How is anal sphincter electromyography (EMG) performed?

Updated: Aug 06, 2019
  • Author: Jasvinder Chawla, MD, MBA; Chief Editor: David C Spencer, MD  more...
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The procedure takes 45-60 minutes, including the pudendal nerve conduction study. With the patient in the lateral decubitus position or the knee-chest or modified lithotomy position, which allows the best examination in infants, digitally examine the sphincter tone. A gloved finger, still in place, can guide the needle, inserted through the perianal skin adjacent to the mucocutaneous junction.

The conventional concentric or monopolar needle suffices for routine clinical use. The tip of the electrode should enter perpendicular to the skin surface close to the anal orifice, 0.5-1 cm from the ring. [2] The ring of the anal orifice has 4 parts: anterior and posterior quadrants on both sides. A complete study consists of exploration of the 4 quadrants with the anal sphincter at rest and while contracted voluntarily or reflexively (see the images below).

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Needle electrode is considered the most accurate method, since the electrodes are inserted directly into the muscle, using needles to guide placement. For male patients, a gloved finger is inserted in the rectum, and then needles with wires attached are inserted through the skin between the anus and the scrotum. For female patients, the needles are inserted around the urethra.

During testing, the patient is instructed to contract the sphincter as though attempting to hold a bowel movement. The motor unit potentials range from 5.5-7.5 ms in duration and from 200-500 mV in amplitude. [38] See the image below for motor unit action potential. Digital examination of the anus, coughing, or crying elicits reflex activity of the sphincter. A full interference pattern should accompany a normal maximal contraction, whether induced voluntarily or reflexively. Reliability of grading the degree of such discharge, as in the skeletal muscles of the limb, depends on patient cooperation. [39, 40] Experienced electromyographers, however, can usually correlate electrical activity and sphincter tone with reasonable accuracy.

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Given their noninvasive nature, skin electrodes are the most commonly used method of recording information. The skin where the electrodes will be placed is cleaned and shaved, and an electrically conductive paste is applied. The electrodes are then taped in place. For female patients, the electrodes are taped around the urethra, while for male patients they are placed between the scrotum and the anus.

The technique involving anal plug electrodes is also used. The tip of an anal plug is lubricated and inserted into the rectum as the patient relaxes the anal sphincter. Electrodes are then attached to the anal plug. Once the electrodes are in place and attached to the recording device, the patient is asked to alternately contract and relax the external sphincter muscle.

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