Abstract
Objectives:
Chronic constipation is a common gastrointestinal disorder, and management is crucial. Computed tomography (CT) is useful for evaluating rectal fecal mass but limited owing to radiation exposure, cost, and inaccessibility at certain facilities. Ultrasonography (US) avoids these pitfalls, but it is unknown whether it accurately assesses rectal feces. In this study, we evaluated the diagnostic performance of US compared with CT as the gold standard for assessing rectal feces.
Methods:
We prospectively evaluated rectal fecal mass retention using US performed within 1 h of CT to assess the degree of agreement between methodologies. Rectal stool findings were evaluated on three levels: no stool (R1), presence of stool (R2), and hard stool filling (R3).
Results:
The sample included 100 patients (55 men, 45 women), of whom 47 were constipated. The kappa coefficients for rectal content detection were excellent between US and CT (p <0.001). Eighty-two cases (R1: 46 cases; R2: 28 cases; R3: 8 cases) were matched with CT and US findings, and 18 were not. Cases that did not match had low urine or high gas volumes. CT and US findings showed high agreement in constipation (kappa coefficient 0.674, p <0.001) and non-constipation groups (kappa coefficient 0.677, p <0.001). All cases with R3 on CT were found in the constipation group, while more than half of the cases with R1 on CT were in the non-constipation group.
Conclusions:
CT and US showed high agreement in evaluating rectal fecal mass retention, indicating that US can substitute CT.
Keywords: chronic constipation, computed tomography, rectal fecal mass retention, ultrasonography
Introduction
Chronic constipation is a common gastrointestinal disorder, with a prevalence of 14% in the general population and a gradual increase in prevalence with age[1]. The aging of the population in Japan has been rapid, with 28.8% over 65 years and 14.9% over 75 years in 2021[2]. Individuals with chronic constipation are expected to increase with the rapidly aging population in Japan.
Some methods to evaluate chronic constipation include anorectal structure and function testing (balloon expulsion test, anorectal manometry, and defecography) and colonic transit testing (radiopaque marker test, wireless motility capsule test, scintigraphy, or colonic manometry)[3]. However, these methods have problems, such as radiation exposure, equipment costs, and complexity of the procedure; thus, they are infrequently used in assessing chronic constipation in Japan. On the other hand, ultrasonography (US) is noninvasive, low-cost, simple, and repeatable, and its clinical application for chronic constipation is attracting attention. Several reports have described the clinical application of extracorporeal US for treating chronic constipation[4-6].
Digital disimpaction is performed by breaking up feces; hence it should be done when feces are in the rectum. Therefore, it is important to evaluate fecal mass retention in the rectum. Computed tomography (CT) is useful in this regard, but there are associated problems, such as radiation exposure, cost, and inaccessibility in certain facilities. To address this, we proposed a simple, functional assessment method using rectal US to treat chronic constipation[6]. In this method, the state of fecal impaction in the rectum can be classified into three patterns. This method helps if the US shows no feces in the rectum; therefore, an enema can be avoided. Moreover, if hard stools are found, a laxative can be administered after the enema to avoid complications of serious intestinal perforation. Thus, knowing whether stool is present in the rectum using US is meaningful for determining the pathological condition and treatment options.
However, it is still unknown whether rectal feces can be accurately assessed using US. Studies have evaluated US by comparing fecal mass in the rectum to discharged stool as the point of reference[7], but no studies have compared US to CT or other imaging modalities. In this study, we evaluated the diagnostic performance of US compared with CT as the gold standard in assessing rectal feces.
Methods
Study design
This was a prospective, single-center observational study. Patients seen at the Yokohama City University Hospital between August 2020 and June 2022 were recruited as study participants.
The study protocol complied with the Declaration of Helsinki and Ethics Guidelines for Clinical Research published by the Ministry of Health, Labor, and Welfare, Japan. This study was approved by the ethics committee of Yokohama City University Hospital in August 2020. This trial has been registered in the University Hospital Medical Information Network (UMIN) Clinical Trials Registry as UMIN000042705. Written informed consent for participation in the study was obtained from all participants.
