The mechanism of faecal incontinence following low anterior resection (LAR) has been speculative and the role of disordered neorectal dynamics difficult to quantify. Using a new methodology which quantifies rectal response to rapid and ramp inflation, in combination with anal physiology, we have evaluated 25 LAR-7 with major incontinence and 5 with minor incontinence. The three groups had comparable age, duration post surgery and anastomotic distance from the puborectalis. The resting anal canal pressure (RAP) did not related to the anastomotic distance (R2 = 0.09). With the anastomosis at and below 3 cms from the puborectalis, the rectoanal inhibitory reflex (RAIR) was a sustained drop in the mid anal canal pressure, in contrast to the normal pattern of recovery above this level. Major incontinence was characterised by a subnormal anal defence, hypersensitive neorectal dynamics and high amplitude contractile wave while minor incontinence was characterised by a hypernormal anal defence and a lesser degree of neorectal hypersensitivity. The mathematical viscoelastic rectal model, defined an increasing longitudinal smooth muscle tone and a decreasing functional collagen with increasing severity of incontinence as well as a high and low circular smooth muscle (CSM) tone with major and minor incontinence respectively. This correlated with previous in vitro studies on myenteric plexus denervation and localised damage to the inferior mesenteric plexus respectively. Based on the findings in this study, we conclude that major incontinence is secondary to neurotenesis of the inferior mesenteric ganglia and the hypogastric plexus, whereas minor incontinence represents a localised neurotenesis/neuropraxia of the inferior mesenteric plexus.
Rectal compliance has theoretical and practical drawbacks resulting in wide variation in the reported normal values. Slope of stress-strain relation, a measure of rectal wall stiffness (Incremental elastic modulus; IEM) may be an effective alternative. The aim of this study was to compare IEM with rectal compliance during ramp inflation. In a group of 15 normal adults [Median age 51.5 years (range 31 to 74); 11 female and 4 male], these parameters were assessed at three rates of inflation, using a proctometrogram-catheter-balloon (PCB) complex and endorectal ultrasound scanner (7 Mhz; B & K) to measure intrarectal pressure and radius respectively. IEM had a linear relation with strain (Slope 33.55; R2 = 0.9815) in contrast to compliance (R2 = 0.0088). Series elastic component (SEC), a measure of passive viscoelasticity, was rate independent elasticity [Slope 1.02 (0.98 to 1.15); P = ns] and a rate dependent viscosity (P = 0.004; One way ANOVA). Both IEM and SEC were reproducible (R = 0.985; P < 0.01). This study emphasises the importance of rectal wall viscoelastic evaluation based on stress and strain rather than compliance.
Evaluation of rectal sensations should be confined to CS and UD because MTT is painful and does not contribute any additional information, and FS is not a true rectal phenomenon.
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