Anal Sphincter Electromyography and Sphincter Function Profiles

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