A BRIEF REVIEW OF MYASTHENIA GRAVIS

Abstract

Myasthenia gravis, the most common of the disorders of the neuromuscular junction, is an autoimmune disease due to the production of antibodies against the acetylcholine receptors and other protein molecules of the post-synaptic membrane of the striated muscle fibers. Clinically it is characterized by transient or more stable weakness of muscles or muscle groups, by aggravation of weakness during effort and its recovery at rest.
A significant rise of the prevalence of MG has been noted during the last hundred years and particularly the last few decades, due mainly to the augmentation of the number of aged patients, along with a concomitant improvement of prognosis that can be attributed to better management of respiratory failure and to the benefits of immunomodulatory therapies.
The ultimate action of the autoantibodies, whether concerning directly the acetylcholine receptors or other elements of the post-synaptic membrane, lead to loss of the available receptors, to reduction of the safety factor of the neuromuscular junction, the inability to produce an action potential and, thus, to a diffuse functional loss of muscle fibers.
Onset of myasthenia gravis is frequently insidious, with transient episodes of weakness. It can occur at any age, but tends to be earlier in females than in males. The disease tends to fluctuate in severity during the first 3-5 years with episodes of spontaneous remission and exacerbation and with greater probability for the occurrence of myasthenic crisis. The topographical distribution of weakness is also unstable, with a trend towards generalization from an initially restricted area, usually concerning the extraocular muscles and the levators of the lids, as well as the muscles of mastication and phonation. Later, the majority of patients develop a chronic form of the disease that tends to diminish in severity, eventually attaining improvement and a more stable level of weakness.
Diagnosis, which can prove difficult in the beginning of the disease, is mainly based on clinical findings, especially concerning ophthalmic myasthenia. Pharmacological testing (edrophonium test), electrophysiological examination (repetitive nerve stimulation, single fiber electromyography), the detection of specific autoantibodies and imaging of the thymus are also essential for the diagnosis.