Mestinon is a drug used to treat a rare auto-immune condition called myasthenia gravis, a neuromuscular disorder that causes weakness and fatigue of the muscles under voluntary control. It works by preventing the breakdown of a chemical called acetylcholine, which sends nerve impulses to muscles, thereby increasing voluntary control. Mestinon may cause toxic side effects, which are usually related to overdose.
Muscarinic Side Effects
The most common side effects are known as muscarinic side effects. Acetylcholine stimulates a type of receptor called a muscarine receptor that is commonly found throughout the body and especially in muscles. Mestinon stimulates these receptors to cause muscle contraction; however, when doses are too high, excessive stimulation of these receptors causes a number of problems. Nausea, vomiting, diarrhea and abdominal cramps are common. Increased production of saliva, tears and bronchial secretions, constriction of the pupils (miosis), increased involuntary contraction of the esophagus (peristalsis) and excessive sweating may also occur.
Atropine may sometimes be successfully used to counteract these side effects.
Nicotinic Side Effects
Myasthenia gravis causes antibodies to form against receptors that take up acetylcholine (called nicotinic postsynaptic receptors) at the junction where muscle fibers meet motor neurons. These antibodies prevent acetylcholine from stimulating receptors to cause voluntary muscle contraction. Mestinon counteracts this by boosting acetylcholine production and stimulating the nicotinic receptors. Excessive stimulation of acetylcholine at nicotinic junctions, however, can cause muscle cramps, muscle twitching or contraction and muscle weakness. In extreme cases, muscle paralysis can occur when muscles become flabby and very weak.
Cholinergic Crisis
Overdoses of Mestinon can lead to a serious condition known as a cholinergic crisis. Cholinergic crises are characterized by increasing muscle weakness that causes severe impairment in the muscles of breathing or respiration. It may also cause muscle tone loss or paralysis in other muscles. If not treated right away, respiratory problems can lead to death.
Individuals with myasthenia gravis who experience a severe depletion of acetylcholine may experience a myasthenic crisis. A myasthenic crisis affects the muscles that control breathing and may require emergency treatment with a respirator or other measures that prevent the person from taking too much air into the lungs. These symptoms closely resemble the symptoms of a cholinergic crisis and can be difficult to differentiate. It is important to distinguish, however, because taking Mestinon or increasing the dose of Mestinon during a myasthenic crisis can worsen symptoms and lead to more serious complications.
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