Patient eligibility criteria
The inclusion criteria for the study were as follows: age over 20 years as of the date of informed consent, US data collected within 1 h of CT, and willingness to provide written informed consent.
Experimental protocol
US was performed within 1 h of CT, with patients not defecating between CT and US. Background information (age, sex, body mass index, and thickness of subcutaneous fat), history (presence of chronic constipation and diabetes mellitus), and rectal stool findings (US and CT) were evaluated. We measured the thickness of subcutaneous fat at the umbilical level using CT. Patients diagnosed with functional constipation according to the Rome IV criteria by experienced gastroenterologists[8] or those already under treatment for functional constipation were classified as part of the constipation group.
US observation procedure
For US devices, a convex probe was selected, preferably with a bandwidth of 2-5 MHz. In this study, we used wireless US devices (iViz wireless, FWT C5-2, convex probe, Fujifilm, Tokyo, Japan) in all the cases (Figure 1).
Figure 1.
Wireless ultrasonography devices (iViz air®, Fujifilm, Tokyo, Japan).
For examinations, the patients were supine, and their heads were propped at approximately 10°, with a bath towel or cushion placed under their necks to relieve tension in the abdomen, and the knee joints were gently flexed.
In transverse and longitudinal scans, the US probe was placed at the superior border of the pubic bone, and the ultrasound beam was tilted caudally by 10-30° to visualize the bladder. The bladder was used as the acoustic window, and the rectal visualization was deeper than that of the bladder.
Evaluation of stool findings in the rectum
With reference to a previous report, we evaluated stool findings in the rectum at three levels with CT: no stool (CR1), presence of stool (CR2), and hard stool filling (CR3) (Figure 2)[7]. On US, if there is no fecal retention or gas in the rectal lumen, an obvious hyperechoic area will not be observed. In a transverse image, a circumferential hypoechoic area may be observed as an empty intestine (UR1). When fecal retention occurs in the rectal lumen, ultrasound waves will be reflected from the surface of the contents at a depth deeper than the bladder (anechoic area), and a hyperechoic area with a half-moon shape will be observed in the transverse image (UR2). In addition, when there is hard stool accumulation, an acoustic shadow will be observed in the transverse image of the crescent-shaped hyperechoic area (UR3) (Figure 3).
Figure 2.
Three levels of computed tomography findings in the rectum (white arrows): no stool (CR1), presence of stool (CR2), and hard stool filling (CR3).
Figure 3.
Three levels of ultrasonography (US) findings in the rectum (white arrows): UR1, the circumferential hypoechoic area is observed as an empty intestine; UR2, ultrasound waves are reflected from the surface of the contents at a depth deeper than the bladder (anechoic area) and a hyperechoic area with a half-moon shape is observed in the transverse image; UR3, an acoustic shadow is observed in the transverse image of the crescent-shaped hyperechoic area.
Statistical analysis
Data on the age and body mass index frequency were analyzed using the unpaired Student's t-test. Data on sex, the presence of diabetes mellitus, and the history of gastrointestinal surgery were compared using the chi-square test. The degree of agreement between the CT and US findings was analyzed using Cohen's kappa. Statistical significance was set at p <0.05. Data are shown as means and standard deviations unless otherwise stated. All statistical analyses were performed using SPSS software, version 28.0.1.0 (IBM Corp., Armonk, NY, USA).
Results
Clinical characteristics of the patients
A total of 100 patients (55 men and 45 women) were included in the study, of which 47 were constipated. No defecation was observed between the CT and US. No significant differences in clinical characteristics were found between the two groups. The demographic data of the patients are shown in Table 1.
Table 1.
