5.18: Myasthenia Gravis: Diagnostic Tests
Myasthenia gravis is an autoimmune condition affecting neuromuscular transmission, causing generalized weakness in skeletal muscles. Initial diagnoses rely on patients' signs, symptoms, and medical history. The challenge lies in distinguishing myasthenia from other muscular dystrophies. An important diagnostic feature is the significant improvement of symptoms after administering anticholinesterase inhibitors.
The edrophonium test is a diagnostic tool for myasthenia gravis. It involves administering intermittent intravenous doses of edrophonium chloride, an acetylcholinesterase inhibitor. The test begins with a 2 mg dose, followed by an additional 8 mg dose after 45 seconds if no effects are observed. If there is a noticeable improvement in muscle strength for 5 minutes following the second dose, the diagnosis is considered positive for myasthenia.
Detecting antibodies against nicotinic receptors in plasma samples or muscle biopsies can aid in diagnosing myasthenia gravis. These antibodies disrupt the function of the receptors. Imaging tests, such as magnetic resonance imaging (MRI) or computed tomography (CT), help identify thymus gland tumors, which have a significant role in the pathology of myasthenia gravis. Electrodiagnostic studies, including electromyography and repetitive nerve stimulation, can confirm muscle weakness and assess the impairment of neuromuscular transmission. These tests evaluate nerve conduction and muscle contraction.
