Lambert-Eaton Myasthenic Syndrome
Introduction[edit | edit source]
Lambert-Eaton Myasthenic Syndrome (LEMS) is a rare autoimmune disorder of presynaptic neuromuscular transmission with impaired release of acetylcholine (ACh). Clinically, it is often mistaken for myasthenia gravis, by the untrained eye.
Etiology[edit | edit source]
There is a strong association with small-cell lung carcinoma, although other auto-immune disorders have also been associated with LEMS.
Pathophysiology[edit | edit source]
Calcium plays an important role in the release of ACh from its vesicles within the presynaptic membrane of the neuromuscular junction. In LEMS, antibodies are created against the voltage-gated calcium channels. This blocks the normal flow of calcium, thereby preventing the release of ACh from its vesicles. There is little to no ACh being released into the synaptic cleft. Therefore, there will be minimal to no muscle contraction.
In contrast to myasthenia gravis, continuous or repeated contractions result in improvement of strength. This occurs due to an increased influx of calcium in the presynaptic membrane, by repetitive muscle contractions. This calcium buildup facilitates the release of ACh by binding with multiple vesicles. In time, the excess calcium is cleared off by mitochondria. Thus, this effect is short-lived.
