Overview of Electrodiagnostic Evaluation of Myopathy
Electrodiagnostic testing is a main tool used by doctors to diagnose muscle diseases, which is also known as myopathy. This testing process includes nerve conduction studies (NCS) and electromyography (EMG). These tests are still very important, even with the growth of genetics and improved imaging techniques. In fact, they’re seen as an extension of the physical check-up and are crucial for diagnosing muscle conditions.
NCS is often the first step and it gives useful details about the working of sensory and motor nerve fibers. NCS can help rule out conditions that might look like muscle diseases but are in fact something else. In certain cases, more specialized tests are used to check for issues with the junction where the nerve and muscle meet, which could be another reason behind muscle weakness.
EMG is the next step where a needle-like electrode is inserted into the muscle to analyze the electrical signals at rest and when the muscle is active. The muscles to be tested depend on the patient’s condition and any technical limitations. Doctors often choose to test muscles that are visibly weak as it is more likely to give useful results. Most muscle conditions affect muscles close to the centre of the body, so arm, leg and back muscles are usually tested. However, in some types of muscle disease, the muscles in the extremities such as hands and feet are affected causing not only abnormal EMG tests but abnormal NCS results due to muscle wasting.
These tests are very useful, not just for diagnosing muscle diseases, but they also help in identifying their causes. They can even guide doctors where to take a muscle sample from, or help in deciding about further genetic testing.
Anatomy and Physiology of Electrodiagnostic Evaluation of Myopathy
Skeletal muscle, the type of tissue responsible for movement in the body, is composed of many parts. Individual muscle fibers are grouped together to form bundles, known as fascicules. Each muscle fiber is a cell with many nuclei, and contains structures called myofibrils, which play a critical role in muscle contraction. The entire structure is enveloped by a special type of cell membrane known as the sarcolemma.
These muscle fibers and fascicules are surrounded by three layers of connective tissue, which help to connect and protect the muscle. The endomysium surrounds individual muscle fibers, the perimysium lies between different fascicules, and the epimysium covers all of the fascicules. The epimysium then connects to a tendon or aponeurosis – tough bands of tissue that attach the muscle to bone, skin, or other connective tissue.
A motor unit is a set of one motor neuron and all the muscle fibers it activates. This is an important structure that helps to control the way muscles contract and generate force. Muscles that perform precise movements, like the muscles in your hand or the muscles that move your eyes, have fewer fibers per motor neuron. This is different from larger muscles, such as your thigh or calf muscles, where each motor neuron controls more fibers. The distribution of motor units and their density help generate information that is recorded during medical tests, such as an electromyogram (EMG).
Normally, a resting muscle is “electrically quiet”—it doesn’t generate electrical activity. However, when a motor neuron is activated, it sends an electrical signal, known as an action potential, to the muscle fibers. This causes a change in the electrical charge across the sarcolemma, which makes the muscle fiber contract. This process, where activation of a motor neuron leads to a motor unit action potential, is key in understanding how muscles work, and assessing these signals is a vital part of an EMG test.
Why do People Need Electrodiagnostic Evaluation of Myopathy
If a patient is showing symptoms of weakness, it might be necessary to do what’s called an “electrodiagnostic study.” This is a type of test that checks if a disease of the muscles, known as a “myopathy,” is causing the weakness. This test can also help rule out other conditions that might look like a myopathy but aren’t. Sometimes, the results of this test can even suggest specific kinds of myopathies that could be causing the problem.
This test can also be useful for guiding doctors on where to take a tissue sample, or biopsy. It can show if there are muscles that are affected by the disease, even if they’re not causing any weakness. It can also point out what muscles shouldn’t be used for a biopsy, because they are too damaged.
Electrodiagnostic testing is a key step in diagnosing myopathies. However, there are some exceptions. For example, if a patient has a family history of genetic myopathies, then it might be better to skip this test and go straight to genetic testing.
When a Person Should Avoid Electrodiagnostic Evaluation of Myopathy
Normally, the small electrical shocks used in nerve conduction studies (NCS), which test how well your nerves are working, are safe. But for people with temporary pacemakers, these little shocks can cause dangerous irregular heartbeats. So, these tests should not be done until the temporary pacemaker wires are removed. Permanent pacemakers, devices that correct abnormal heart rhythms, and central lines (tubes placed into a large vein to give medication or collect blood) are safe. However, extra care should be taken when giving the shocks near these devices.
There’s no definite reason why a person can’t have a needle EMG, which measures the electric activity in your muscles. However, if a person is on blood thinners, this can increase the risk of bleeding, so it’s seen as a potential hazard. Therefore, precautions should be taken to avoid bleeding complications.