Clinical Characteristics of the Patients (SD, Standard Deviation).
| Constipation group (n = 47) | Non-constipation group (n = 53) | p-value | |
|---|---|---|---|
| Age (years), mean (SD) | 70.8 (12.5) | 67.9 (13.2) | 0.264 |
| Sex (Men:Women) | 21:26 | 34:19 | 0.051 |
| BMI†, mean (SD) | 23.8 (4.4) | 22.5 (3.6) | 0.117 |
| Thickness of subcutaneous fat | 18.3 (7.9) | 15.7 (6.6) | 0.085 |
| Diabetes mellitus, n (%) | 9 (19.1) | 9 (17.0) | 0.778 |
| History of gastrointestinal surgery, n (%) | 17 (36.2) | 26 (49.1) | 0.194 |
| Presence of ascites, n (%) | 2 (4.3) | 5 (9.4) | 0.442 |
BMI, body mass index
Analysis of correspondence between CT and US for detection of rectal contents
The kappa coefficients for rectal content detection were excellent between US and CT (kappa coefficient 0.685, p <0.001). Eighty-two cases (R1: 46 cases, R2: 28 cases, R3: 8 cases) were matched with the CT and US findings, and 18 were not matched (Table 2). The reasons for a lack of match were as follows: low urine output (five cases), high gas volume (four cases), a large amount of ascites (one case), and an unknown cause (eight cases).
Table 2.
Compatibility of CT and US for Rectal Content.
| US | ||||
|---|---|---|---|---|
| UR1 | UR2 | UR3 | ||
| CT | CR1 | 46 | 3 | 1 |
| CR2 | 11 | 28 | 1 | |
| CR3 | 1 | 1 | 8 | |
CT, computed tomography; US, ultrasonography
CR1, no stool; CR2, presence of stool; CR3, hard stool filling.
UR1, the circumferential hypoechoic area is observed as an empty intestine; UR2, ultrasound waves are reflected from the surface of the contents at a depth deeper than the bladder (anechoic area), and a hyperechoic area with a half-moon shape is observed in the transverse image; UR3, an acoustic shadow is observed in the transverse image of the crescent-shaped hyperechoic area.
Analysis of the compatibility of CT and US in detecting rectal contents in constipation and non-constipation groups
CT and US findings showed high agreement in the constipation and non-constipation groups (constipation group: kappa coefficient 0.674, p <0.001; non-constipation group: kappa coefficient 0.677, p <0.001). All cases with CR3 were found in the constipation group, whereas more than half of the patients with CR1 were found in the non-constipation group (Table 3A, 3B).
Table 3A.
Compatibility of CT and US for Rectal Content in the Constipation Group.
| US | ||||
|---|---|---|---|---|
| UR1 | UR2 | UR3 | ||
| CT | CR1 | 16 | 0 | 0 |
| CR2 | 7 | 13 | 1 | |
| CR3 | 1 | 1 | 8 | |
CT, computed tomography; US, ultrasonography
CR1, no stool; CR2, presence of stool; CR3, hard stool filling.
UR1, the circumferential hypoechoic area is observed as an empty intestine; UR2, ultrasound waves are reflected from the surface of the contents at a depth deeper than the bladder (anechoic area), and a hyperechoic area with a half-moon shape is observed in the transverse image; UR3, an acoustic shadow is observed in the transverse image of the crescent-shaped hyperechoic area.
Table 3B.
Compatibility of CT and US for Rectal Contents in the Non-Constipation Group.
| US | ||||
|---|---|---|---|---|
| UR1 | UR2 | UR3 | ||
| CT | CR1 | 30 | 3 | 1 |
| CR2 | 4 | 15 | 0 | |
| CR3 | 0 | 0 | 0 | |
CT, computed tomography; US, ultrasonography
CR1, no stool; CR2, presence of stool; CR3, hard stool filling.
UR1, the circumferential hypoechoic area is observed as an empty intestine; UR2, ultrasound waves are reflected from the surface of the contents at a depth deeper than the bladder (anechoic area), and a hyperechoic area with a half-moon shape is observed in the transverse image; UR3, an acoustic shadow is observed in the transverse image of the crescent-shaped hyperechoic area.
Discussion
In this study, we compared CT and US results to determine the validity of rectal US. We have shown that CT and US show a high degree of agreement in evaluating fecal mass retention in the rectum. In addition, many cases in the non-constipation group showed no stool in the rectum, whereas many cases in the constipation group showed stool accumulation in the rectum. To our knowledge, this is the first study to evaluate the diagnostic performance of US for rectal feces in comparison with CT as the gold standard.
