What are the Symptoms of MS?
In multiple sclerosis (MS), damage to the myelin in the central nervous system (CNS), and to the nerve fibers themselves, interferes with the transmission of nerve signals between the brain and spinal cord and other parts of the body. This disruption of nerve signals produces the primary symptoms of multiple sclerosis, which vary depending on where the damage has occurred.
Over the course of the disease, some symptoms will come and go, while others may be more lasting.
MS lesions may be present without symptoms.
Because investigators speculate that the breakdown of the blood/brain barrier is the first step in the development of MS lesions, it is important to distinguish new lesions from old. To do this, physicians give patients injections of gadolinium, a chemical contrast agent that normally does not cross the blood/brain barrier, before performing a scan. On this type of scan, called T1, the appearance of bright areas indicates periods of recent disease activity (when gadolinium is able to cross the barrier).
The ability to estimate the age of lesions through MRI has allowed investigators to show that, in some patients, lesions occur frequently throughout the course of the disease even when no MS symptoms are present.
Can Life Events Affect the Course of MS?
While there is no good evidence that daily stress or trauma affects the course of multiple sclerosis, there is data on the influence of pregnancy.
Since multiple sclerosis generally strikes during childbearing years, a common concern among women with the disease is whether or not to have a baby. Studies on the subject have shown that multiple sclerosis has no adverse effects on the course of pregnancy, labor, or delivery; in fact symptoms often stabilize or remit during pregnancy.
This temporary improvement is thought to relate to changes in a woman's immune system that allow her body to carry a baby: because every fetus has genetic material from the father as well as the mother, the mother's body should identify the growing fetus as foreign tissue and try to reject it in much the same way the body seeks to reject a transplanted organ. To prevent this from happening, a natural process takes place to suppress the mother's immune system in the uterus during pregnancy.
However, women with MS who are considering pregnancy need to be aware that certain medications for the treatment of MS should be avoided during pregnancy and while breast feeding. These drugs can cause birth defects and can be passed to the fetus via blood and to an infant via breast milk. Among them are prednisone, corticotropin, azathioprine, cyclophosphamide, diazepam, phenytoin, carbamazepine, and baclofen.
Unfortunately, between 20 and 40 percent of women with multiple sclerosis do have a relapse in the three months following delivery. However, there is no evidence that pregnancy and childbirth affect the overall course of the disease one way or the other. Also, while multiple sclerosis is not in itself a reason to avoid pregnancy and poses no significant risks to the fetus, physical limitations can make child care more difficult. It is therefore important that MS patients planning families discuss these issues with both their partner and physician.
MS and the Immune System
To understand what is happening when a person has multiple sclerosis, it is first necessary to know a little about how the healthy immune system works.
The immune system is a complex network of specialized cells and organs that defends the body against attacks by "foreign" invaders such as bacteria, viruses, fungi, and parasites. It does this by seeking out and destroying the interlopers as they enter the body. Substances capable of triggering an immune response are called antigens.
The immune system displays both enormous diversity and extraordinary specificity. It can recognize millions of distinctive foreign molecules and produce its own molecules and cells to match up with and counteract each of them. In order to have room for enough cells to match the millions of possible foreign invaders, the immune system stores just a few cells for each specific antigen. When an antigen appears, those few specifically matched cells are stimulated to multiply into a full-scale army. Later, to prevent this army from overexpanding, powerful mechanisms to suppress the immune response come into play.
T cells, so named because they are processed in the thymus, appear to play a particularly important role in MS. They travel widely and continuously throughout the body patrolling for foreign invaders. In order to recognize and respond to each specific antigen, each T cell's surface carries special receptor molecules for particular antigens.
T cells contribute to the body's defenses in two major ways. Regulatory T cells help orchestrate the elaborate immune system. For instance, they assist other cells to make antibodies, proteins programmed to match one specific antigen much as a key matches a lock. Antibodies typically interact with circulating antigens, such as bacteria, but are unable to penetrate living cells. Chief among the regulatory T cells are those known as helper (or inducer) cells. Helper T cells are essential for activating the body's defenses against foreign substances. Yet another subset of regulatory T cells acts to turn off, or suppress, various immune system cells when their job is done.
