Constipation affects approximately 9% of all children (1). It is emotionally distressful, stigmatizing for the child and parents, and has great implications for functioning and quality of life (2). Despite being so frequent and with such an impact, the knowledge of the physiology and pathophysiology of constipation is limited. In an estimated 90% of constipated children, no obvious organic cause is identified and the condition is consequently categorized as “functional” (3). Pediatric constipation is a symptom-based diagnosis, a fact that reflects the general lack of knowledge of mechanisms and etiological factors. A number of factors have been identified as abnormal in constipated children. These include defecation dynamics (contraction of the external anal sphincter at defecation) (4,5), increased colorectal transit time (6,7), and increased rectal compliance (8). Despite the rectum's function both as a reservoir and as a conduit, the role of motility and tone in children is sparsely investigated, and comparisons between healthy and constipated children are lacking.
Rectal motility patterns are traditionally investigated using pressure catheters, rectal balloons, or bags; however, these techniques have several methodological shortcomings. Strain gauges and perfused catheters only detect contractions that change luminal pressure, and in a noncoaptive system wall proximity may lead to insufficient activity detection. Barostats detect changes in tone but may not register localized contractions. In contrast, rectal impedance planimetry measures cross-sectional areas (CSAs) and avoids most of the errors associated with other methods. Recently, the method was modified to allow continuous measurement of CSA at 3 levels concurrent with measurement of rectal and anal pressures (9,10). This allows detailed description of rectal motility and determination of rectal dimensions and biomechanical parameters. The aim of the present study was to compare rectal motility and wall properties in healthy and constipated children.
In the study period 16 children ages 7 to 12 years and diagnosed with constipation according to the Rome III criteria (11) were consecutively referred to our center for incontinence. All of the children were otherwise healthy and had no known organic cause of constipation. Twelve children (1 girl), mean age 8.8 (standard deviation [SD] ± 1.2), agreed to participate in the study. Data from 2 were excluded from analysis, 1 because of technical problems with the impedance planimetry system and 1 because he was diagnosed as having milk allergy. This left data from 10 patients. Results were compared with those of 10 healthy children of similar age (5 girls), mean age 9.9 (SD ± 1.5) years, recruited among the children of employees at the Department of Pediatrics. None of the healthy controls had a history of bowel dysfunction or took medications affecting the gastrointestinal system.
The study was approved by the local ethics committee and the Danish Data Protection Agency. Written, informed consent was obtained from the participants and their parents before any procedures were initiated.
Impedance planimetry is based on Ohm's law and uses the fact that the potential difference, V, between 2 electrodes within a conductive fluid can be transformed into CSA (12). Rectal contractions or relaxations can be seen as changes in CSA.
A specially designed pediatric probe was constructed for simultaneous measurement of 3 CSAs and rectal pressure (Fig. 1). Two excitation electrodes were placed on a 15-cm catheter tube with a diameter of 0.8 cm. The distance between the excitation electrodes was 6 cm, and between these we placed 3 pairs of detection electrodes. Hence, it was possible to detect rectal contractions at 3 levels and to determine the direction of propagation. The distance between the detection electrodes in each pair was 2 mm. The excitation electrodes were connected to a generator producing an alternating current of 0.1 mA at 10 kHz. The electrodes were all within a noncompliant bag with a maximum CSA of 7850 mm2 (corresponding to a diameter of 11 cm). The bag was filled and emptied through central infusion channels, and the height of column of water in a level container determined the applied balloon pressure. The intraluminal rectal pressure was recorded using a water-perfused catheter within the bag. The catheter was connected to an external transducer (Baxter, Irving, CA) (Fig. 1). As previously described, a multipoint calibration was performed before each experiment (13).
All of the medication affecting gastrointestinal function was withdrawn for at least 2 weeks before the study. A thorough medical history and a physical examination were obtained, and all of the children filled out a 1-week bowel diary. To ensure that the rectum was empty, a microenema was given the evening before the investigation. Following a 12-hour fast, experiments started at 09.00 hours.
