Overview of Electrodiagnostic Evaluation of Neuromuscular Junction Disorder
Neuromuscular junction disorders refer to a group of conditions that make your muscles weak. These can be due to your body’s immune system attacking its own cells (autoimmune), present from birth (congenital), due to metabolic issues, or toxin-related. There are three main types of these disorders: myasthenia gravis, Lambert-Eaton myasthenic syndrome (LEMS), and botulism. These conditions lead to problems with signal transmission between the nerve ending and the muscle fiber. This is where communication happens to integrate the movements of our body.
In myasthenia gravis, the problem occurs on the muscle side of this communication channel (postsynaptic membrane). However, with Lambert-Eaton myasthenic syndrome and botulism, the issue lies on the nerve side (presynaptic membrane). These conditions result in symptoms like muscle tiredness and weakness that can vary, often getting worse after activity. Yet, these conditions don’t affect the sensory system (how you feel sensations in your body), because this system doesn’t have this type of nerve-muscle communication point (neuromuscular junction).
Patients usually have more weakness in the muscles close to the center of the body (proximal) than those further away (distal). Common complaints might include weakness in muscles that help to speak, swallow, or control eye movement (bulbar or extraocular muscles). Myasthenia gravis often presents with these kind of muscle weaknesses as well as weakness in the limbs. Another important point of investigating is if the patient has ever had thymoma, a tumor in the thymus gland.
Patients with Lambert-Eaton syndrome may not often complain about bulbar weakness, but they typically show a widespread weakness in the limbs that are close to the body’s center (proximal limb weakness). It’s also important to note that Lambert-Eaton syndrome strongly links with a type of lung cancer called small cell carcinoma.
Botulism is an uncommon condition caused by a toxin made by the bacterium Clostridium botulinum. In the US, botulism is most commonly seen in infants.
Why do People Need Electrodiagnostic Evaluation of Neuromuscular Junction Disorder
If your doctor thinks you might have a condition that affects the connection between your nerves and muscles (called a neuromuscular junction disorder), they will rely on several tests to confirm this. First, they will look into your medical past and check your symptoms. This will then be followed by a series of specific tests.
One common test is called a repetitive nerve stimulation. This involves delivering mild electrical shocks to a nerve in your body multiple times to see how your muscles respond. Exercise testing might also be performed, where your muscle response will be measured after you exercise.
The most reliable test for neuromuscular junction disorders is the single-fiber electromyography (SFEMG). This involves inserting a very thin needle into a muscle to record its electric activity. This test has been found to identify these disorders with up to a 99% accuracy rate.
If you’re taking any medication that stops the breakdown of a chemical called acetylcholine (anticholinesterase medication), your doctor will ask you to stop taking them anywhere from 8 to 24 hours before these tests. This is to ensure that the medication doesn’t interfere with the results.
When a Person Should Avoid Electrodiagnostic Evaluation of Neuromuscular Junction Disorder
There are only a few situations where it’s not safe to perform certain tests used to diagnose problems with the muscles and nerves. One such test, the needle EMG, uses tiny needles and shouldn’t be done on people who have serious problems with bleeding. We also shouldn’t use these needles on areas where there is a skin infection.
Another test, NCS, can’t be done on patients who have a heart defibrillator, which is a device that corrects abnormal heart rhythms. If they’re using an external defibrillator, this test is also not recommended. It’s important to ask the patients if they have a pacemaker, and if so, we should avoid performing electrical stimulation directly on or near the pacemaker. This is because the electrical stimulation can interfere with the functioning of the heart device.
How is Electrodiagnostic Evaluation of Neuromuscular Junction Disorder performed
Before starting any medical test, it’s important to gather all the necessary information about the patient’s medical history, their symptoms, and perform a thorough physical exam. The doctor in charge of the test, known as the diagnostician, must explain to the patient why they need the test and what it includes. They must also go over the potential risks and benefits. After getting the patient’s approval (a process known as informed consent), the test can begin.