Killer T cells, on the other hand, directly attack diseased or damaged body cells by binding to them and bombarding them with lethal chemicals called cytokines. Since T cells can attack cells directly, they must be able to discriminate between "self" cells (those of the body) and "nonself" cells (foreign invaders). To enable the immune system to distinguish the self, each body cell carries identifying molecules on its surface. T cells likely to react against the self are usually eliminated before leaving the thymus; the remaining T cells recognize the molecular markers and coexist peaceably with body tissues in a state of self-tolerance.
In autoimmune diseases, such as MS, the detente between the immune system and the body is disrupted when the immune system seems to wrongly identify self as nonself and declares war on the part of the body (myelin) it no longer recognizes. Through intensive research efforts, scientists are unraveling the complex secrets of the malfunctioning immune system of patients with MS.
Components of myelin such as myelin basic protein have been the focus of much research because, when injected into laboratory animals, they can precipitate experimental allergic encephalomyelitis (EAE), a chronic relapsing brain and spinal cord disease that resembles MS. The injected myelin probably stimulates the immune system to produce anti-myelin T cells that attack the animal's own myelin.
Investigators are also looking for abnormalities or malfunctions in the blood/brain barrier, a protective membrane that controls the passage of substances from the blood into the central nervous system. It is possible that, in MS, components of the immune system get through the barrier and cause nervous system damage.
Scientists have studied a number of infectious agents (such as viruses) that have been suspected of causing MS, but have been unable to implicate any one particular agent. Viral infections are usually accompanied by inflammation and the production of gamma interferon, a naturally occurring body chemical that has been shown to worsen the clinical course of MS. It is possible that the immune response to viral infections may themselves precipitate an MS attack. There seems to be little doubt that something in the environment is involved in triggering MS.
MS and Genetics
In addition, increasing scientific evidence suggests that genetics may play a role in determining a person's susceptibility to multiple sclerosis.
Some populations, such as Gypsies, Eskimos, and Bantus, never get MS. Native Indians of North and South America, the Japanese, and other Asian peoples have very low incidence rates. It is unclear whether this is due mostly to genetic or environmental factors.
In the population at large, the chance of developing MS is less than a tenth of one percent. However, if one person in a family has MS, that person's first-degree relatives-parents, children, and siblings-have a one to three percent chance of getting the disease.
For identical twins, the likelihood that the second twin may develop MS if the first twin does is about 30 percent; for fraternal twins (who do not inherit identical gene pools), the likelihood is closer to that for non-twin siblings, or about 4 percent. The fact that the rate for identical twins both developing MS is significantly less than 100 percent suggests that the disease is not entirely genetically controlled. Some (but definitely not all) of this effect may be due to shared exposure to something in the environment, or to the fact that some people with MS lesions remain essentially asymptomatic throughout their lives.
Further indications that more than one gene is involved in MS susceptibility comes from studies of families in which more than one member has multiple sclerosis. Several research teams found that people with MS inherit certain regions on individual genes more frequently than people without MS. Of particular interest is the human leukocyte antigen (HLA) or major histocompatibility complex region on chromosome 6. HLAs are genetically determined proteins that influence the immune system.
The HLA patterns of MS patients tend to be different from those of people without the disease. Investigations in northern Europe and America have detected three HLAs that are more prevalent in people with MS than in the general population. Studies of American MS patients have shown that people with MS also tend to exhibit these HLAs in combination-that is, they have more than one of the three HLAs-more frequently than the rest of the population. Furthermore, there is evidence that different combinations of the HLAs may correspond to variations in disease severity and progression.
Studies of families with multiple cases of MS and research comparing genetic regions of humans to those of mice with EAE suggest that another area related to MS susceptibility may be located on chromosome 5. Other regions on chromosomes 2, 3, 7, 11, 17, 19, and X have also been identified as possibly containing genes involved in the development of MS. These studies strengthen the theory that MS is the result of a number of factors rather than a single gene or other agent.
Development of MS is likely to be influenced by the interactions of a number of genes, each of which (individually) has only a modest effect. Additional studies are needed to specifically pinpoint which genes are involved, determine their function, and learn how each gene's interactions with other genes and with the environment make an individual susceptible to MS. In addition to leading to better ways to diagnose MS, such studies should yield clues to the underlying causes of MS and, eventually, to better treatments or a way to prevent the disease.
Reference: National Institute of Neurological Disorders and Stroke (NINDS)