Anal Sphincter Electromyography and Sphincter Function Profiles Technique
- Author: Jasvinder Chawla, MD, MBA; Chief Editor: David C Spencer, MD more...
EMG studies can help quantitate sphincter dysfunction in neurologic disorders. They help establish or rule out the possibility of agenesis of the striate sphincter in the preoperative assessment of the newborn with an imperforate anus. Electrical studies not only localize the sphincter precisely, if it is present, but also determine its functional capacity. The anal sphincter may sustain traumatic injury in several conditions, including during parturition, prostatectomy, or rectal surgery for repair of an anal fistula or prolapse. EMG helps determine the extent of damage in such cases and aids in establishing the differential diagnoses for fecal incontinence.
The anal and external urethral sphincters share a common segmental derivation. Thus, confirming the integrity of the anal sphincter provides an important, albeit indirect, guide in ilioconduit surgery for prominent urologic dysfunction. EMG of the urethral sphincter ideally involves the help of urologists working in a laboratory equipped with tools for urodynamic investigations.
Abnormality of the striated sphincter may occur from a lesion of the peripheral nervous system (PNS), central nervous system (CNS), or a combination of both. Weakness from central lesions causes reduction in voluntary discharges with preservation of reflexive activation. The interference pattern is incomplete, with motor unit potentials of normal amplitude but discharging at a low frequency. With complete loss of voluntary activity, the low frequency discharge normally seen at rest continues during maximal effort contraction. Lesions involving the PNS are generally localized to the cauda equina or the sacral or pudendal plexus.
Needle examination of the subcutaneous external anal sphincter (EAS) muscles alone likely suffices in most patients with conus medullaris or cauda equina disorders and those with symptoms related to central nervous system disorders.[2, 19] However, examination of the deeper muscles is necessary when evaluating for sphincteric dysfunction. The deeper muscles are typically found with a needle insertion perpendicular to the mucosal surface slanted at about 30° to the anal canal axis, usually at a depth of 15-25 mm.
In an incomplete paralysis, a volitional effort recruits a few motor units that fire at a high frequency. In contrast to central paralysis, the surviving units show a polyphasic waveform and a long duration. In an acute cauda equina syndrome, the initial paralysis may result from a functional block. Axonal degeneration, if present, gives rise to fibrillation potentials, positive sharp waves, and complex repetitive discharges.
Patients often have a mixture of central and peripheral paresis in congential malformation, vascular disease, or traumatic injury of the conus medullaris. Spina bifida with meningomyelocele characteristically affects both upper and lower motor neurons. EMG of the anal sphincter in these cases reveals absent or markedly reduced voluntary activity. Reflexive contraction, if present, shows isolated high-frequency discharges of a few motor units. Complete damage to the sacral segment of the conus medullaris precludes sphincter response either voluntarily or reflexively. Spontaneous potentials recorded in these cases indicate the involvement of the anterior horn cells.
Amyotrophic lateral sclerosis (ALS) typically spares the sphincter, even when the limb muscles show evidence of conspicuous denervation. In contrast, abnormal spontaneous activity serves as a specific marker for neuronal degeneration of Onuf’s nucleus in multiple system atrophy and progressive supranuclear palsy. In one series of 126 patients in whom multiple system atrophy was suspected, 82% of those with definite diagnosis had abnormal sphincter study findings. Abnormal findings also help differentiate multiple system atrophy from Parkinson disease.
In recent studies,[31, 32] utilization of urethral sphincter electromyography (US-EMG) as a supplement to the external anal sphincter EMG (EAS-EMG) was proposed for routine electrophysiological method in patients with a suspicion of MSA. The differences of multiple parameters of EAS-EMG were more significant than those of US-EMG for MSA cases.
Reports on autonomic systems like lower urinary tract and bowel functions in patients with critical illness polyneuropathy (CIP) are not available in medical literature. Reitz, in a recent study, has revealed that sensory and motor pathways controlling the lower urinary tract, including detrusor overactivity and detrusor overactivity incontinence, might be affected by CIP.
In a study by Podnar, it has been shown that the current study complements previous reports in men, supporting the high clinical utility of sacral neurophysiologic studies in confirmation and exclusion of sacral neuropathic lesions.
Anal Sphincter Electromyography Procedure
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. 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).
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. 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.[36, 37] Experienced electromyographers, however, can usually correlate electrical activity and sphincter tone with reasonable accuracy.
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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