1 Anorectal investigation Alexis M.P. Schizas Andrew B. Williams Introduction rectum forms the internal anal sphincter, which terminates distally with a well-defined border at a Many tests are available to investigate anorectal variable distance from the anal verge. Continuous disorders, each only providing part of a patient's with the outer layer of the rectum, the longitudinal assessment, so results should be considered together layer of the anal canal lies between the internal and alongside the clinical picture derived from a and external anal sphincters and forms the medial careful history and physical examination. edge of the intersphincteric space. The longitudinal Investigations provide information about structure muscle comprises smooth muscle cells from the rectal alone, function alone, or both, and have been directed wall, augmented with striated muscle from a variety to five general areas of interest: faecal incontinence, of sources, including the levator ani, puborectalis constipation (including Hirschsprung's disease), and pubococcygeus muscles. Fibres from this layer anorectal sepsis, rectal prolapse (including solitary traverse the external anal sphincter forming septa rectal ulcer syndrome) and anorectal malignancy. that insert into the skin of the lower anal canal and adjacent perineum as the corrugator cutis ani muscle. The striated muscle of the external sphincter Anatomy and physiology of surrounds the longitudinal muscle and between these the anal canal lies the intersphincteric space. The external sphincter is arranged as a tripartite structure, classically The adult anal canal is approximately 4 cm long and described by Holl and Thompson and later adopted begins as the rectum narrows, passing backwards by Gorsch and by Milligan and Morgan. In this between the levator ani muscles. There is in fact wide system, the external sphincter is divided into deep, variation in length between the sexes, particularly superficial and subcutaneous portions, with the anteriorly, and between individuals of the same sex. deep and subcutaneous sphincter forming rings of The canal has an upper limit at the pelvic floor and a muscle and, between them, the elliptical fibres of lower limit at the anal opening. The proximal canal the superficial sphincter running anteriorly from is lined by simple columnar epithelium, changing to the perineal body to the coccyx posteriorly. Some stratified squamous epithelium lower in the canal consider the external sphincter to be a single muscle via an intermediate transition zone just above the contiguous with the puborectalis muscle, while dentate line. Beneath the mucosa is the subepithelial others have adopted a two-part model. The latter tissue, composed of connective tissue and smooth proposes a deep anal sphincter and a superficial muscle. This layer increases in thickness throughout anal sphincter, corresponding to the puborectalis life and forms the basis of the vascular cushions and deep external anal sphincter combined, as well thought to aid continence. as the fused superficial and subcutaneous sphincter Outside the subepithelial layer the caudal of the tripartite model. Anal endosonography (AES) continuation of the circular smooth muscle of the and magnetic resonance imaging (MRI) have not 1 Chapter 1 resolved the dilemma, although most authors report adequately to produce pressures sufficient to a three-part sphincter where the puborectalis muscle prevent leakage of flatus, liquid and solid stool. For is fused with the deep sphincter.1,2 The external anal effective defecation there needs to be coordinated sphincter is innervated by the pudendal nerve (S2– relaxation of the striated muscle components with S4), which leaves the pelvis through the lower part an increase in intra-abdominal pressure to expel the of the greater sciatic notch, where it passes under the rectal contents. The structure of the anorectal region pyriformis muscle. It then crosses the ischial spine should prevent herniation or prolapse of elements and sacrospinous ligament to enter the ischiorectal of the anal canal and rectum during defecation. fossa through the lesser sciatic notch or foramen via Because of the complex interplay between the pudendal (or Alcock's) canal. the factors involved in continence and faecal The pudendal nerve has two branches: the inferior evacuation, a wide range of investigations is needed rectal nerve, which supplies the external anal for full assessment. A defect in any one element sphincter and sensation to the perianal skin; and of the system in isolation is unlikely to have great the perineal nerve, which innervates the anterior functional significance and so in most clinical perineal muscles together with the sphincter situations there is more than one contributing factor. urethrae and forms the dorsal nerve of the clitoris (or penis). Although the puborectalis receives its main innervation from a direct branch of the fourth Rectoanal inhibitory reflex sacral nerve root, it may derive some innervation Increasing rectal distension is associated with from the pudendal nerve. transient reflex relaxation of the