"What You Should Know"
Many technicaI factors can affect the test
Juan L. Joy, M.D.
Clinical electromyography (EMG) consists of nerve conduction studies (NCS) and needle electromyography (needle EMG). In the strict sense of the word, EMG only refers to the needle electrode examination of muscles, however is has traditionally been used to refer to both NCS and needle EMG. EMC is a very powerful diagnostic modality for the evaluating the peripheral nervous system which, in competent hands, can provide invaluable information that may not be obtainable with any other diagnostic test. It can also complement the information obtained from other sources, such as imaging studies, for optimal localization of a lesion.
More so than most electrodiagnostic tests, EMG is extremely dependent on the skill of the examiner. There are many technical factors which can affect the test and produce erroneous results if not recognized5,6,8. It is also important to note that EMC is not a standard test like electrocardiography. There are dozens of muscles and nerves that can be tested, and it is the electromyographer's job to tailor the examination to the patient's particular clinical problem. What is tested is as important as how competently it is done.
Peripheral nerves transmit electrical impulses and can be simplistically thought of as extremely efficient conductive wires. For NCS studies, a noninvasive stimulator applies brief electrical impulses to a peripheral nerve transcutaneously, the nerve then transmits the impulse and a response is recorded by electrodes at some distance away. The time it takes for the stimulus to reach the recording electrodes (latency) can be accurately measured and a velocity of transmission calculated. Both motor and sensory nerves can be examined. Healthy nerves will transmit the electrical impulse faster than diseased ones.
Needle EMG does not introduce any electrical stimulation, instead it records the intrinsic electrical activity of skeletal muscle fibers. The needle is quite slim (about a 25 gauge) and produces minor discomfort which most patients can tolerate. Needle EMG findings suggestive of denervation include:
Abnormalities pointing to a myopathy include small short-duration MUP. Since the root and peripheral nerve supply to the muscles is known, needle EMG can be used to localize peripheral nerve or root lesions by noting which muscles show denervation changes.
For most conditions, NCS and needle EMG complement each other and are performed at the same session.
CTS is the single most common referring diagnosis in our EMG laboratory and by far the most common entrapment neuropathy2,9. Electrophysiological features include:
Although false negatives do occur, they are so infrequent that the diagnosis should be reconsidered. An asymptomatic contralateral CTS can be seen in 32% of patient 8.
The second most frequent entrapment neuropathy is ulnar neuropathy at the elbow2,9. Entrapment at the wrist is much less frequent. An useful clinical tip in making a distinction is that when the lesion is at the wrist, only the sensation in the palm is affected, whereas with a lesion at the elbow both the palm and the dorsum of the hand are numb. This is so because the ulnar nerve branch which supplies sensation for the dorsum of the hand (dorsal ulnar cutaneous nerve) leaves the main trunk of the ulnar nerve at the forearm so it would not be involved in lesions at the wrist.
The cardinal EMG findings in elbow involvement are:
Electrodiagnostic studies are positive in 73-91% of cases8.
In this group of conditions the peripheral nerves are affected in a diffuse fashion and more or less symmetrically. This is distinguished from entrapment neuropathies where only one nerve is affected. In the USA the most common etiologies are diabetes mellitus (by far the most frequent), alcohol abuse, and uremia. In third world countries, leprosy is one of the leading causes8.
EMG features include:
Apart from establishing the diagnosis, NCS can also be helpful in suggesting the etiology of the neuropathy. Peripheral neuropathies are classified as "axonal" or "demyelinating", depending on which part of the nerve is most affected by the disease process. The great majority of polyneuropathies are of the axonal type. Only a handful are demyelinating, so finding NCS features of demyelination really narrows down the differential diagnosis6,8. Examples of demyelinating peripheral neuropathies include: Guillain-Barre syndrome, chronic demyelinating neuropathy, and polyneuropathy associated with gammopathy.
This the second common referral diagnosis (after CTS). Needle EMG is essential to establish the diagnosis. NCS are usually normal in radiculopathies, but are required to exclude concurrent peripheral neuropathy or mononeuropathy which may affect the needle EMG examination.
In adults, polymyositis is the most common cause of acquired myopathy3. Needle EMG is essential for diagnosis. NCS are performed for the same reasons as in radiculopathies.
Small-short polyphasic motor unit potentials are the EMG hallmark6.
As mentioned above, EMG examination can be performed in many nerves and muscles and must be tailored to the particular clinical situation. Be as specific as you can in your referral diagnosis. In cases of radiculopathy, if a specific root level is suspected, mention it. For entrapment neuropathies, include the nerve and the suspected site of involvement (wrist, elbow, etc.). A short pertinent clinical history is also very helpful. Important conditions to mention are diabetes mellitus, alcoholism, uremia, cancer, and collagen vascular diseases. In general, any severe systemic disease is worth mentioning. It is best to request both NCS and needle EMG since they usually complement each other.
The time of onset of symptoms is very important since needle EMC evidence of denervation takes at least 7-10 days to develop following nerve injury 7. Thus, a negative needle EMG examination done within 10 days of the onset of symptoms should be repeated later. A two weeks interval is recommended to be certain that enough time has passed for needle EMG findings to be detectable.
The following is a example of a short but powerful referring history: 50 year-old previously healthy female to rule out a right C6 radiculopathy. Symptoms started 2 months ago.
Both NCS and needle EMG are only mildly uncomfortable to most patients. No special preparation is required for the test, except to avoid applying oils or lotions on the skin as it interferes with electrode adherence.
As with any other ancillary medical test, clinical correlation is crucial. Ask yourself, does the EMG diagnosis makes sense with the clinical picture? An inadequately performed or interpreted EMG can become a nightmare and lead to unnecessary procedures and even surgery1. If the EMG diagnosis is confusing, discuss it with the electromyographer or request a formal neurological or neurosurgical consultation. As with everything else in medicine, common sense is the key.