Multiple sclerosis symptoms tend to evolve according to four different courses. Each of these may be further described as "mild", "moderate" or "severe".
Relapsing-Remitting Multiple Sclerosis (RRMS)
Relapsing-Remitting Multiple Sclerosis (RRMS) is the most common "type" of MS, affecting 85% of people with multiple sclerosis. Multiple sclerosis symptoms appear as an "attack" with a sudden worsening of symptoms. These attacks (also called "flare-ups" or "exacerbations") are followed by a complete or partial remission of symptoms. There may be be periods of stability between these attacks that last months or years during which time there is no disease progression.
Primary-Progressive Multiple Sclerosis (PPMS)
Primary-Progressive Multiple Sclerosis (PPMS) affects about 10% of people with MS. It defines a type of multiple sclerosis that results in slowly worsening function from the time of disease onset with no distinct relapses nor remissions. The rate of progression for those with PPMS can change over time, with occasional periods of stability and temporary minor improvements.
Secondary-Progressive Multiple Sclerosis (SPMS)
Secondary-Progressive Multiple Sclerosis (SPMS) describes a type of MS that may develop in those with relapsing-remitting MS in which MS symptoms worsen more steadily. Before the advent of interferons and other multiple sclerosis treatments, about one-half of people with RRMS developed SPMS within 10 years.
Progressive-Relapsing Multiple Sclerosis (PRMS)
Progressive-Relapsing Multiple Sclerosis (PRMS) is relatively uncommon. People with PRMS experience a steady worsening of symptoms from the onset of symptoms with additional attacks that lead to declines in neurological functioning. There may or may not be some recovery following these attacks, but the disease continues to progress without full remissions.
PPMS, SPMS and PRMS are sometimes lumped together as described as "chronic progressive multiple sclerosis" to distinguish them from relapsing-remitting MS.
20% of people with multiple sclerosis have a benign form of the disease in which symptoms show little or no progression after the initial attack. These individuals remain fully functional.
A few patients experience "malignant MS", defined as a swift and relentless decline resulting in significant disability or even death shortly after disease onset. However, multiple sclerosis is very rarely fatal and most people with multiple have a fairly normal life expectancy.
Reference: National Institute of Neurological Disorders and Stroke (NINDS)