What is electroneurography (ENG)?
ENG is a method used to test nerve conduction of the peripheral nerves and thus determine the speed at which a nerve transmits electrical signals (nerve conduction velocity). It also measures how well an electrical nerve stimulus is transmitted to a certain muscle (neuromuscular conduction).
What is electromyography (EMG)?
EMG is a method for testing a muscle’s electrical activity. It provides clues as to whether there is a disorder of the muscle itself or whether the nerve that communicates with the muscle is not functioning properly.
How does ENG work?
Motor neurography is used to examine the nerve by electrically stimulating at least two points along its path and measuring the time it takes for the nerve stimulus to reach the target muscle, i.e. to trigger a response (contraction). This takes just a few thousandths of a second and can only be measured electronically. The muscle contraction is recorded by a computer connected to the surface electrodes.
The nerve is stimulated at two different points. The difference in the conduction times and the length between the two stimulation points is used to calculate the speed of nerve conduction (NLG).
How does EMG work?
The doctor disinfects the skin and inserts an ultra-thin needle electrode directly into the muscle of the patient. Amplifiers make it possible to detect the activity of the individual muscle fibers inside a muscle. A computer records the measured voltage fluctuations and generates a visualization of the signal. These fluctuations can also be converted into auditory signals heard as noise and rattling through a speaker. A skilled doctor will be able to make conclusions about the type of damage based on the sounds alone. The detailed analysis is then carried out on the computer. Here the doctor will specifically look for:
- Electrical signals produced directly after the needle prick
- The generation of spontaneous signals when the muscle is in a relaxed state (spontaneous activity)
- Signals generated when carefully tensing the muscle during the examination
Why is ENG used?
An ENG helps doctors to determine the type and severity of various peripheral nerve disorders. The method is used to diagnose:
- The severity of a polyneuropathy, which is a peripheral nerve disorder that has many possible causes (including diabetes or chronic alcohol overconsumption).
- The exact site and severity of nerve damage caused by an injury.
- The appearance of articular constriction in the nerves of the legs and arms, e.g. carpal tunnel syndrome (CTS) in the wrist.
What is the purpose of the EMG?
An EMG helps the doctor to determine the type and severity of various muscle and nerve disorders. This should always be preceded by a thorough neurological examination in order to reach a tentative diagnosis. Then the doctor can then conduct a closer examination of certain muscles. This is to the patient’s benefit, as the needle EMG is not a very pleasant procedure.
A muscle normally does not show any electrical activity in its relaxed state. However, even a slight contraction produces electrical signals, which further increase with more intense muscle movements. Certain disorders of the muscles, peripheral nerves or nerve roots at the spinal outlet result in abnormal or specific patterns of electrical activity in the muscle being tested with the needle.
EMG makes it possible to resolve the following questions:
- Is the muscle weakness due to a problem in the muscle or the associated nerve?
- In the case of myoparalysis resulting from an injury or inflammation of the supplying nerve, an EMG helps determine the likelihood of recovery (prognosis). It is important to detect whether there is any residual activity in the muscle or signs that the nerve is recovering through regeneration of nerve fibers (reinnervation).
- A close examination is performed to see which muscles are affected by nerve damage and thus to pinpoint the site of the nerve damage. This makes it possible to visualize the affected region of the body, for example, using computer tomography.
What needs to be considered before using ENG?
It is first necessary to carry out a thorough neurological examination in order to minimize the number nerves that need to be tested. The reason is that electrical stimulation of the nerves is sometimes unpleasant for the patient.
What needs to be considered before performing an EMG?
An EMG should not be performed on patients who are taking blood thinners (e.g. anticoagulant medications) or who have a coagulation disorder. If absolutely necessary, the doctor can still examine specific, smaller muscles in the hands and feet. This does, however, involve a risk of hemorrhage in the muscle.
Although only disposable needles are used in our laboratory, the patient must still notify the doctor of any infectious diseases they have which can be contracted by contact with blood (e.g. hepatitis B or C, HIV).
How is the ENG carried out?
The nerve is electrically stimulated at least at two points along the nerve path. For nerves supplying the hand muscles, these points in the upper arm are found in the elbow or hand muscles, for example.
The surface electrodes for recording the muscle contraction are attached to the finger muscles in this case. When stimulated, the patient will feel an electrifying sensation.
The examination will yield two important results:
- Disorders of the myelin sheaths that envelop the nerves cause a slowing of nerve conduction velocity.
- Disorders affecting the interior of the nerves may show normal nerve conduction velocity, but the level of electrical nerve response (amplitude of the nerve potential) is less.
The strength of the muscle contraction also makes it possible to draw conclusions about possible nerve damage.
What is the procedure for the EMG?
The patient lies down on an examination table and remains as relaxed as possible. When performing a needle EMG, the doctor disinfects the area of skin and inserts a thin needle electrode directly into the muscle. The doctor will then wait until the muscle has relaxed again after inserting the needle. Next, the patient gently tenses the muscle and then fully tenses it. The computer records all electrical activities in the muscle.
Possible complications of ENG and EMG
Patients often find the electrical impulses during the electrical neurography examination unpleasant but tolerable. No serious complications are known.
Unfortunately, the needle EMG is not a painless examination. Most patients, however, find the pain of inserting the ultra-thin needle tolerable. In very rare cases, infections or bruising may occur in the region of the puncture.