When is anal sphincter electromyography (EMG) indicated?

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

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.

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