Lambert-Eaton myasthenic syndrome (LEMS) is a rare autoimmune disease that mainly affects muscles in the lower limbs. It is named after Edward Lambert and Lee Eaton, two neurologists at the Mayo Clinic in Rochester, Minnesota, who first described the syndrome back in the 1950s and 60s.
Causes of LEMS
LEMS is an autoimmune disease, a disorder in which the immune system attacks the body’s own tissues. In the case of LEMS, this immune attack is directed against a specific type of calcium channels found in nerve cell endings that play a key role in muscle movement control. Calcium channels are required for the release of acetylcholine, a chemical messenger that triggers muscle contraction. When the levels of acetylcholine are too low to elicit proper muscle contraction, the end result is muscle weakness.
This specific type of myasthenic disease may or may not be associated with an underlying cancer. A particular type of lung cancer called small-cell lung cancer or SCLC accounts for the majority of cases in which LEMS is associated with a malignancy. This happens because the calcium channels that are attacked by the immune system in people with LEMS are also present on the surface of some cancer cells. It is thought that in cancer patients, the antibodies generated to eliminate cancer cells mistakenly attack the calcium channels found on nerve cells, leading to the onset of LEMS.
Symptoms of LEMS
Characterized by muscle weakness that starts in the lower limbs, LEMS makes walking and climbing stairs more difficult. As the disease progresses, the arms and shoulders also are affected and everyday tasks become increasingly challenging.
Due to muscle weakness, some patients may also experience fatigue, or abnormal tiredness, and lack reflexes or have very poor ones. Weakness in the muscles that are needed for breathing can also cause breathing problems. In severe cases, such problems may lead to respiratory failure.
LEMS also can cause problems with speech, chewing, and swallowing. Eye muscle weakness can lead to double vision and eyelid drooping. Most patients also have signs of autonomic dysfunction, such as dry mouth and eyes, constipation, and impotence.
Diagnosis of LEMS
The diagnosis of LEMS involves the thorough assessment of a patient’s clinical signs and symptoms.
The extent of muscle weakness is typically assessed during a physical exam, as are tendon reflexes. Electromyography (EMG) also can be performed to better evaluate muscle-nerve communication.
The presence of antibodies against calcium channels is highly specific for this condition, and detectable in about 85% of LEMS patients. These can be found in patients with and without SCLC.
People without this cancer who are positive for calcium channel antibodies will likely undergo a chest CT scan to look for tumors in the lung. The presence of antibodies against a protein called SOX1 also may help distinguish patients with LEMS associated with SCLC from those with non-SCLC LEMS.
Treatment for LEMS depends on whether or not the condition is associated with cancer. This is because cancer treatment is a priority, and usually helps ease LEMS symptoms.
Firdapse (amifampridine) is an FDA-approved medication for the relief of LEMS symptoms in patients ages 17 and older. It is a potassium channel blocker that increases calcium influx in nerve endings, stimulating the release of acetylcholine, and therefore muscle contraction.
Cholinesterase inhibitors, such as pyridostigmine, block the enzyme that normally breaks down acetylcholine (cholinesterase), so that acetylcholine remains available for a longer period of time to trigger muscle contraction. While these medications are not specifically approved to treat LEMS, they are sometimes used off-label to help manage its symptoms.
Immunosuppressive medications like prednisone also can be used to control the body’s immune system and ease symptoms in patients with more severe forms of LEMS.
Plasmapheresis, a procedure in which plasma — the liquid portion of blood — is replaced or treated to remove harmful antibodies, also can help relieve LEMS symptoms. Intravenous immunoglobulin (IVIG) replacement therapy, which involves the injection of nonspecific antibodies (immunoglobulins) from donated blood, is thought to temporarily inhibit the production of autoantibodies, thereby easing LEMS symptoms.
Last updated: Oct. 26, 2021
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