Limitations of EMG

Neurophysiology only. With rare exceptions, EMG is only capable of identifying a lesion and categorizing it. For example, you may learn that there is an axonal lesion involving the upper portion of the brachial plexus. The patient’s clinical presentation and other studies will be necessary to determine what type of pathology is present.

False negatives. As with all procedures, EMG can be normal despite the presence of a neuropathic or myopathic lesion. Depending on the condition, the false negative rate is 10-20 percent.

Cutaneous nerves. Many cutaneous nerves cannot be tested. Therefore, patients can have areas of numbness that cannot be evaluated by EMG. It may still be helpful to perform the test to rule out involvement of a more proximal or major nerve.

Onset of denervation. Spontaneous discharges indicative of denervation take one to three weeks to appear in tested muscles from the time that the axon is involved. In other words, cervical paraspinal muscles will take at least one week to appear abnormal in a cervical radiculopathy, and leg musculature may require three weeks to appear abnormal in a radiculopathy. Certain conduction studies (H reflexes, F waves) may be of diagnostic assistance immediately after onset of a proximal neuropathic process.

Disappearance of spontaneous denervation potentials. Fibrillations and/or positive waves can persist for months to years after the resolution of a problem. This fact may obviously limit the value of studies that are done to rule out a recurrent problem or document the resolution of a prior problem.

Quantitative assessment of a radicular lesion. Generally speaking, EMG is not useful for quantitating the degree of involvement in a radiculopathy.

Performance of the EMG

The EMG is considered an extension of the clinical examination. Therefore, relevant history and examination should precede the test. The clinical examination will help determine which specific nerves and muscles are studied.

The testing procedure is somewhat uncomfortable; this depends on the emotional state of the patient, specific procedures that must be done, diameter of needles used, and "touch" of the electromyographer. On rare occasions, the patient is uncomfortable for several hours after the test and may require minor analgesics.

The test may take 20 to 90 minutes and averages 45 minutes. A good deal of this time is utilized by recording data and setting the machine or electrodes.

Nerve conduction velocity studies involving small electrical shocks are generally performed by the physician. However, standard NCVs may be performed by a technician and reviewed and interpreted by a physician. In the state of New Jersey, the electromyographic portion of the test (EMG), which involves the insertion of fine needles, must be performed by a physician. Decisions regarding which nerves and muscles to examine are determined during the course of the test. Although images of action potentials and motor units can be saved, they are rarely useful for purposes of review. A review and interpretation of the recorded data culminates in the EMG report.

Complications are extremely unusual. Although infections can occur theoretically, we have not seen one yet. Most electromyographers currently use disposable needles. The non-disposable needles are sterilized with the operating room equipment. A pneumothorax can occur while needing certain thoracic muscles, although this is rare.