Lambert-Eaton myasthenic syndrome (LEMS) is a rare autoimmune disease characterized by muscle weakness, especially in the arms, legs, shoulders, and hips. LEMS patients also may have weakness in the eye and throat muscles.

LEMS can be associated with cancer or can occur spontaneously. It is caused by antibodies produced by the immune cells that mistakenly attack the neuromuscular junction, the point where nerve and muscle cells meet, disrupting the communication between the nervous and muscular systems. This results in weakening of the muscles because they do not receive constant signals from the nervous system to contract or relax.

How nerve cells communicate with muscle cells

The neuromuscular junction is the area where a nerve cell and a muscle cell meet. At this junction, a nervous signal that has been traveling along the nerve fiber as an electrical signal is converted into a chemical signal. When the electrical signal, also called an “action potential,” reaches the end of the nerve cell, it triggers the opening of so-called voltage-gated calcium channels (VGCC), which allow the uptake of calcium into the nerve terminal. The calcium ions bind to a protein that is present on the vesicles that contain a neurotransmitter called acetylcholine. This triggers the release of acetylcholine from the nerve terminal into the space at the neuromuscular junction.

The released acetylcholine binds to specific receptor proteins found on muscle cells called acetylcholine receptors. These receptor proteins are in the form of a pore (a channel). The binding of acetylcholine opens up the pore and allows the inward flow of charged ions into the muscle cell, triggering muscle contraction.

How LEMS affects the neuromuscular junction

In LEMS patients, immune cells produce autoantibodies (or antibodies against self) against VGCC protein on the nerve cells, blocking their function and subsequently the transmission of messages from the nerve cells to the muscle cells. Since the muscle cells are not constantly receiving messages to contract and relax, they weaken over time.

Cancer-related and non-cancer related LEMS

The production of autoantibodies, which is the underlying cause of LEMS, can be cancer-related or spontaneous (non-cancer related).

Nearly 50% of LEMS cases are associated with cancer, especially small-cell lung cancer. In such cases, it is thought that antibodies produced against cancer cells mistakenly attack the nerve cells. This usually occurs later in life, around age 60, and coincides with the manifestation of cancer itself.

Non-cancer associated LEMS can occur at any age, but also is more common in older people. The reasons for the production of autoantibodies is not known.

Last updated July 17, 2019

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