About 60,000 Americans suffer the chronic neuromuscular disorder myasthenia gravis, an autoimmune disorder in which the immune system attacks the pathways nerve impulses travel to reach the muscles, weakening them. The impulses are ordinarily carried by a chemical called acetylcholine. Muscle fibers have acetylcholine receptors, areas where the chemical is detected and processed so that the muscle contracts. However, when a patient has myasthenia gravis, the immune system treats these receptor sites as foreign invaders, creating antibodies that can destroy as much as 80 percent of the sites.
When this happens, most commonly in men over 60 and women under 40, it can cause difficulty breathing, soft speech or hoarseness, trouble chewing or swallowing, facial muscle paralysis, double vision, problems looking people in the eye, and a feeling of having to work harder than usual to move, stand from a chair, or climb up steps. An unusual symptom that appears in many cases is diminished olfactory sensitivity—patients lose some ability to smell, and therefore also to taste food. This is an interesting sign because myasthenia gravis does not appear to cause cognitive impairment, but the sense of smell is not connected to the muscles in any obvious way.
Myasthenia gravis is a rare example of an autoimmune disease that isn’t hereditary. In some cases, it’s caused by an overactive thymus gland—which plays a major role in immune system regulation—resulting from a non-cancerous tumor, but explanations for the remainder of cases are still emerging. Recently, scientists found an antibody to a neurotransmitter that appears to be responsible for a significant proportion of cases.
Myasthenia gravis generally responds well to treatment. As with many autoimmune diseases, immune-system suppressing drugs are the most common treatment approach, though anti-inflammatory drugs are not used, because inflammation is not the problem. Another treatment approach is medication to boost the activity of the neurotransmitter, to compensate for the damage to the receptors. Patients undergoing an attack, called a myasthenic crisis, are given treatments that operate directly on the antibodies responsible; these treatments last a few weeks. In severe cases, such as when there is a tumor, the entire thymus gland is surgically removed.