What is the role of sphincter EMG in urologic management of neurologic disease?

Updated: Aug 08, 2019
  • Author: Frances M Dyro, MD; Chief Editor: Robert A Egan, MD  more...
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Electromyographic (EMG) recordings from the urethral and anal sphincters were first made in the 1930s by Beck, [3] who recorded from the anal sphincter using fine wires. Petersen and Franksson performed studies of the urethral sphincter in 1955. [4]

Since then, measurements of sphincter activity using conventional electrodes, wires, surface electrodes, and catheter-mounted electrodes have become an important part of the evaluation of patients with voiding dysfunction. [5] The procedure is not done routinely unless a neurologic abnormality is suspected as the cause of voiding dysfunction. Some indications would be voiding dysfunction after pelvic or spinal cord trauma and spinal stenosis potentially affecting the cauda equina or conus.

Occasionally, needle EMG may be used to evaluate pelvic pain syndromes. EMG is usually not performed in cases of stress urinary incontinence or cases in which a central nervous system (CNS) lesion is producing the incontinence.

Muscle fibers in the periurethral striated muscle are predominantly type Ia, the slow-twitch, fatigue-resistant fibers. The rhabdosphincter fibers are generally smaller than those of other striated muscle. No stretch receptors have been found in the striated muscle immediately related to the urethra. A curious property of the urethral sphincter is that it has a tonic firing pattern that silences only during voiding.

Positioning of electrodes for recording from the urethral sphincter is performed most easily with the patient in the lithotomy position. The periurethral tissue may be made analgesic with ethyl chloride spray or lidocaine ointment.

The electrode is inserted just lateral to the urethral meatus in women. Sphincter muscle is encountered at a depth of about 15 mm. In the male, the electrode is inserted in the perineum between the scrotum and the anal verge. It may be positioned by feeling for the prostate gland with a finger in the rectum and directing the electrode toward the apex of the gland. The EMG activity should be monitored with a speaker during the positioning.

In females, a 25-mm electrode is used; in adult males, a 75-mm electrode is necessary. If a shorter electrode is used, EMG activity in the more superficial bulbocavernosus muscle is recorded. Other techniques of needle insertion include placement under cystoscopic control and placement via a vaginal approach in women. Although some discomfort is associated with insertion, it is short-lived. Most patients tolerate the procedure if well prepared. The author prefers to use monopolar needle electrodes.

At rest, continuous baseline activity consists of motor unit potentials of 100-500 µV firing at rates of 1-4/sec. This activity is at its lowest when the patient’s bladder is empty. As the bladder fills, the firing rate increases slowly until bladder fullness is perceived. At that point, the type II fibers are contracted voluntarily in a graded way. When bladder capacity is reached, some voluntary units as large as 3 mV are seen.

The motor units of the sphincters are of shorter duration than those of skeletal muscles and resemble the units of facial muscles. Individual motor unit analysis using a delay line showed that 93% of the motor units in the sphincter are less than 6 msec in duration and less than 2 mV in amplitude. [6]

Generally, about 10-15% of units are polyphasic. The presence of fibrillations and positive sharp waves may be difficult to determine. If present, these potentials are best seen during voiding, when the sphincter motor units are silent. Fasciculation potentials are extremely difficult to appreciate when tonic muscles are active. Complex repetitive discharges are more common in the sphincters than in other striated muscles. Their presence has been associated with urinary retention in women [6] and enuresis in children. [7]

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