Electrically evoked activity in the human external anal sphincter

Neurophysiological testing in anorectal disorders

The neurophysiological techniques currently available to evaluate anorectal disorders include concentric needle electromyography (EMG) of the external anal sphincter, anal nerve terminal motor latency (TML) measurement in response to transrectal electrical stimulation or sacral magnetic stimulation, motor evoked potentials (MEPs) of the anal sphincter to transcranial magnetic cortical stimulation, cortical recording of somatosensory evoked potentials (SEPs) to anal nerve stimulation, quantification of electrical or thermal sensory thresholds (QSTs) within the anal canal, sacral anal reflex (SAR) latency measurement in response to pudendal nerve or perianal stimulation, and perianal recording of sympathetic skin responses (SSRs). In most cases, a comprehensive approach using several tests is helpful for diagnosis: needle EMG signs of sphincter denervation or prolonged TML give evidence for anal motor nerve lesion; SEP/QST or SSR abnormalities can suggest sensory or autonomic neuropathy; and in the absence of peripheral nerve disorder, MEPs, SEPs, SSRs, and SARs can assist in demonstrating and localizing spinal or supraspinal disease. Such techniques are complementary to other methods of investigation, such as pelvic floor imaging and anorectal manometry, to establish the diagnosis and guide therapeutic management of neurogenic anorectal disorders.

Anourethral reflex. Description of a reflex and its clinical significance: preliminary study

A new reflex which I call the 'ano-urethral reflex' is described and its clinical significance discussed. It was studied in 16 volunteers with normal vesical and rectal functions. A concentric needle electrode was introduced into the external and sphincter (stimulating), and another one into the external urethral sphincter (recording). The reflex response was recorded by EMG apparatus. The external urethral sphincter basal activity was increased with stimulation of the external anal sphincter. This reflex response was absent when the external and sphincter was anesthetised. The average latency of the reflex was 129.5 ms. Reflex urethral sphincter contraction at defecation guards against involuntary micturition which may result from rectal distension and levator contraction. The anourethral reflex can be a valuable diagnostic tool in routine investigations of patients with urological and proctological complaints.

[Physiology of fecal continence and defecation]

Defaecation is a complex function that requires interactions between the somatic and autonomic nervous systems. Moreover, social living standards lead to self-control of continence involving supraspinal nervous structures. After a brief description of the anatomy of the hind gut, the various reflex mechanisms underlying faecal continence and defaecation are exposed; the specificities of the nervous control of the smooth and striated muscles brought into play in these events are detailed. Then the main useful investigations allowing to localize the nervous structures at the origin of the anorectal disorders are reviewed.

External anal sphincter function in spinal patients. Electromyographic and manometric study

Six patients with complete transection of the spinal cord and six healthy volunteers were examined by using anorectal manometry together with electromyographic (EMG) recording of the external anal sphincter composed of striated muscle. Anal pressure and EMG activity of the external anal sphincter were continuously recorded at rest and during gradual rectal distention (10, 20, 30, 40, and 50 ml) by means of an air-filled balloon eliciting a rectoanal inhibitory reflex (RAIR) at the upper part, and an inflation reflex (IR) at the lower part of the anal canal. All patients and controls had a RAIR for each rectal distention volume. A relationship between the duration of the RAIR and the rectal distention volume was present in controls only. In controls the IR was present for each rectal distention volume, whereas it was present in only one patient for a 40-ml volume. During the resting period, all controls showed continuous tonic EMG activity of the external anal sphincter, but after 30 minutes all the patients showed a decrease and ultimately in five cases a disappearance of the tonic EMG activity of the external anal sphincter. In spinal patients, the presence or absence of EMG activity of the external anal sphincter did not modify the anal canal pressure. These results indicate that: 1) the tonic EMG activity of the external anal sphincter seems to be under the control of supraspinal structures, because in spinal patients it disappears in the absence of sensitive inputs toward the spinal cord; 2) the absence of EMG activity at rest indicates that the external anal sphincter is not implicated in the RAIR disturbances observed in spinal patients; 3) the IR is not a spinal reflex but is under voluntary control, because it is not present in spinal humans; 4) in spinal humans the tonic EMG activity of the external anal sphincter does not play a role in the maintenance of the anal pressure at rest.

Neurobiological aspects of the pelvic floor muscles involved in defecation

Neurobiological aspects of the organization of pelvic floor musculature are reviewed. Evolutionary considerations on the origin of these muscles indicate that they develop with specific attachments and function, i.e., do not derive from preexisting muscles such as the ones from the tail. Anatomically, pelvic floor muscles can be divided into 1) true sphincters and related muscles and 2) muscles which flank the visceral outlets. While in quadrupedal mammals the EAS behaves as a fast twitch muscle, in man this muscle has slow twitch characteristics. Like some epaxial muscles the EAS has a strong connectivity with its surrounding skin. In further analogy with some epaxial muscle the EAS, although endowed with muscle spindles, is devoid of the phasic, monosynaptic component of the stretch reflex. Onuf's nucleus which innervates pelvic floor muscles receives an important group of suprasegmental afferents including, probably, direct corticospinal fibers. Pelvic floor muscles play a fundamental role in signaling arrival of feces to the perineum. While sphincteric activity is important for continence, other mechanisms such as the anorectal angle and anal cushions are also of relevance. Although emphasis has been put on motor factors, fecal incontinence can also result from impairments in sensory mechanisms of the anorectal system. In diseases like amyotrophic lateral sclerosis, Werdnig Hoffman's and others there is selective sparing of neuropathology in Onuf's nucleus. In contrast, the nucleus is affected in some autonomic visceromotor neuronal disorders, e.g., Shy Drager syndrome, Fabry's disease. It has been suggested that Onuf's nucleus occupies an intermediate position between visceral and somatic nuclei.

Electrophysiological observations on the human pudendo-anal reflex

A reproducible electrophysiological technique is described to determine the latency of reflex contraction of the external anal sphincter in response to stimulation of the dorsal genital nerve: the pudendo-anal reflex. This was studied in 38 asymptomatic control subjects and 20 women with neurogenic faecal incontinence, supplemented by determination of the mean motor unit potential duration (MUPD) of the external anal sphincter and anorectal manometry. The reflex latency in the control group was 38.5 +/- 5.8 (SD) ms and appeared to be independent of age or sex. Three patients with faecal incontinence had absent reflexes; the remainder showed significant prolongation of latency (56 +/- 12.2 SD ms) and diminution of amplitude. MUPD was prolonged in incontinence and showed significant correlation with the corresponding reflex latency determination (tau = 0.56, p less than 0.001). The latency of this polysynaptic spinal reflex hence provides a reliable index of neuropathy of the external anal sphincter.