Myasthenia gravis is the most common known disorder of neuromuscular transmission, thought to affect fifteen to twenty persons per hundred thousand in the U.S. population. The disorder is felt to be an antibody-mediated attack on acetylcholine receptors at the neuromuscular junctions, resulting in poor skeletal muscular contraction, weakness and fatigability as a result.
While clinical presentation occurs usually at younger ages (second and third decades) in women and more advanced ages (sixth to eighth decades) in men, there is significant variability. Presentation may include weakness of the eye muscles (so called ocular disturbances) most often characterized by patients as a lid lag, where the upper lid may droop or "double vision." Difficulty chewing and/or swallowing (oropharyngeal weakness) may occur. Less commonly initially, but with advancing symptomatology, weakness of the pelvic muscles and/or limbs (upper and/or lower extremities) may occur. Weakness typically fluctuates from day to day and may be exacerbated by prolonged use of affected muscles. The course of the disorder is variable, but is usually progressive if untreated.
The thymus gland, which normally resides beneath the upper portion of the sternum, seems to be clearly involved in myasthenia gravis, but the nature of the association of the gland and the disease is not yet fully understood. Ten to fifteen percent of patients with myasthenia gravis have a thymic tumor; fifty percent of patients with thymic tumors (thymomas) have myasthenia gravis. Fully seventy to eighty percent of patients with myasthenia gravis, who do not have a thymoma, have exuberant germinal centers when the thymic gland is microscopically examined, indicating some form of active immunologic process.
The diagnosis of myasthenia is often delayed months and years, as mild symptomatology may be sloughed off by patients. Once the diagnosis is entertained, a variety of neuromusculature and electromyographic tests may be performed. Probably the single most useful general testing is the discovery of serologic markers whereby acetylcholine receptor or muscle-specific receptor antibodies may be determined. Not all patients, however, have such antibodies; fully ten percent of patients may not, and the diagnosis is then delineated on a clinical basis. Magnetic resonance imaging studies of the brain are important to rule out other conditions that may mimic, such as demyelinating disease of the central nervous system.
The treatment of myasthenia gravis is a specialized topic, which is best approached and delineated by the treating neurologist. While medical treatments include esterase inhibitors, such as Mestinon, most patients in less than advanced stage and condition are referred for removal of the thymus gland with a significant percentage of patients finding that over time all medication may be discontinued secondary to improvement over time by the mere removal of the thymic gland. Of course, responses to thymectomy are quite variable. The most favorable response is generally felt to occur two to five years out from surgery, but the results are anything but predictable. For completeness sake, while myasthenia gravis may occur in some patients acutely, the treatment, at least as addressed by surgery is rarely an emergency. Most importantly, in patients with significant symptomatic weakness, plasmapheresis (plasma exchange preceding surgery) may be very useful, the surgery to follow usually several days after plasmapheresis whereby the patients' clotting mechanisms are able to be restored in the interim.
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