Experimental studies with balloon distension have shown that the rectum is much more sensitive and discriminating than the colon. However, several stud
Fig. 4. Example of manovolumetry of a low anterior resection (LAR), before operation (upper panel, left image) and after the covering stoma closure (right image). Post-operative rectal volume is significantly reduced, there are involuntary contractions of the external sphincter and the recto-anal inhibition reflex (RIRA) is present. On the lower panel, a manovolumetry in a case of low rectal cancer before (left) and after (right) low anterior resection and straight colo-anal anastomosis is shown. The rectal volume is reduced to a third; when an involuntary rectal contraction appears, the reservoir function seems completely lost. This aspect coupled with a weak sphincter pressure might explain the post-operative faecal incontinence, which was complained of by the patient. Moreover the RIRA disappears and threshold for urgency increases ies have shown evidence that a colonic segment brought down in the pelvis to restore continuity after complete rectal excision acquires an imperfect sort of rectal sensation thought to depend on receptors in the pelvic floor muscles. Pelvic sepsis and resulting fibro-sis renders such activation more difficult and may explain the unsatisfactory functional results after such a complication. Studies in animals have shown that there are specific relaxatory fibres in the parasympa-thetic pelvic nerves, which might explain the unique ability of the rectum to accommodate a sustained distension. This phenomenon is illustrated in Figure 4
showing the effect of continuous rectal distension on rectal volume and anal pressure before and after surgery (own experience). Before surgery, rectum slowly expands to accommodate the balloon used for distension. In contrast, the post-operative recordings are characterised by a low volume and marked recurrent volume decreases (i.e., contractions). Such vigorous contractions might explain why some patients are incontinent despite normal anal pressures.
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