One common procedure is a nerve conduction study. This is a test to see if the nerves are working properly. As a first step, the doctor makes sure no other nerve problems exist that could interfere with the final results.
One part of nerve conduction study is called a repetitive nerve stimulation (RNS) test. This test involves stimulating a motor nerve (the nerve that makes a muscle move) several times in a row to see how the muscles react, or how the muscles’ action potentials change. If there is more than a 10% decrease in the muscle response, it might suggest there’s a problem with the nerve. Ideally, the test is done on the muscle that seems to be most affected, but practical considerations (like how easy it is to isolate this muscle) may determine which muscle is tested. It’s important that the muscle and the test electrodes stay still, or else the results could be inaccurate. The temperature of the limb being tested should be kept at 32 degrees Celsius (about 90 degrees Fahrenheit), as cooler temperatures can affect the test results.
The RNS test also involves an exercise component. If the initial part of the test doesn’t show a significant decrease in the muscle response, the patient is then asked to exercise for one minute. Following this, the RNS test is repeated after each minute for 3 to 4 minutes to see how the exercise might have affected the muscle response. The same thing is repeated but this time after the patient’s muscle has been made to contract forcefully for 10 seconds.
Another type of nerve test is called single-fiber electromyography (SFEMG). This test is considered the most accurate for diagnosing disorders related to the neuromuscular junction (the place where the nerve connects with the muscle). Performing this test is safe but can be technically challenging. It requires special training and expertise.
To perform an SFEMG, the doctor uses a special needle that can record muscle activity from a single muscle fiber or a small group of fibers. This test looks for variations in the muscle fiber responses, referred to as ‘jitter’. Most modern equipment calculate this automatically. If the average jitter value goes beyond the expected normal value, it could suggest a nerve or muscle disorder.
Possible Complications of Electrodiagnostic Evaluation of Neuromuscular Junction Disorder
When doctors perform electrodiagnostic studies, which are tests that check the electrical activity of your muscles and nerves, complications are rare as long as necessary precautions are taken. However, there’s a tiny chance that you might experience bleeding or get an infection if needles are used in the studies. These risks are part of why doctors take precautions during these tests.
What Else Should I Know About Electrodiagnostic Evaluation of Neuromuscular Junction Disorder?
In medical tests that study the motor nerves, which are the nerves that control muscle movement, doctors can often identify diseases affecting the meeting point of the nerve and the muscle, known as the neuromuscular junction (NMJ).
In NMJ disorders, nerve signals move at normal speeds since the myelin – a layer that insulates nerve fibers – is unaffected. However, the strength of these signals, or amplitudes, can be altered. Monitoring these amplitudes can help doctors differentiate between two NMJ disorders: myasthenia gravis and Lambert-Eaton myasthenic syndrome. In Lambert-Eaton, the nerve signal strength decreases, while in myasthenia gravis, it remains normal.
Tests of sensory nerves, the nerves responsible for feelings like touch or pain, typically show normal results in NMJ disorders, because these nerves don’t have the neuromuscular junction.
In another test known as repetitive nerve stimulation, doctors can identify NMJ disorders by a greater than 10% decrease in nerve signal strength between the first and the fourth stimulation. For instance, in myasthenia gravis, there is usually a decrease in nerve signal strength. In contrast, in Lambert-Eaton myasthenia syndrome, nerve signal strength typically increases after brief muscle contraction, but then decreases due to exhaustion.
Another type of test called single-fiber EMG can reveal increased jitter, or variability in signal transmission, in NMJ disorders. While the test is extremely sensitive in identifying NMJ disorders, it isn’t specific to any particular disorder.
There are also tests known as ‘late responses’, but these are generally not useful in a study of the neuromuscular junction.
In a part of the exam called needle EMG, the doctor should examine all weak muscles in the patient. The test could show unstable muscle signals with normal recruitment, the process to enlist more muscle fibers to increase muscle strength. Most NMJ disorders don’t show abnormal spontaneous activity, except for a disease known as botulism. Therefore, certain findings such as muscle denervation or myotonic discharge do not signify an NMJ disorder.