Anal Sphincter Electromyography and Sphincter Function Profiles

Updated: Aug 06, 2019
Author: Jasvinder Chawla, MD, MBA; Chief Editor: David C Spencer, MD 



Anal sphincter electromyography may be clinically useful in the evaluation of patients with urinary, bowel, and sexual dysfunction. It should be considered for all patients with a history and clinical examination suggestive of a central or peripheral sacral neuropathic lesion. The external anal sphincter (EAS), innervated by the pudendal nerve, is the best muscle for detection of neuropathic lesions in lower sacral myotomes.[1, 2] Its circular superficial location and muscle bulk allows for easier access and less painful needle insertion in order to diagnose particularly proximal sacral nervous system disorders.[1, 3]

Surface recordings from the sphincter have shown increased activity with body actions and decreased activity in sleep. Although surface EMG has been studied, needle EMG is clearly superior.[4] This article describes the clinical utility, commonly used technique, and role of EMG in various neurological disorders associated with anal sphincter abnormalities.


Currently, no guidelines exist for referral of a patient for anal sphincter EMG. However, one can consider the utility of this procedure in all patients with neuropathic abnormalities in the lower sacral segments. Common patient complaints leading to EMG include difficulties with bladder emptying, “clustering” of sacral symptoms (urinary, bowel, and sexual symptoms), and perineal sensory loss.[5] Peripheral sacral parasympathetic nerve fiber damage is a common cause of detrusor failure, leading to bladder dysfunction.[1]

Those most likely to have abnormal study findings include, particularly, those with bladder emptying difficulties, perineal sensory loss, and lower-limb focal neurologic signs.[1] For those in whom cauda equina lesions are suspected, bilateral EMG is far more sensitive for detecting abnormalities than unilateral examination and should be performed in all cases.[6] As is true for all EMGs, the study helps to confirm a suspected diagnosis and is not useful as a screening test to exclude a neuropathic lesion.[1]

Quantitative EMG of the external anal sphincter (EAS) is highly useful in the diagnosis of patients with suspected neuropathic sacral lesions.[7] It is useful for the confirmation or exclusion of cauda equina or conus medullaris lesion in the context of appropriate clinical and other laboratory findings.[7] However, no single diagnostic criterion has both satisfactory sensitivity and satisfactory specificity.[6] Combined with neurophysiologic measurement of sacral reflexes, it is highly sensitive (94-96%) for diagnosing chronic cauda equina or conus medullaris lesions.

Quantitative anal sphincter EMG is likely of greater value in women, for whom sacral reflex testing is less useful.[8] EAS EMG has also been shown to be abnormal, with evidence of denervation or reinnervation, in postpartum women with fecal incontinence. EMG could possibly be used to identify those at risk for pelvic floor disorders.[9]

Anal sphincter EMG has also proven useful in earlier detection of pudendal neuropathies and even possibly for preclinical markers for future development of pelvic floor disorders. Clouds analysis may be particularly helpful for evaluating the pelvic floor, as it can be used irrespective of the force of muscle contraction. This is particularly important for tonically contracting pelvic floor muscles which, like most facial muscles, do not move bones through a measurable range of motion.[10]

Anal sphincter EMG may also be useful in the evaluation of patients with multisystem atrophy (MSA).[11, 12]  Patients with MSA have degenerative changes in interomediolateral cell columns and Onuf’s nucleus in the spinal cord, innervating both urethral and anal sphincters. Abnormal EMG of the EAS is strongly suggestive of an atypical parkinsonian syndrome such as MSA. Patients with MSA typically have longer mean duration motor unit potentials (MUPs) than healthy control subjects. Patients with idiopathic Parkinson disease do not show marked sphincter EMG abnormalities. Therefore, these abnormalities can be used to distinguish MSA from idiopathic Parkinson disease in the first 5 years after disease onset.[13]

In contrast, similar sphincter EMG abnormalities are found in some, although not many, patients with dementia with Lewy bodies, pure autonomic failure, progressive supranuclear palsy, and spinocerebellar ataxia type 3.[14] Sphincter EMG and relevant sacral autonomic tests are diagnostic tools for autonomic disorders, reflecting the common and significant involvement of the sacral cord in MSA.[15]

In amyotrophic lateral sclerosis (ALS), the Onufrowicz nucleus in the medial sacral spinal cord, which innervates the sphincter and pelvic floor muscles, is spared; therefore, urinary and bowel incontinence is not a feature of even advanced cases.[16] However, abnormalities including increased fiber density, abnormal jitter, and fibrillations have been reported in some patients with ALS.[17]

The most burdensome and disabling problem affecting patients with myotonic dystrophy may be fecal incontinence as a result of sphincter involvement. Up to two thirds of patients with myotonic dystrophy have occasional fecal incontinence, and more than 10% report fecal incontinence one or more times a week.[18] Pudendal nerve terminal motor latencies are normal in these patients, confirming the absence of a neurogenic lesion.[19] Herbaut et al reported myopathic units with decreased duration and amplitude of the motor units in the external anal sphincter and puborectalis muscle of patients with myotonic dystrophy and fecal incontinence.[18, 20, 21] Eckardt et al reported myopathic potentials with myotonia.