internal anal The autonomic supply to the anal canal and sphincter and contraction of the external anal pelvic floor comes from two sources. The fifth sphincter, known as the rectoanal inhibitory reflex lumbar nerve root sends sympathetic fibres to the (Fig. 1.1). The exact neurological pathway for this superior and inferior hypogastric plexuses, and the reflex is unknown, although it may be mediated via parasympathetic supply is from the second to fourth the myenteric plexus and stretch receptors in the sacral nerve roots through the nervi erigentes. Fibres pelvic floor. Patients with rectal hyposensitivity have of both systems pass obliquely across the lateral higher thresholds for rectoanal inhibitory reflex; it surface of the lower rectum to reach the region of is absent in patients with Hirschsprung's disease, the perineal body. progressive systemic sclerosis, Chagas' disease, The internal anal sphincter has an intrinsic nerve and initially absent after a coloanal anastomosis, supply from the myenteric plexus together with an although it rapidly recovers. additional supply from both the sympathetic and parasympathetic nervous systems. Sympathetic nervous activity is thought to enhance and The rectoanal inhibitory reflex may enable rectal parasympathetic activity to reduce internal sphincter contents to be sampled by the transition zone contraction. Relaxation of the internal anal sphincter mucosa to allow discrimination between solid, liquid may be mediated by non-adrenergic, non-cholinergic and flatus. The rate of recovery of sphincter tone after nerve activity via the neural transmitter nitric oxide. this relaxation differs between the proximal and distal Anorectal physiological studies alone cannot canal, which may be important in maintaining separate the different structures of the anal canal; continence.5 instead, they provide measurements of the resting and squeeze pressures along the canal. Between 60% Further studies investigating the role of the and 85% of resting anal pressure can be attributed rectoanal inhibitory reflex in incontinent patients 3 The to the action of the internal anal sphincter. show that as rectal volume increases, greater external anal sphincter and the puborectalis muscle sphincter relaxation is seen, whereas constipated generate the maximal squeeze pressure.3 Symptoms patients have a greater recovery velocity of the of passive anal leakage (where the patient is resting anal pressure in the proximal anal canal. unaware that episodes are happening) are attributed There is a longer recovery time back to resting to internal sphincter dysfunction, whereas urge 6 pressure in patients with faecal incontinence. symptoms and frank incontinence of faeces are due to external sphincter problems.4 Faecal continence is maintained by the complex Manometry interaction of many different variables. Stool must be delivered at a manageable rate from the colon A variety of different catheter systems can be used into a compliant rectum of adequate volume. The to measure anal pressure and it is important to note consistency of this stool should be appropriate that measurements differ depending on which is and accurately sensed by the sampling mechanism. employed. Systems include microballoons filled with Sphincters should be intact and able to contract air or water, microtransducers and water-perfused 2 Anorectal investigation Review rectoanal Help= F1 Press 3b(4) 97 1 60 40 136 132 131 127 * 20 0 Press 2 60 40 126 112 121 118 * 20 97 0 Press 164 168 165 3 60 148 40 * 20 0 90 Press 175 184 60 147 4 40 * 20 117 0 Press 5 60 40 * 20 122 0 09 Press 69 6 60 141 40 * 20 0 70 81 Press 7 40 20 * 0 74 78 Press 62 72 65 8 40 20 10 * 0 C=13:23:53 T=2.0 s Figure 1.1 • Normal rectoanal inhibitory reflex. catheters. These may be hand-held or automated. Hand-held systems are withdrawn in a measured stepwise fashion with recordings made after each step (usually of 0.5–1.0-cm intervals); this is called a station pull-through. Automated withdrawal devices allow continuous data recording (vector manometry). Water-perfused catheters use hydraulic capillary infusers to perfuse catheter channels, which are arranged either radially or obliquely staggered. Each catheter channel is then linked to a pressure transducer (Fig. 1.2). Infusion rates of perfusate (sterile water) vary between 0.25 and 0.5 mL/min per channel. Systems need to be free from air bubbles, which may lead to inaccurate recordings, and must avoid leakage of perfusate onto the perianal skin, which may lead to falsely high resting pressures due to reflex external sphincter action. Perfusion rates should remain constant, because faster rates are associated with higher resting pressures, while larger diameter catheters lead to greater recorded Figure 1.2 • Perfusion system used for anorectal pressure.7 manometry. Standard water perfusion set-up plus Balloon systems may be used to overcome some computer interface for anorectal manometry. The screen of these problems and may be more representative shows a vector volume profile. of pressure generated within a hollow viscus than recordings using a perfusion system.