Multiple sclerosis (MS) is a progressive neurological disorder characterized by demyelination, which is the loss of the insulating cover around nerve fibers called the myelin sheath. This loss leads to delayed or missing nerve signals.
MS is classified into four main types by the National MS Society Advisory Committee on Clinical Trials in MS. The types are based on the phase and severity of disease progression. The four types are: clinically isolated syndrome (CIS); relapsing-remitting MS (RRMS); secondary progressive MS (SPMS), and; primary progressive MS (PPMS)
Clinically isolated syndrome (CIS)
In CIS, a patient first experiences MS-like symptoms resulting from nerve inflammation and demyelination lasting more than 24 hours, such as vision problems, difficulty walking, or numbness. Patients with CIS may never have another episode of symptoms, or they could progress into MS at a later date. Patients are considered to have CIS until physicians find more evidence for MS. If MRI imaging shows a lesion in the nervous system, it is likely the symptoms will reoccur and the patient will be diagnosed with MS. The 2017 diagnostic criteria state that a diagnosis of MS requires two nervous system attacks at different locations and at two different times. Sometimes attacks can occur without symptoms and MRI imaging can be used to find other regions of the nervous system that might have been affected.
Relapsing-remitting MS (RRMS)
The most common form of MS is RRMS estimated to account for 80-85 percent of all cases. This form of the disease is characterized by periods of neurodegeneration where new symptoms occur or old symptoms worsen (relapsing), and periods of rest where some or all normal function is restored (remitting). While patients may recover during the remission periods, the disease generally progresses over time and some symptoms are likely to persist even during remission periods.
Secondary progressive MS (SPMS)
RRMS often continues to worsen and eventually may change to SPMS, at which time the disease transitions from relapse and remission phases into a steady progression of symptoms. There still may be periods when the disease is said to be active or not active based on MRI evidence of increasing nervous system damage, and with or without progression based on changes in symptoms. A diagnosis of SPMS usually comes after reviewing the progression of the disease, but there currently are no clear criteria for determining when MS changes from RRMS to SPMS.
Primary progressive MS (PPMS)
In PPMS, the disease starts in the attack phase and there are very few periods of remittance (recovery). However, there sometimes may be periods of stability when symptoms do not worsen. As in the case of SPMS, there are times when the disease can be active or not active, with or without progression. Periods of remission may occur later in the disease. It is estimated that 10-15 percent of patients have PPMS.
Other terms are sometimes used to describe rare versions of MS progression.
Malignant MS is a form of MS similar to PPMS in which the disease does not have periods of relapse and remission, but instead progresses very rapidly. Other names for this form are fulminant/fulminate, Marburg, aggressive, or advanced MS.