Myastenia gravis is a disease of neuro-muscular junction transmission. In this disorder, signals to muscles are not conducted because of decreased number of receptors on post synaptic membranes i.e on the membranes of muscles. These decreased receptors are due to autoimmune disorder. Antibodies are formed against these acetylcholine receptors that destroy them. And this results in decreased muscles power. Because when action potential reaches pre synaptic neuron, it causes release of acetylcholine. This acetylcholine attaches to some of the receptor but as most of the receptors are destroyed, there is not enough effect or we can say end plate potential is not generated and most of the acetylcholine is degraded by enzyme acetylcholine esterase.
Patients of myasthenia gravis are treated with cholinergic agonists i.e the ones that act indirectly. It means those cholinergic agonists are given to those patients that inhibit enzyme acetylcholine esterses. This treatment can cause relief in myasthenia gravis symptoms. But if can also result in two different worse situations that is called cholinergic crisis and myasthenic crisis. The difference between these two are discussed below.
Difference Between Cholinergic Crisis And Myasthenic Crisis:
Cholinergic crisis and myasthenic crisis are due to two different situations but both have the same symptoms. First we will discuss the mechanism of both and then the similarity of symptoms.
It means that the cholinergic activity has increased too much. Cholinergic activity can increase only if the enzymes that break down acetycholine are deactivated. And this can happen in myasthenia gravis when the patient is taking drugs like neostigmine or pyridostigmine.
This overstimulation at neuromuscular junctions will lead to over activation of receptors and thus receptors will be unable for further stimulation thus causing muscle weekness.
This occurs when the condition of patient of mysthenia gravis worsen and too much receptors are destroyed by immune system. Thus the remaining receptors are unable to cause muscle contractions. This condition also leads server muscle weekness.
Cholinergic Crisis Vs Myasthenic Crisis:
The difference between these tho crisis is that:
- In cholinergic crisis there is overstimulation at neuro-muscular junction while in myasthenia crisis their is under stimulation.
- Cholinergic crisis could be due to taking increase dose of anti cholinesterses or some poisons while myasthenic crisis is due to large number of destroyed receptors.
- Cholinergic crisis can be exaggerated by taking anti cholinesterases while myasthenic crisis can be improved. And this is the basis for differentiating both.
- There is fasciculation and week muscles in cholinergic crisis while only week muscles in myasthenic crisis.
- There is bradycardia in cholinergic crisis while tachycardia in myasthenic crisis.
- Secretions are normal in myasthenic crisis while increased in cholinergic crisis.
- Pupil is normal or dilated in myasthenic crisis while constricted in cholinergic crisis.
- Skin is warm and flushing in cholinergic crisis while faint and cold in myasthenic crisis.
Similarities between both is that in both of these conditions the muscle become extremely weak.
Test For Differentiating Cholinergic crisis from Myasthenic Crisis:
One of the most important question that may be asked you in exam etc is how to differentiate cholinergic crisis from myasthenic crisis? For this, you should check the symptoms give above but the more accurate way is Edrophonium test.
In this test a small amount of drug called edrophonium which is also called tensilon is injected and the response is recorded. If the patient improvement in symptoms it means that the person has myasthenic crisis. And this patient should be given drugs like tensilon, or neostigmine or pyridostigmine.
But if the patient symptoms exaggerate, it means that he/she has cholinergic crisis. In this case you would have to give atropine for reversal of this crisis.
To learn more about this test, you would have to read our separate complete guide about tensilon test. We have added the principle, procedure, results, precautions, indications, contraindication and side effects of edrophonium test. You can search that in site or click the link above to visit that guide.
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