symptoms of MS. Such cases are termed RIS. Someone with RIS should be monitored and may be diagnosed with MS later if more symptoms appear.
Another form of MS is inactive or benign MS. This form of the disease is a variant of RRMS that shows little or no change over many years, but could still change at a later date. A person is typically said to have this form if they have gone about 15 years or longer with few symptoms and little disability. Another term sometimes used for inactive MS is “burned-out” MS, in which the disease progression appears to slow after a long period of worsening.
Pediatric MS (POMS)
While MS usually occurs in adults, it can affect children younger than 18, in which case it is referred to as pediatric MS or pediatric-onset MS (POMS). Children with POMS have RRMS exclusively and tend to have more frequent relapses than adults with early MS, though children also tend to recover from relapses more quickly.
Last updated: June 17, 2021
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