The electromyogram is a test that helps doctors understands what state the nerves in the body are. With this test you can study how the information flows through the wiring system that is the nerves. You can also study the muscles to see how the signal arrives. As for the nerves, we can study the motor nerves (the ones that carry the signal to the muscles to move) and the sensory ones (those that pick up the sensitivity). Can we have a normal electromyogram if these nerves are injured?
Having done this initial summary, let’s see what happens when we study sciatica. The electromyogram has a number of technical limitations that must be known when drawing conclusions in a report. What I am going to tell next generically refers to the sciatica caused by a herniated disc in the lower back.
The strengths of this test in sciatica are in the study of muscles. We pinch different muscles with a needle electrode and we detect if the signal that arrives is normal. In these cases, if we have a normal electromyogram we can safely say that the motor nerves have not been affected. Look, I mean motors. The signal that has to move the muscles is preserved and therefore the lesion is less severe a priori. One small consideration, for this result to be reliable must have been more than three weeks since the injury.
The roots that carry sensitivity are studied by measuring how the nerve conducts electricity. Basically we give a cramp in the nerve, we collect the signal in another point and thus we see if it is healthy. If the result is normal electromyogram, in these cases it is not telling us that the nerve is healthy. We will see why:
– On the one hand the motor nerves are formed by several roots. In sciatica a nerve root is damaged, not the entire nerve that is much larger. The roots that leave the lumbar area join together and form the nerves. In the electromyogram we study the nerves already formed in its passage through the legs. With the lesion being higher, we study a nerve that is practically healthy and does not give alterations during the test.
– As for the sensitive nerve happens another very interesting phenomenon. It’s a bit complicated to explain but I’ll try. They are very technical questions so do not overwhelm if you do not understand, many doctors do not know either.
The sensitive root is formed by neurons that have two axons. That is, on the one hand we have the nerve cell containing the mechanisms to keep alive and, on the other hand, there are two pelillos called “axons” that carry the electrical signal. These two pelillos are alive while they are connected to the body of the nervous cell. Well, the body of the cell is outside of where the hernias occur. One of the two axons goes to the leg and picks up the signals from there and the other axon goes to the marrow to then go up and carry the signal to the brain. The herniated disc damages the axon that goes to the brain and this is the one that “dies” when losing the connection with the cell. However, the other axon that goes to the leg is healthy as an apple. What the patient notes is that he has tingles, Pain or loss of sensation in an area of the leg. What we see in the test is that the nerve is healthy, because we are evaluating the part that has not been damaged.
The true utility of the electromyogram in the case of sciatica is achieved by pricking the muscles. We also measure the current of nerves in the legs in case there are other injuries that have gone unnoticed (not always what we feel is due to a herniated disc). Although we know that we cannot detect a loss of sensation or sciatic pain due to a hernia with this test.
In summary, a normal electromyogram in sciatica indicates that the motor roots are probably healthy and have not been damaged by the hernia but we do not know how the sensory nerves are. In other words, there may be loss of sensation in areas of the leg and intense sciatic pain because the nerve root is trapped by the herniated disc and at the same time have a normal electromyogram.
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