Contraindications to anal sphincter EMG are similar to contraindications for performing limb or cranial EMG. EMG is usually contraindicated in persons receiving anticoagulant therapy. It also may be contraindicated in persons with extensive skin infections because of the risk of spreading infection from the skin to the muscle.

Other risks may exist, depending upon the specific medical condition (eg, edema, bleeding, burns) or surgical site.

EMG should not be performed from the site of anticipated muscle biopsy.

Technical Considerations

Best Practices

Needle electrode is considered the most accurate method, since the electrodes are inserted directly into the muscle, using needles to guide placement.

The superficial EAS muscle has the highest diagnostic yield. The superficial EAS can be localized on needle EMG by its higher number of low threshold motor units that fire continuously during relaxation.[2] It can be reached with a sharp angle of insertion relative to the mucosal surface.[2]


The anal sphincter is innervated by the pudendal nerve, which derives from the anterior division of the S3, S4, and, occasionally, also S2 spinal nerves. The pudendal nerve, which innervates the external anal sphincter (EAS), is formed primarily by the ventral rami of the second and third sacral spinal nerves, with a frequent contribution from the first sacral spinal nerve.[2] Although the superficial EAS is circular in nature, it should be considered to be 2 separate muscles. The EAS is divided into 3 striated subcutaneous, superficial, and deep muscles. The superficial EAS muscle, only 2-5 millimeters below the skin surface just outside the anal canal (usually 10-12 mm from the anal orifice), has the highest diagnostic yield.[2, 22] The superficial EAS is usually more severely affected by proximal nerve lesions than the deeper EAS muscles.[22]

Interdigiitation of muscle fascicles across the midline results in substantial overlap of innervation between the two sides. This enables partial reinnervation from the contralateral side after unilateral pudendal neurectomy.[23] The anal sphincter, which normally is under volitional control, shares similar physiologic properties with the skeletal muscles of the limbs.

The superficial EAS can be localized on needle EMG by its higher number of low-threshold motor units that fire continuously during relaxation.[2] It can be reached with a sharp angle of insertion relative to the mucosal surface.[2] The role of the superficial EAS is to protect the anal orifice. The deeper muscles serve intrinsic sphincter functions.[22]


Periprocedural Care

Patient Education & Consent

Most clinical neurophysiology laboratories use a standard template for the procedure documenting the procedure and its explanation to the patient. The authors’ lab uses the following statement built into the template: “Prior to starting the procedure, the patient’s identity was verified and the nature of the procedure explained. The appropriate sites of the examination were confirmed directly with the patient, and a preprocedural pause was performed for final verification of all of the above.” A written informed consent is not a part of the standard practice at most institutions. A verbal consent as mentioned above with clear explanation to the patient about the risks involved is sufficient.

Pre-Procedure Planning

The presence of physiologic tone without volitional effort in the anal sphincter, in contrast to peripheral skeletal muscles, makes study of this muscle very challenging for an electromyographer. The subject at rest maintains sustained firing of isolated motor unit potentials at a low rate. This activity varies considerably with changes in subject position. The activity continues during sleep, although the discharge rate drops substantially compared with that during wakefulness. Sphincter activity ceases completely only during attempted defecation. The presence of physiologic tonic activity at rest makes detection of abnormal spontaneous potentials difficult in a partially denervated muscle. In contrast, the paretic sphincter may reveal abundant fibrillation potentials, positive sharp waves, and complex repetitive discharges, as in any denervated limb muscle.


See the list below:

  • The conventional concentric or monopolar needle is commonly used for routine clinical purposes.

  • Skin electrodes are the most commonly used method of recording information.

  • Anal plug electrodes may also be used.

Patient Preparation

Patients are asked to put on a surgical gown and lie on the examining table.


Patients are typically asked to lie in the lateral decubitus position. The patient may assume the knee-chest or modified lithotomy position, which allows the best examination in infants.

Monitoring & Follow-up

Detailed management of patients with sphincter-related abnormalities is beyond the scope of this article. Treatment is based upon the severity as well as the nature of the underlying disorder. One of the major aspects of treatment belongs to nonpharmacotherapy group. A combination approach involving volumetric rehabilitation, electroanal stimulation, kinesiotherapy, and biofeedback can be effective in patients without severe damage to the pelvic floor muscle.[24, 25, 26, 27]



Approach Considerations

EMG studies can help quantitate sphincter dysfunction in neurologic disorders.[28] 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.[29]

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, 22] However, examination of the deeper muscles is necessary when evaluating for sphincteric dysfunction.[2] 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.[2]

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.[30] 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.[30]

Amyotrophic lateral sclerosis (ALS) typically spares the sphincter, even when the limb muscles show evidence of conspicuous denervation.[31] In contrast, abnormal spontaneous activity serves as a specific marker for neuronal degeneration of Onuf’s nucleus in multiple system atrophy[32] and progressive supranuclear palsy.[33] 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.[28]

In recent studies,[34, 35] 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,[36] 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,[37] 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.[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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