PURPOSE: Parasympathetic afferent nerves are thought to mediate rectal filling sensations. The role of sympathetic afferent nerves in the mediation of these sensations is unclear. Sympathetic nerves have been reported to mediate nonspecific sensations in the pelvis or lower abdomen in patients with blocked parasympathetic afferent supply. It has been reported that the parasympathetic afferent nerves are stimulated by both slow ramp (cumulative) and fast phasic (intermittent) distention of the rectum, whereas the sympathetic afferent nerves are only stimulated by fast phasic distention. Therefore, it might be useful to use the two distention protocols to differentiate between a parasympathetic and sympathetic afferent deficit. METHODS: Sixty control subjects (9 males; median age, 48 (range, 20-70) years) and 100 female patients (median age, 50 (range, 18-75) years) with obstructed defecation entered the study. Rectal sensory perception was assessed with an "infinitely" compliant polyethylene bag and a computer-controlled air-injection system. This bag was inserted into the rectum and inflated with air to selected pressure levels according to two different distention protocols (fast phasic and slow ramp). The distending pressures needed to evoke rectal filling sensations, first sensation of content in the rectum, and earliest urge to defecate were noted, as was the maximum tolerable volume. RESULTS: In all control subjects, rectal filling sensations could be evoked. Twenty-one patients (21 percent) experienced no sensation at all in the pressure range between 0 and 65 mmHg during either slow ramp or fast phasic distention. The pressure thresholds for first sensation, earliest urge to defecate, and maximum tolerable volume were significantly higher in patients with obstructed defecation (P < 0.001). In each subject, the pressure thresholds for first sensation, earliest urge to defecate, and maximum tolerable volume were always the same, regardless of the type of distention. CONCLUSION: Rectal sensory perception is blunted or absent in the majority of patients with obstructed defecation. The observation that this abnormality can be detected by both distention protocols suggests that the parasympathetic afferent nerves are deficient. Because none of the patients experienced a nonspecific sensation in the pelvis or lower abdomen during fast phasic distention, it might be suggested that the sympathetic afferents are also deficient. This finding implies that it is not worthwhile to use different distention protocols in patients with obstructed defecation.
