nerve conduction study

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nerve conduction study

Neurology A noninvasive method for assessing a nerve's ability to carry an impulse, which quantifies latency periods and conduction velocities; larger peripheral motor and sensory nerves are electrically stimulated at various intervals along a motor nerve. See Carpal tunnel syndrome, F wave, H-reflex, Jitter, Latency period.

nerve conduction study

Abbreviation: NCS
An electrodiagnostic test to determine whether the conduction of impulses along specific nerves is normal or pathologically slowed. In the test an electrical shock is given to a nerve that controls a particular muscle. The time for the muscle to contract and the distance for the electrical stimulus to travel along the nerve are recorded. In patients with neuropathies, the expected velocity of impulse conduction will not be met; slowing will be evident. Patients with cut or injured nerves will show maximal slowing of impulse conduction.
See also: study

Electroneurography

Synonym/acronym: Electrodiagnostic study, nerve conduction study, ENG.

Common use

To assess peripheral nerve conduction to assist in the diagnosis of diseases such as diabetic neuropathy and muscular dystrophy.

Area of application

Muscles.

Contrast

None.

Description

Electroneurography (ENG) is performed to identify peripheral nerve injury, to differentiate primary peripheral nerve pathology from muscular injury, and to monitor response of the nerve injury to treatment. A stimulus is applied through a surface electrode over a nerve. After a nerve is electrically stimulated proximally, the time for the impulse to travel to a second or distal site is measured. Because the conduction study of a nerve can vary from nerve to nerve, it is important to compare the results of the affected side to those of the contralateral side. The results of the stimulation are shown on a monitor, but the actual velocity must be calculated by dividing the distance in meters between the stimulation point and the response point by the time between the stimulus and response. Traumatic nerve transection, contusion, or neuropathy will usually cause maximal slowing of conduction velocity in the affected side compared with that in the normal side. A velocity that is greater than normal does not indicate a pathological condition. This test is usually performed in conjunction with electromyography in a combined test called electromyoneurography.

This procedure is contraindicated for

  • high alert Patients with a bleeding disorder when performed in addition to electromyography.

Indications

  • Confirm diagnosis of peripheral nerve damage or trauma

Potential diagnosis

Normal findings

  • No evidence of peripheral nerve injury or disease. Variable readings depend on the nerve being tested. For patients age 3 yr and older, the maximum conduction velocity is 40 to 80 milliseconds; for infants and the elderly, the values are divided by 2.

Abnormal findings related to

  • Carpal tunnel syndrome
  • Diabetic neuropathy
  • Guillain-Barré syndrome
  • Herniated disk disease
  • Muscular dystrophy
  • Myasthenia gravis
  • Poliomyelitis
  • Tarsal tunnel syndrome indicated by decreased conduction time
  • Thoracic outlet syndrome

Critical findings

    N/A

Interfering factors

  • Factors that may impair the results of the examination

    • Inability of the patient to cooperate or remain still during the procedure because of age, significant pain, or mental status.
    • Age-related decreases in electrical activity.
    • Poor electrode conduction or failure to obtain contralateral values for comparison.

Nursing Implications and Procedure

Pretest

  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient this procedure is performed to measure the electrical activity of the muscles.
  • Obtain a history of the patient’s complaints or symptoms, including a list of known allergens, especially allergies or sensitivities to latex or anesthetics.
  • Obtain a history of the patient’s neuromuscular system, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
  • Review the procedure with the patient. Inform the patient that it may be necessary to remove hair from the site before the procedure. Address concerns about pain related to the procedure and inform the patient the procedure may be uncomfortable because of a mild electrical shock. Advise the patient that the electrical shock is brief and is not harmful. Inform the patient the procedure is performed in a special laboratory by a health-care provider (HCP) and takes approximately 15 min to complete but can take longer depending on the patient’s condition.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.
  • Instruct the patient to remove jewelry and other metallic objects from the area to be examined.
  • Make sure a written and informed consent has been signed prior to the procedure and before administering any medications.

Intratest

  • Potential complications: N/A
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient.
  • Ensure the patient has removed all external metallic objects from the area to be examined prior to the procedure.
  • Instruct the patient to void prior to the procedure and to change into the gown, robe, and foot coverings provided.
  • Place the patient in a supine or sitting position, depending on the location of the muscle to be tested.
  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Use clippers to remove hair from the site if appropriate, and cleanse the skin thoroughly with alcohol pads.
  • Apply electrode gel and place a recording electrode at a known distance from the stimulation point. Measure the distance between the stimulation point and the site of the recording electrode in centimeters.
  • Place a reference electrode nearby on the skin surface.
  • The nerve is electrically stimulated by a shock-emitter device; the time between nerve impulse and electrical contraction, measured in milliseconds (distal latency), is shown on a monitor.
  • The nerve is also electrically stimulated at a location proximal to the area of suspected injury or disease.
  • The time required for the impulse to travel from the stimulation site to location of the muscle contraction (total latency) is recorded in milliseconds.
  • Calculate the conduction velocity. The conduction velocity is converted to meters per second (m/sec) and computed using the following equation: Conduction velocity (m/sec) = [distance (m)] / [total latency − distal latency]
  • When the procedure is complete, remove the electrodes and clean the skin where the electrodes were applied.
  • Monitor electrode sites for inflammation.

Post-Test

  • Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
  • If residual pain is noted after the procedure, instruct the patient to apply warm compresses and to take analgesics, as ordered.
  • Instruct the patient to resume usual diet, medication, and activity, as directed by the HCP.
  • Recognize anxiety related to test results, and be supportive of perceived loss of independent function. Discuss the implications of abnormal test results on the patient’s lifestyle. Provide teaching and information regarding the clinical implications of the test results, as appropriate.
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

  • Related tests include acetylcholine receptor antibody, biopsy muscle, CK, EMG, evoked brain potentials (SER, VER), fluorescein angiography, fundus photography, glucose, glycated hemoglobin, insulin, microalbumin, and plethysmography.
  • Refer to the Musculoskeletal System table at the end of the book for related tests by body system.
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