Before insertion of the impedance planimetric probe, transabdominal ultrasound was performed to confirm that the rectum was empty (14). The children were also asked to defecate and empty their bladders. During the procedure, the children lay in the left lateral position. First, rectal baseline pressure was determined using a small water-perfused catheter (Unomedical, Copenhagen, Denmark). This baseline pressure also served as zero point for the distension protocol. The lubricated probe was then gently placed into the rectum without the use of endoscopy. Rectal CSAs were determined approximately 4, 5.5, and 7 cm from the anal verge. The rectal bag was filled with saline at 37°C to a pressure level of 10 cmH2O above baseline pressure and a 5-minute adaptation period began. This was followed by 30-minute recording of rectal resting motility. Finally, to determine compliance a 14-minute pressure, controlled phasic distension protocol began. First, the pressure was lowered to baseline pressure for 2 minutes. This was followed by phasic distensions of pressures set at 10, 20, and 30 cmH2O above baseline pressure. Each distension lasted 2 minutes and was separated by 2-minute intervals during which pressure was lowered to basic rectal pressure.
The calculation of rectal size and compliance was based on the CSA from the central set of detection electrodes located 5.5 cm from the anal verge. We defined phasic rectal contractions as changes in CSA of a least 10% and lasting >2 minutes. Rectal size was described as the area under the central CSA curve during the last 20 minutes of recordings at rest. The rectal compliance was calculated as the change in CSA divided by the change in pressure (ΔCSA/ΔP) between baseline pressure and at the end of the 2-minute distension 30 cmH2O above baseline pressure. All of the data were analyzed independently by 2 investigators (I.M.J. and L.F.). L.F. was blinded as to which group the data belonged. If disagreement occurred, the evaluation performed by the blinded investigator was used.
Stata 11 (StataCorp, College Station, TX) was used for the statistical analysis. Results are reported as median and range or mean, with SD whenever appropriate. The Student t test or the Wilcoxon signed-rank test was used for comparison between groups. P < 0.05 was considered statistically significant.
The rectal catheter was inserted with ease in all of the cases, and no adverse events occurred during the study. Baseline rectal pressure was 6 cmH2O in constipated and 6 cmH2O in healthy children (P = 0.87).
The rectum exhibited 2 different patterns of contractions: a basic low-amplitude rhythm and phasic contractions of higher amplitude.
Basic Low-Amplitude Rhythm
A basic low-amplitude rhythm with a frequency of 6 to 8/minute was observed in all of the children, healthy as well as constipated. It was present all of the time and seen as minor fluctuations (3%–5%) in CSA. It only occasionally caused rectal pressure changes and was, therefore, rarely detected by manometry.
Phasic Rectal Contractions
In 8 patients and 7 healthy children periods of phasic contractions with CSA changes of >10% of baseline were observed (Fig. 2). Their frequency was 2 to 8/minute and they were superimposed on the basic low-amplitude rhythm. The phasic rectal contractions increased rectal pressure. Some were localized only to be detected by 1 set of detection electrodes, whereas others propagated. In all of the subjects except 1, the propagating contractions moved aborally. During longer runs of phasic rectal contractions, the contractions fused, reducing baseline CSA for longer periods (Fig. 3). There was significantly more time with phasic rectal contractions in constipated children (median 38% of time range [0–100]) compared with healthy children (median 8.8% of time range [0–57]) (P < 0.05).
Rectal Dimensions and Compliance
The mean CSA was higher in constipated children (median 1802 mm2 [range 1106–2945 mm2]) than in healthy children, median 1375 mm2 [range 437–1861 mm2]) (P < 0.05) (Fig. 4). There was no difference in rectal compliance between constipated (38 mm2/H2O [range 12–86 mm2/H2O]) and healthy children (33 mm2/H2O [range 20–45 mm2/H2O]) (P = 0.30) (Fig. 5).
