Electrodiagnostic Study of Anorectal Dysfunction
Abstract & Commentary
By Michael Rubin, MD, Professor of Clinical Neurology, NewYork-Presbyterian Hospital, Cornell Campus. Dr. Rubin is on the speaker's bureau for Athena Diagnostics, and does research for Pfizer and Merck.
Synopsis: Future research will demonstrate whether these tests can also document and monitor the benefit of specific therapeutic interventions.
Source: Lefaucheur JP. Neurophysiological Testing in Anorectal Disorders. Muscle Nerve. 2006;33:324-333.
Electrodiagnostic examination of the anogenital region may be unpleasant and awkward for both patient and physician, but valuable information, of both diagnostic and prognostic value, can be garnered by a variety of techniques in the work-up of suspected peripheral or central disorders of anorectal dysfunction.
Electromyography (EMG) of the external, striated, anal sphincter muscle is performed by needling the 4 quadrants of the muscle. Automated computerized programs have enhanced the diagnostic sensitivity of this test, and guidelines have been standardized to quantify the findings (Clin Neurophysiol. 2000;111;2200-2207). At rest, the normal sphincter demonstrates tonic activity, whereas active denervation may be documented by the presence of fibrillation potentials and high frequency repetitive discharges. Chronic denervation with reinnervation is evidenced by a reduced interference pattern comprising large amplitude, long duration, motor unit potentials. Amyotrophic lateral sclerosis generally spares the external anal sphincter, whereas, it is involved in multiple system atrophy (MSA). Controversy exists as to whether its involvement differentiates MSA (affected) from Parkinson's disease (spared). EMG of the internal anal sphincter muscle remains predominantly a research tool. Surface EMG, which would significantly facilitate study of this region for both examiner and examinee, has yet to achieve the sensitivity of needle EMG. When combined with anorectal manometry, however, surface EMG has demonstrable utility, exhibiting abnormal patterns of rectal muscle activation, whether increased, explaining chronic constipation, or deceased, explaining fecal incontinence.
Single fiber EMG (SFEMG) can objectively document the presence of reinnervation within the anal sphincter muscle by measuring fiber density, which increases in the chronic phase of neurogenic injury, analogous to fiber type grouping. It remains infrequently used. SFEMG measurement of jitter, which increases in neuromuscular junction disorders such as myasthenia, is of little utility in anogenital disorders, given that they are spared in these conditions.
Transrectal electrical stimulation of the pudendal nerve, performed by insertion of the index finger into the rectum while wrapped in a glove attached to a St. Mark's electrode, allows for measurement of the terminal motor latency (TML) of this nerve. Reportedly useful in predicting whether surgical repair of perineal tears will benefit the (usually) postpartum patient, the technique remains less sensitive than needle EMG for documenting anal sphincter denervation. Questions of the true validity of the test have also been raised based on the surprisingly short latency values obtained using the St. Mark's electrode. Muscle artifact may seriously interfere with accurate recording, values vary with age, and findings do not correlate with anorectal manometry. TML recordings by this technique are, thus, of questionable value. Sacral magnetic stimulation of the sacral roots at the sacral foramina, to evoke anal sphincter compound muscle action potentials, analogous to routine motor nerve conduction studies, may offer a viable alternative. Less painful and less uncomfortable, magnetic stimulation also allows for simultaneous study of both central and peripheral sphincter motor pathways.
Transcranial magnetic stimulation of the motor cortex, with recording of external anal sphincter evoked motor responses, permits assessment of central motor pathway conduction times and excitability of cortical motor circuitry. To date, the clinical relevance of these studies remains to be established.
Reliable somatosensory evoked responses (SEP) may be elicited by stimulating the rectal wall, anal canal, or anal verge, while recording over the cortex, but stimulation of the dorsal nerve of the clitoris and penis are more sensitive. Yet, small diameter spinothalamic pathways are not studied by SEP, and further research to overcome this limitation is ongoing.
By electrically stimulating the anal mucosa in a graded, stepped manner, and asking the patient to report when (s)he feels the stimulus, it is possible to quantify the patient's perception threshold, so-called quantitative sensory threshold testing. Reproducible, accurate, and, when abnormal, associated with rectal incontinence, it is nonspecific, altered by age and feces in the anal canal, and does not correspond to natural stimuli. Thermal sensory threshold testing can overcome some of these drawbacks but may not correlate with anal incontinence. Neither method differentiates central from peripheral causes, and both depend on patient cooperation, making them, in this sense, subjective.
Sacral anal reflex latency measurements are age and sex dependant, but have been obtained by electrically stimulating either the pudendal nerve (dorsal nerve of penis or clitoris) or the perianal region, and recording from both the bulbocavernosus muscle and external anal sphincter. Reproducibility in the latter is poor, and onset latency is difficult to measure accurately. Findings cannot differentiate central from peripheral causes.
Sympathetic skin responses (SSR) may be obtained by electrically stimulating the face or arm and recording from the perineal region. Central from peripheral causes of anal dysfunction may then be differentiated, by comparing the perineal response to palmar and plantar SSR recordings, but SSRs lack sensitivity and reproducibility.
Electrophysiologic testing of anorectal dysfunction is an extension of the clinical examination. Various available tests complement anorectal manometry and each other. Future research will demonstrate whether these tests can also document and monitor the benefit of specific therapeutic interventions.
Commentary
Even after correct diagnosis, anorectal disorders can remain therapeutically challenging. Botulinum toxin, useful in so many others areas of neurology, has recently been used to treat spasticity of the anal sphincter, as well as paradoxical puborectal syndrome, a disorder where there is insufficient relaxation of the sphincters with straining on defecation. Anal fissure can also be cured with Botox. Where will they put it next?
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