Multiple sclerosis is a neurodegenerative disorder that damages the nerves in the brain and spinal cord, leading to problems with muscle movement, balance and vision. The illness is an example of a demyelinating disease, where the protective coating called myelin that surrounds nerve fibres becomes damaged.
Multiple sclerosis follows a different course in every individual but there are three main ways in which the disease can progress depending on which form of the illness a patient has.
Relapsing remitting multiple sclerosis
Around 80% of all individuals with multiple sclerosis have the relapsing remitting form of the disease. These individuals have periods where their symptoms are mild or absent (remission), followed by periods of symptom relapse. Symptoms may occur suddenly and in acute bouts or exacerbations.
During these periods of relapse, symptoms may become worse each time and the relapsing remitting form of this condition may eventually progress to secondary progressive multiple sclerosis, where there are few or no periods of remission. Relapsing remitting multiple sclerosis may be diagnosed when two episodes of relapse are separated by more than 30 days or there has only been one relapse but there is MRI evidence of newly scarred or damaged myelin three months later.
Secondary-progressive multiple sclerosis
Patients with this form of multiple sclerosis often experience phases of relapse followed by remission at first, but this later gives way to progressive disease, characterized by worsening symptoms and few or no periods of remission.
Primary-progressive multiple sclerosis
The least common form of multiple sclerosis is the primary progressive form which occurs in about 10% to 15% of all cases and usually in people aged over 40 years. In this form of the condition, symptoms get worse over time rather than occurring in bouts or as sudden attacks.
Primary progressive multiple sclerosis may be diagnosed if there have been no previous symptoms of relapse but the patient has become increasingly disabled over a period of at least one year.