Do you have experienced anytime muscle weakness and fatigue? That’s a common point in everyone’s life they met with weakness and fatigue due to either stressful workloads or lack of energy. Fortunately, this weakness is temporary and goes off when one is under rest or on more nutritional intake of diet.
But if this muscle weakness persists for a longer period even with a complete balance of nutrition and little workloads, then something going wrong with your muscles. Myasthenia gravis is such a disease which brings muscle weakness that persists for longer periods and even worsens with time.
Muscle weakness is may be of so many reasons and among them how could you identify it is whether due to myasthenia gravis or not? For instance, muscle weakness can be produced from a simple reason such as malabsorption to a more serious reason such as hyperthyroidism and multiple sclerosis.
So, in order to diagnose Myasthenia gravis, we need a specific test that can differentiate this disease out of the crowd. Tensilon test is such a quick diagnostic test, in which edrophonium injection is given to assess the possibility of Myasthenia gravis in the patient.
Before going to the details of test, first let’s know what happens in Myasthenia gravis.
It is an autoimmune disorder where the immunity of the patient destroys cells of own resulting loss of cholinergic transmission. This mainly results in the muscle weakness in the patient that can affect skeletal muscles of various groups at different organs. For example, it can affect the muscles at eye resulting in double vision impairing the clear vision in the patient. Similarly, it can affect the larynx muscles resulting slurred speech. Particularly, skeletal muscle group at arms and legs are more affected in muscle weakness and fatigue.
That’s fine, now how can identify myasthenia gravis in the patient. Simply by using a drug that improves cholinergic transmission. That’s the principle involved in tensilon test. Edrophonium is given by injection and improvement of muscle strength is checked in the patient. If muscle strength is improved, the patient may have myasthenia gravis otherwise the muscle weakness is due to any other reason.
At the neuromuscular junction, skeletal muscle is equipped with nicotinic acetylcholine receptors (nAChRs) which are ionotropic receptors coupled with the sodium channels. Normally when acetylcholine is released by calcium mediated exocytosis from cholinergic neurons, they occupy these nAChRs resulting in depolarization of skeletal muscle leading to contraction. But in patients with Myasthenia gravis, due to immune response, few of the nAChRs are lost by the action of antibodies against these receptors. Now, in these patients the acetycholine that is released from neurons has to act on the remaining nAChRs that are unaffected. At the same time, the probability of this binding can be limited by acetylcholinesterase enzyme which cleaves the acetycholine into choline and acetate. By all of these effects, there will be little chance of binding of acetylcholine to nAChRs resulting in decreased muscle strength producing muscle weakness as one of the key symptom in myasthenia gravis.
In this context, any drug that increases cholinergic transmission at skeletal muscle can improve the symptoms. Acetylcholinesterase inhibitors can play this role by increasing the ACh levels at the synaptic cleft. Among these, edrophonium is the drug that is used for diagnosis of myasthenia gravis by inhibiting the enzyme.
We have so many drugs that increase cholinergic transmission such as physositgmine and neostigmine but why we use edrophonium in tensilon test? Before going to that let’s see the structure of edrophonium.
It is a simple aromatic compound with phenyl ring attached to a quaternary ammonium group. It also contains hydroxyl group which is essential for interaction with acetylcholinesterase enzyme. Now let’s see why edrophonium is selected for the test. First, the drug can’t cross the blood brain barrier so that it shows mainly peripheral actions. Since the purpose of the drug here is to test the improvement in muscle strength peripherally, central actions of the drug are not essential. Other drugs like physostigmine can show both central as well as peripheral actions.
Second reason is that edrophonium has fast onset and short duration of action. When edrophonium injection is given, it show onset of action within 10 to 30 sec and it shows its action for a duration of around 10 min. Again this is an advantage with the drug. Since for diagnosis of myasthenia gravis muscle strength should be improved for a short period, here edrophonium best fits into the purpose compared with other agents.
Acetylcholine esterase has two important binding sites such as ester binding site and cation binding site. Ester binding site is associated with serine which has hydroxyl group as side chain. On the other hand, cation binding site is made up of glutamate which has carboxylate side chain which can interact with cations. Now, edrophonium has a quaternary ammonium group therefore it can bind to the glutamate residue at cation binding site. Similarly the hydroxyl group of edrophonium can form a hydrogen bond with the hydroxyl group of serine site. In this way, the two important sites of acetylcholine esterase are blocked by edrophonium resulting in inhibition of its activity.
But here which site is more important for enzymatic activity? Undoubtedly, it is serine site where acetylcholine is cleaved in to choline and acetate. So edrophonium blocks this site by formation of H-bonding. Is that hydrogen bonding strong and long acting? No, As the H-bond is relatively weaker than covalent bond, the inhibition produced by edrophonium is weak, reversible and lasts for only a short period.
Even the purpose of edrophonium is for a quick diagnosis, still it can produce few of the side effects by stimulation of cholinergic system. It can slow the heart rate resulting in bradycardia or it can produce a bronchoconstriction leading to difficulty in breathing. Due to enhanced cholinergic response, blood pressure may be reduced in the patient. Finally, muscle can also be paralyzed due to depolarization block. All these side effects are mainly due to stimulation of cholinergic system which should be thoroughly checked before going for real diagnosis.
We have seen that edrophonium can produce side effects by stimulation of cholinergic system. Then how can we antagonize these side effects? Is there any antidote for this? Yes, fortunately we have an antidote atropine, which is a non-selective muscarinic antagonist.
Before going to the tensilon test for diagnosis of myasthenia gravis, the patient should be checked for development of any possible side effects with edrophonium. For this purpose, a low dose of edrophonium such as 2g is given by injection through intravenous route and side effects are thoroughly checked.
If no significant side effect is found, then edrophonium can be given at dose of 8mg by IV route and any signs of improvement in the muscle strength are checked. This can be done by various short exercise procedures as well as based on the inputs given by the patient regarding how they feel in improvement in muscle strength after the injection. If the data suggests a clear improvement in the muscle strength, the underlying cause for muscle weakness in the patient is mainly due to myasthenia gravis.
So, that’s about the tensilon test used for diagnosis of myasthenia gravis. Edrophonium is an ideal candidate here for diagnosis as it is short acting. If you like the post, please share this article with your friends.