Lambert-Eaton Myasthenic Syndrome (LEMS)
There is no cure for LEMS, as scientists have not yet figured out how to selectively stop the autoimmune attack on motor nerve terminal calcium channels and other nerve terminal proteins targeted by LEMS. Therefore, symptomatic treatments for neuromuscular weakness that results from LEMS are favored.
Potassium Channel Blockers
The potassium channel blocker Firdapse (also known as amifampridine, 3,4-diaminopyridine, 3,4-DAP) is an FDA-approved therapy that allows the electrical activity that passes through the neuromuscular junction to continue for a longer period, thereby increasing calcium influx into the nerve ending and the release of acetylcholine.
DAP is available in Europe as a modified version called amifampridine phosphate (brand name Firdapse). Firdapse was approved in the United States in November 2018 and is commercialized by Catalyst Pharmaceuticals. Firdapse was tested in multiple clinical trials in adults living with LEMS. The studies measured the effect of Firdapse on the Quantitative Myasthenia Gravis score (a physician-rated scale used to assess muscle weakness) and the Subject Global Impression scale (a scale where patients rate their overall impression of the effects of the treatment). Patients receiving Firdapse showed a greater benefit on these measures as compared to those on placebo.
The most common side effects experienced by patients in the clinical trials were burning or prickling sensation (paresthesia), upper respiratory tract infection, abdominal pain, nausea, diarrhea, headache, elevated liver enzymes, back pain, hypertension and muscle spasms. Firdaspe can also cause seizures and is contraindicated in patients with a history of seizures.
The most commonly used cholinesterase inhibitor is pyridostigmine (Mestinon). These drugs slow the breakdown of acetylcholine.
Cholinesterase inhibitors keep acetylcholine around longer at the neuromuscular junction but also in the autonomic nervous system (which controls involuntary bodily functions). Sometimes the drugs can cause diarrhea, abdominal cramps and/or excessive saliva. At high doses, they can increase weakness. Imodium can be used to treat diarrhea caused by pyridostigmine.
Cholinesterase inhibitors are usually only mildly effective for LEMS, but in rare cases they can prove sufficient for managing the disease.
Additionally, cholinesterase inhibitors that are used in myasthenia gravis (MG), are also used in LEMS to help with symptoms by keeping the chemical acetylcholine around longer at the neuromuscular junction. Cholinesterase inhibitors block the action of acetylcholinesterase, the enzyme that breaks down acetylcholine. The drug that is typically used is pyridostigmine.
Guanidine, a potassium channel blocker, is sometimes used in conjunction with pyridostigmine, but guanidine is mostly avoided due to toxicity that can involve kidney and bone marrow. If it is used, doses should be low (5 to 10 mg/kg per day, given in three to four divided doses to start) and close monitoring of side effects is necessary.
Drugs that suppress the immune system can be used to attack the autoimmune nature of the disease, but they have side effects and increase susceptibility to infections and, possibly, cancer.
Intravenous immunoglobulin (IVIG) therapy can be used to block the binding of antibodies.
These treatment options need to be considered carefully, and close follow-up is necessary. Your doctor or MDA Care Center physician can help you determine which treatments are appropriate for you.
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For individuals who need additional therapy, immunosuppression or immune modulation is usually the next step.
The treatment options are similar to those discussed for MG (see Myasthenia Gravis Medical Management), with prednisone and azathioprine most often being used. Mycophenolate mofetil may be a substitute for azathioprine.
Immune modulation options also include intravenous immunoglobulin (IVIG) and plasma exchange. Plasma exchange removes antibodies from the blood temporarily. IVIG is usually preferred, with benefits that peak at two to four weeks. IVIG therapy is essentially an injection of nonspecific antibody (immunoglobulin) that might work by dialing down the immune system's production of its own antibodies, much as warm air tells a thermostat to stop pumping out heat. It is also thought that IVIG blocks the binding of antibodies, such as calcium channel antibodies, from their targets. It probably also dilutes the antibodies and other immune system products that damage cells. IVIG can be given in combination with other immunotherapies and as a bridge while waiting for azathioprine to become effective.