Anorectal physiology and function

The resting pressure of the anal canal should be high enough to main­tain an air- and watertight seal during rest and activity. Penninckx et al. (4) estimated that the resting tone was generated by nerve- induced activity of the internal sphincter (45%), myogenic tone of the internal sphincter (10%), tone of the external sphincter (35%), and the anal cushions (15%).

When the rectum is distended there should be a compensatory relaxation of the internal sphincter muscle. This is known as the rectoanal inhibitory reflex and is mediated by intrinsic nerves as evi­denced by the fact that the reflex remains after spinal cord transec­tion. The reflex is probably mediated by nitric oxide and possibly by other non-adrenergic and non-cholinergic neurotransmitters. The reflex is absent in Hirschsprung’s disease.

The squeeze pressure is mediated by the external sphincter; it can be voluntary, induced by cough or an increase in intra-abdominal pressure, or by digitating the anal canal. At the time of defecation the external sphincter relaxes.

Puborectalis is tonically active and maintains the resting anorectal angle. Further contraction will occur in response to a sudden rise in abdominal pressure and this reduces the anorectal angle further and helps preserve continence.

Measurement

Anorectal physiology

The function of the anus and rectum can be measured using a var­iety of physiological tests. HRAM can be performed using water- perfused or solid-s tate catheters. The catheter provides a single pressure, averaged around the circumference at 6 mm intervals, and measures values along the entire length of the anal canal; it is also fitted with an inflatable balloon at the end.

Anal ultrasound

The structure of the anal canal can be visualized using three­dimensional anal ultrasound (Figure 58.3). Ultrasound is useful, safe, and well tolerated by patients. Anal ultrasound is the pre­ferred investigation to detect sphincter defects but insertion of the

Figure 58.2 High -resolution anorectal manometry.

probe can distort the anal canal. Normal anal sphincter morph­ology and anal sphincter measurements can also be obtained using transvaginal and transperineal scanning (5).

Proctography

The dynamics of defecation can be assessed radiologically by evacu­ation proctography or magnetic resonance (MR) proctography (Figure 58.4).

Figure 58.3 Three-dimensional anal ultrasonography showing a defect in the external sphincter.

straining. Fluoroscopic proctograms use rectal contrast and contrast may also be placed in the vagina, bladder, and small bowel, helping with better visualization of abnormalities such as an enterocele and rectocele. The patients are then fluoroscopically screened as they sit on the toilet and attempt to evacuate their rectum. MR defecography avoids radiation exposure and is better at visualizing bony landmarks. However, the patient is required to lie in the supine position and expel the gel onto a pad and this sometimes inhibits their normal defecatory push. This test visualizes the uterus, vagina, and anterior compartment as well as the rectum and any potential enterocele. A recent study suggested that MR proctograms may under-report pelvic floor abnormalities especially where there has been poor rectal evacuation (6).

Colon transit studies

The simplest way of studying a colon transit is ingesting a capsule of markers and taking a plain abdominal X-ray after 72 and then 120 hours. The distribution of the markers in the colon provides a crude measure of colonic transit. A normal study should show at least 80-90% of markers are eliminated by 120 hours in the ma­jority of people. The markers have completely disappeared by this day. It is possible to use scintigraphic techniques to study both seg­mental colonic transit and small bowel transit and to give an idea of the flow of markers through the bowel. Some centres use three different-shaped markers on three different days and take further X-rays during the week.