This study demonstrated that squeeze pressure in female patients with FI and CRAI was significantly lower in the erect position than in the left-lateral position, although this finding was not true of those with non-CRAI. Resting pressure, DDV, and MTV were significantly higher, and anal canal length was significantly shorter in the erect position than in the left-lateral position in either the CRAI or non-CRAI patients.
Previous studies on the effect of posture on physiological measurements are limited. Johonson et al. [9] reported that posture did not affect squeeze pressure in the study of 27 healthy individuals. This finding was true for all 80 patients with FI in this study however, upon separate examination of the CRAI and non-CRAI groups, squeeze pressure was lower in the erect position than in the lateral position in the CRAI patients. The reason for this finding is unclear however, squeezing the anal canal may be physically inhibited by the intra-anal apex of the “circumferential” RAI but may not be suppressed by “anterior” RAI alone entered during the erect position. Also, it is possible that the higher intra-abdominal pressure created during daily life may expose the lower rectal wall to forces consistent with CRAI development. Notably, CRAI occasionally continued to be observed at rest immediately after CRAI had been observed during straining effort on defecography (Fig. 6). This finding may support the possible existence of the intra-anal apex of RAI at the erect position.
FI was the most common presenting symptom in patients with RAI, in line with previous reports [1, 3, 17]. The cause of FI in patients with RAI has been attributed to reduced resting anal pressure [18, 19], reduced rectal sensation [20], and the inappropriate activation of the rectoanal inhibitory reflex caused by RAI [21]. In this study, two-thirds of patients in the CRAI group had passive incontinence suggesting a dysfunction of the internal anal sphincter, and, notably, only one patient had a partial defect of the internal anal sphincter. Our CRAI patients had fewer incidences of urge and/or mixed incontinence, suggesting dysfunction of the external anal sphincter or a reduction in rectal wall compliance due to chronic irritation of the prolapsed rectal wall [3]. Although squeeze pressure was not lower in the erect position than the lateral position in our CRAI patients with urge FI, their median squeeze pressure measured in the left-lateral position (91 cm H2O) was lower than the in-house normal value (150‒300 cm H2O). The urge FI may not be caused by the suppression of voluntary contraction in the erect posture, but the underlying reduced function of the external anal sphincter. Also, although squeeze pressure being lower in the erect posture than in the lateral posture in our CRAI patients with passive FI, the passive FI may not be caused by the inhibition of the voluntary contraction theoretically. Nonetheless, squeeze pressure being lower in the erect posture than in the lateral posture may be another triggering mechanism for FI within daily life in patients with CRAI.
Johonson et al. [9] showed that resting pressure was significantly higher in the erect position than in the left-lateral position in the study of healthy adults and attributed this to a higher intrarectal rectal pressure in the erect position than in the left-lateral position. The authors speculated that the change in pressure in the rectum and anal canal might be linked; rising intrarectal pressure challenges the muscular anal canal continence mechanism causing a corresponding rise in resting anal canal pressure. Thekkinkattil et al. [8] reported similar results in a study of 135 patients with FI, ascribing to it to engorgement and bulking of the anal cushion in the erect position compared with the lateral position because the anal cushions are important in contributing to the rise in pressure. Besides patients with FI, those with non-prolapsing hemorrhoids also have a thicker anal cushion and, thus, a higher resting pressure [22]. This study also showed similar results in all 80 FI patients. However, the difference in the median resting pressure measured in the erect position from that in the left-lateral position was only 7–8 cm H2O in either the CRAI or non-CRAI patients, which would be clinically insignificant to avoid FI especially in the CRAI patients, because the median squeeze pressure was nearly 30 cm H2O lower in the erect position than the lateral position. Anal canal length in the lateral position was longer in healthy subjects compared with female patients with FI in previous studies [23, 24]. Meanwhile, there was no change in the anal canal length with a change in the posture in the healthy adults [9]. In this study, it is unclear why FI patients presented a shorter anal canal length in the erect position compared with the lateral position. However, the difference in the median length was only 3 mm, hence it is uncertain whether this finding might provide additional information on increasing the chance of FI in the erect position.
Studies on the effect of posture on rectal sensation are limited. Halani et al. [25] reported that although there was no significant difference in the rectal volume measured in the left-lateral position compared with the lithotomy position in female FI patients, higher values for the measurements were observed in lithotomy than in the left-lateral position. Another study of urogynecological patients reported similar results [26]. In these studies, testing was performed first in the left-lateral position and then in the lithotomy position. In our study, DDV and MTV were significantly higher in the erect position than in the left-lateral position in the 80 FI patients. The reason is unclear however, but testing was performed in the same sequence as previous studies [25, 26], first in the left-lateral position and then in the erect position in our study, which may have blunted rectal sensation during rectal capacity examination in the erect position. The order of examination should be reversed and re-examined in the future. These results do not seem to have an influence on FI.
This study has several limitations. The major limitation to our manometric technique is the necessity that “pull-throughs” be performed. A manometric device capable of simultaneously measuring radial and longitudinal anorectal pressure profiles would be helpful. The subjects who were continent of feces were not studied as a control group. The involuntary contraction of the internal anal sphincter and the external anal sphincter continence reflex were not measured. The CRAI patients were small in number, and non-CRAI patients had various defecographic findings, including rectocele, which is also regarded as a cause of FI y[27, 28]. Small number of FI patients with IBS were included, possibly because majority of patients with IBS did not consult a surgeon, but a medical physician. Also, the study population came from a single tertiary care center, and our findings may not be generalizable to all patients with CRAI and FI.
In conclusion, voluntary contraction in female FI patients with CRAI was suppressed in the erect position. Further studies that measure the involuntary contraction of the internal or external anal sphincter in these patients are required to explore the whole incontinence machinery.