Equipment used for Electrodiagnostic Evaluation of Myopathy
Modern tools used for diagnosing nerve and muscle problems mix together hardware pieces like a stimulator, amplifier, control panel, and a computer. This computer is special because it has inbuilt programs created to process and store the signals our body’s nervous system sends out. Nowadays, doctors are increasingly using computer tools that help them analyze these signals in greater detail, which makes diagnosis more precise. This kind of nerve testing using computers (Quantitative EMG) is mostly done for research, but sometimes doctors use it in special medical cases too.
The tools that your doctor will use to check your nervous system are called electrodes which can be placed on the surface of your skin or inserted through a needle. Surface electrodes used for nerve conduction studies (tests to see how fast your nerves can send signals) are generally made of silver-silver chloride, platinum, or stainless steel. To stick the electrodes onto your skin and make sure the electric signals from your body are properly picked up, your doctor will apply a special gel, adhesive, paste or saline solution.
Doctors only use throwaway needle electrodes for a test called needle EMG to avoid spreading serious infections like hepatitis, HIV or prion diseases . There are two main types of these needle electrodes:
* Monopolar electrodes: Here, a sharp needle tip which picks up the signals from your body (active electrode) is inserted into your muscle. A separate electrode is placed on the skin to act as a reference electrode.
* Concentric electrodes: In this type, there’s an insulated wire within the needle that performs the function of both the active and reference electrodes. This design helps to reduce interference from unwanted signals (noise) because the two electrodes are very close together.
Who is needed to perform Electrodiagnostic Evaluation of Myopathy?
The American Association of Electrodiagnostic Medicine has outlined certain necessary qualifications for doctors who conduct electrodiagnostic tests. Only a qualified and properly trained doctor should do all needle EMG tests, which are types of nerve studies. A trained assistant can carry out nerve conduction studies, which test the speed and strength of signals traveling through nerves, but they must do this under the guidance of that qualified doctor.
Preparing for Electrodiagnostic Evaluation of Myopathy
There’s no detailed preparation needed for tests related to electrical activity in your body (electrodiagnostic studies). However, it’s important to avoid using things like lotions, creams, or oils on your skin for a few days before the test, or at least on the day of the test. This allows the doctors to get the most accurate results possible.
How is Electrodiagnostic Evaluation of Myopathy performed
To figure out issues with your muscles, your doctor may use two main types of tests: Nerve Conduction Studies (NCS) and Electromyography (needle EMG).
NCS, as the name implies, is a test that measures how well your nerves work. This is done through checking the speed and strength of electrical activity in your nerves. Usually, the results of this test are normal unless there’s another disorder that affects both the nerves and muscles, like certain critical illnesses or conditions like amyloidosis.
On the other hand, Electromyography (EMG) uses needles, or sometimes surface electrodes, to assess the health of your muscles. However, when it comes to detecting muscle disorders, only EMG needles are used. Think of these needles as tiny microphones that can listen to and measure the electrical signals from your muscles. These signals can tell a lot about how your muscles are functioning. And don’t worry, these needles are specifically designed for this purpose and are safe to use.
While testing, the doctor will make sure to pick specific muscles to study based on your symptoms. If they’re planning to take a small sample of your muscle (a biopsy), they will use this test to find the best place to do that. They’ll look for muscles that are affected, but not too weak or too normal. They will only test on one side of your body, leaving the other side untouched in case a biopsy is needed. They analyze the results by looking for any spontaneous activity, the shape of motor unit potential, and the recruitment pattern.
In a healthy muscle, no spontaneous activity should be seen during rest. However, in diseases that affect the muscles, other types of electrical signals can be picked up. Some of these signals resemble the sound of a ticking clock, a jackhammer, or a dive bomber. These sounds and their patterns can provide clues to what kind of muscle disorder you might have.
Another part of the test involves looking at the duration and strength of the electrical activity of your muscles (MUAPs) when you move them. Normally, only a few motor units (a group of muscles and the nerve that controls them) are activated when minimal effort is used. But as you use more force, more motor units have to work. In people with muscle disorders, these motor units have to work a lot harder and faster to generate force, even for simple tasks. This early or rapid recruitment pattern is typical for muscle diseases.
All in all, these tests help your doctor understand what’s going on with your muscles and plan the best course of treatment for you.
Possible Complications of Electrodiagnostic Evaluation of Myopathy
Tests that study the electrical activity of your muscles and nerves are generally safe and usually not a problem for patients. Unwanted side effects caused by the test itself are extremely rare. However, there are a few potential risks to keep in mind.