The principal findings of the present study were that constipated children had phasic rectal contractions for a significantly longer period compared with healthy children, and that constipated children had larger than normal rectal diameters but normal rectal compliance.
The rectal impedance planimetry allows detailed description of contraction patterns and the present study is the first to apply the method in children. Because rectal impedance planimetry determines rectal CSA, contractions can be detected even in the absence of pressure changes. Therefore, we were able to identify contractions not usually identifiable by manometry (15). The basic 6 to 8/minute low-amplitude rhythm probably reflects the slow waves of the smooth muscle cells. It is specific for various segments of the gastrointestinal tract (16,17) and, thus, not surprisingly present in both healthy and constipated children.
In line with previous studies using rectal impedance planimetry in adults, we defined phasic rectal contractions as changes in CSA of at least 10% of baseline. This definition is arbitrary and future use of mathematical pattern recognition may be needed to define specific contraction patterns; however, the fundamental difference between impedance planimetry and manometry explains why the proportion of time with phasic contractions was much higher in the present study compared with previous manometry-based studies. Most systems for manometry will simply miss a number of contractions with low amplitude.
Children and adults with constipation have been reported as having both hyper- and hypomotility (18–21). Thus, Clayden and Lawson (21) described “hypertrophic waves” in children with constipation, and others have found that adults with constipation exhibit slightly more rectal motor activity compared with healthy subjects and patients with diarrhea (20,22,23). In contrast, Loening-Baucke and Younoszai (19) found that childhood constipation with fecal incontinence was associated with decreased rectal motor activity. This finding has since then been confirmed in adults (18,20). The discrepancy between studies could be because of differences in definitions of motility patterns, lack of normative data, and the use of methods with inherent sources of error. Thus, many children with constipation have “megarectum” (24), and contractions in a large noncoaptive reservoir do not result in the same pressure changes as they would in a smaller one (15). We did not find phasic rectal contractions in all of the children, but from a study of 24-hour rectal activity, we know that rectal phasic activity is highly irregular (25).
The pressure profile of longer periods of phasic rectal contractions was similar to that of the preciously described rectal motor complex (RMC) (25–29). Interestingly, we found that rectal tone, estimated from baseline CSA, changed during some runs of phasic rectal contractions and continued to do so after the phasic contractions had ended (Fig. 3). The change in baseline CSA only occasionally led to pressure changes, explaining why this apparent correlation between the RMC and rectal tone has not previously been observed with manometry. The function of RMC is unclear, but it reflects the activity of the enteric nervous system and may contribute to maintaining continence (28,30,31). Rao et al (22) found that RMC was more frequent in adults with slow transit constipation and speculated that they impede the transport of stool; however, our finding of increased phasic rectal contractions corresponding to RMC is also in concert with changes observed in smooth muscle hollow organs subjected to obstruction, and they may simply reflect a wall overloading (32).
In keeping with ultrasound studies, we found that constipated children had abnormally large rectal diameter and capacity (14). The compliance, however, reflects the rectum's ability to distend, which is related to the collagen content and the state of the smooth muscle fibers (33). We did not find that compliance was abnormal in constipated children. This may be a type II error because other studies have reported that 64% to 79% of constipated children have increased compliance (34,35). Another explanation could be that most previous studies have used volume distension method, which tends to overestimate compliance of a megarectum (35–37).
The present study was purely descriptive and future intervention studies are needed to clarify whether the observed increased number of contractions and rectal size are part of the pathophysiology of constipation or a secondary phenomenon caused by rectal overloading.
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Keywords:© 2014 by European Society for Pediatric Gastroenterology, Hepatology, and Nutrition and North American Society for Pediatric Gastroenterology,
child; compliance; constipation; fecal incontinence; healthy; impedance planimetry; motility; rectal motor complexes; rectum