The name "multiple sclerosis" means that a patient has more than one sclerosis. A sclerosis is a plaque, or hardened tissue. As mentioned in the previous section, people with multiple sclerosis have these plaques, or lesions, in the white matter of the brain or spinal cord. While the gray matter contains neurons (nerve cells), white matter contains the axons (nerve fibers) that connect neurons and allow them to communicate with each other. Specifically, MS means that there has been a breakdown of the myelin sheath, which is the protective layer surrounding the axons. This breakdown is known as demyelinization. When the myelin sheath thins and breaks down, the axon can't function correctly and electrical signals can't travel between the neurons. Although the myelin can rebuild itself, the rate of demyelinization caused by MS is too rapid for it to keep up. In some cases, the axon is cut completely.
This results in numerous neurological symptoms that can appear in the form of attacks. The type, duration and severity of the disease depends on the type of MS. It also varies depending on the person, which can make it difficult for a doctor to determine the type and predict its course.
According to the National Multiple Sclerosis Society, there are four main types of MS:
Progressive-Relapsing - Patients with this rare form of MS get progressively and steadily worse and also suffer attacks, or relapses. Sometimes they recover briefly, but they never experience remission (long-term recovery) of the disease.
Relapsing-Remitting - Most MS patients are diagnosed with this form. They have attacks followed by remissions, during which the disease doesn't get worse.
Primary-Progressive - A small percentage of MS patients experience a slow progression of the disease without attacks or remissions. They may temporarily improve, or the disease may stay the same for short periods of time.
Secondary-Progressive - Some patients begin with a diagnosis of progressive-relapsing MS but develop secondary-progressive MS. This means that the disease progresses more quickly, and they may or may not experience attacks, periods of recovery or periods of remission.
Multiple sclerosis is one of a class of diseases known as autoimmune diseases. This means that the immune system, which normally defends us against foreign bodies such as viruses and bacteria, attacks the body instead. In MS, T-cells and monocyctes (specialized types of white blood cells) attack the myelin sheaths of the central nervous system. This is why one of the tests for MS is a spinal tap, to check if the cerebrospinal fluid contains an abnormally high number of antibodies and certain protein byproducts (which indicate a breakdown of myelin, composed of protein and lipids).
MS is an extremely unpredictable disease, because the lesions can occur anywhere in the central nervous system and the stages of demyelinization vary depending on the lesion. In the next section, we'll look at the symptoms of MS, and how MS is treated.