There’s a theoretical risk associated with the electrical currents used in the tests, especially for nerve function. Thankfully, modern equipment comes with safeguards to prevent any electrical injury. It’s important that this equipment is well maintained and used correctly. Although these studies should be carried out with caution in hospital intensive care units due to the presence of other electrical equipment, they are generally safe for those with internal lines for giving medicine, modern heart pacemakers, and defibrillators, provided they have a specific type of lead. However, these tests should not be carried out on patients with temporary pacemakers fitted on their skin.
Another potential risk is the slim chance of a pneumothorax, which is when air builds up between your lung and chest wall, possibly causing a collapsed lung. Even though this is rare, examining certain muscles close to the lungs carries a higher risk. It’s usually best not to test these muscles as a routine, but if it’s necessary, doctors can use ultrasound images to guide where the needle goes for testing.
Bleeding from the needle insertion is extremely rare too. Studies have shown that even patients who’re on blood thinners or taking medication to prevent blood clots are at a very low risk of developing a hematoma, which is a pool of clotted blood. So, there’s usually no need to stop taking these medications before the electrical test. Doctors can use strategies such as using a smaller needle, limiting the number of needle insertions, and avoiding testing muscles near large blood vessels to reduce the risk of bleeding.
Risk of infection is usually not a concern anymore since doctors use disposable needles. As long as doctors follow proper hygiene practices and avoid inserting needles into skin areas that are infected, the risk of infection is very low.
What Else Should I Know About Electrodiagnostic Evaluation of Myopathy?
Electrodiagnostic tests are vital when investigating suspected cases of myopathy (a disease where muscle fibers do not function properly). These tests can help confirm whether a patient has myopathy and sometimes even point towards the cause of this condition. These tests can also guide doctors in choosing which muscle to sample if a biopsy (a procedure to extract sample tissues or cells for laboratory testing) is needed.
* In many cases of myopathy, nerve conduction studies (tests assessing the speed and degree of electrical activity in your nerves) come out as normal. However, in severe cases where the muscles are significantly weak and have shrunk (a condition known as muscle atrophy), the output of your motor nerves (nerves that control muscle activity), can be reduced. Among unique features, a particular pattern known as prolonged Compound Muscle Action Potential (CMAP) duration is noted in critical illness myopathies. Recognizing this pattern can help doctors diagnose this condition.
* Needle Electromyography (EMG, a diagnostic procedure to assess the health of muscles and the nerve cells that control them) can point out standard myopathic features. These include motor aspects like short duration, low amplitude, and many phases with rapid involvement. A reduced spike duration is considered a dependable sign of myopathy.
However, these results aren’t specific to just myopathies and can be seen in disorders of the nerve-muscle junction as well. So, detailed testing, including repetitive nerve stimulation and single-fiber EMG, is necessary when such a disorder is suspected. Some characteristics of needle EMG can help find the cause of myopathy. This could be through identifying spontaneous activity or figuring out which muscles are affected and where.
Abnormal Spontaneous Activity
* Certain patterns of muscle activity are more typical in myopathies with structural changes within the muscle, like protein build-up, vacuoles (empty spaces), inflammation, muscle fiber death, and fiber splitting.
* Specific types of myopathies, such as Polymyositis, Dermatomyositis, and Necrotizing myopathies, often show prominent fibrillation potentials and positive sharp waves in the affected muscles.
Fibrillation potentials (a type of muscle activity) can also occur in some muscle dystrophies and other inherited myopathies. They might also appear in myotonic disorders (muscles unable to relax normally), as well as in disorders where ‘electrical myotonia’ can be detected without any visible myotonia (prolonged muscle contractions).
Topographical Distribution of Affected Muscles in Various Myopathies
The most common pattern for most myopathies is a predominantly weak set of proximal muscles (those close to the center of the body). Comprehensive electrodiagnostic assessment should also focus on neck extensor, distal (far from the center of the body), and facial muscles to lookup often less noted patterns.
In specific myopathies, for example, steroid myopathy (the most common drug-induced myopathy), nerve conduction studies and EMG often come out normal. These myopathies affect a particular type of muscle fiber and can often get worse given certain causes like undertreatment of inflammatory myopathy or development of steroid myopathy.
Lastly, EMG has limited success in diagnosing disorders that impact muscle properties mechanically but not electrically, as seen in some metabolic and congenital myopathies. In these cases, the muscle doesn’t contract properly, but the electrical activity, which is what the EMG test measures, could still appear normal.