The usefulness of CT and magnetic resonance imaging (MRI) for evaluating chronic constipation has been reported in several cases. Mizukami et al. reported that CT was performed on patients with irritable bowel syndrome and healthy controls. They found that patients with irritable bowel syndrome had significantly more abnormal morphology of the sigmoid colon and that colon morphology could be evaluated by colon CT[9]. Inoh et al. reported that an increased diameter of the descending colon was associated with residual stool, an increased diameter of the sigmoid colon with diarrhea, and an increased diameter of the rectum with constipation. They also showed that MRI was useful in evaluating the pathophysiology of colonic motility to determine the appropriate treatment for chronic constipation[10]. However, these modalities have the problems of complexity, test time, and radiation exposure.
Rectal evaluation using US is noninvasive, low-cost, simple, and repeatable, and the results can be used to select the best treatment option. We previously proposed a simple, functional assessment method using rectal US to treat chronic constipation[6]. The current results demonstrate the validity of this protocol. In addition, wireless US was used in our study. In general, wireless US has advantages such as portability; moreover, we showed that wireless US has no problem with image evaluation.
In our study, CR3 was found in the constipation group, whereas more than half of the patients with CR1 were in the non-constipation group. Park et al. reported that rectal gas volume on abdominal CT is greater in patients with constipation and rectal evacuation disorders than in patients without rectal evacuation disorders[11]. However, there have been no previous reports about fecal impaction in the rectum of patients with chronic constipation. Our finding that patients with constipation have stool retention may be because these patients have a loss of defecation desire and impaired fecal evacuation[12].
Our study had a few limitations. First, this was a single-center study with a limited number of examiners; therefore, an inter-examiner evaluation could not be performed. Future multicenter studies are needed to verify our results. Second, there were reasons for the lack of matching between CT and US, such as low urine output and high gas volume, and no breakthrough for this could be demonstrated in this study. Gas in the colon near the surface of the body, such as in the transverse colon, is easily evaluated by US, but gas in the rectum is considered difficult to evaluate by US. Third, the study was unable to show how the results could be used to treat chronic constipation. If the rectum is full of hard stools with no defecation occurring for several days, it may be effective to administer an enema and then regularly administer stool-softening agents, such as osmotic laxatives. Conversely, if there is no fecal mass in the rectum despite no defecation for several days, it may be effective to administer drugs that promote intestinal peristalsis. However, these are only suggestions, and future investigations into therapeutic interventions using this assessment model are needed. Fourth, our study assessed the presence or absence of feces but not the quantity of feces in the rectum.
In conclusion, CT and US showed a high degree of agreement in evaluating fecal mass retention in the rectum, indicating that US can be a substitute for CT in diagnosing rectal stool and constipation. In addition, many patients in the non-constipation group showed no stool in the rectum, whereas many patients in the constipation group showed stool accumulation. In diagnosing rectal stool status, US is a simple, bedside, and objective method that does not involve radiation exposure or other invasive procedures.
Conflicts of Interest
Atsushi Nakajima has received honoraria from PDR Adiopharma Inc. Astellas, Mylan EPD, EA Pharma, Kowa Pharma, Taisyo Pharma and Bioferumin Pharma.
Author Contributions
NM, TH, HS, MK, and AN conceived the study. NM, MM, and MT evaluated the ultrasonography. ST, TY, KA, TK, and HO recruited the study participants. All authors read the final manuscript and approved its submission for publication.
Approval by Institutional Review Board (IRB)
The study protocol complied with the Declaration of Helsinki and Ethics Guidelines for Clinical Research published by the Ministry of Health, Labor, and Welfare, Japan. This study was approved by the ethics committee of Yokohama City University Hospital in August 2020. This trial has been registered in the University Hospital Medical Information Network (UMIN) Clinical Trials Registry as UMIN000042705.
Data Accessibility Statement
Due to the nature of this research, participants did not agree for their data to be shared publicly, so supporting data is unavailable.
Acknowledgements
We thank the staff of the participating institutions for their support in recruiting the patients.
References
- 1.Suares NC, Ford AC. Prevalence of, and risk factors for, chronic idiopathic constipation in the community: systematic review and meta-analysis. Am J Gastroenterol. 2011 Sep; 106(9): 1582-91. [DOI] [PubMed] [Google Scholar]
- 2.Annual Report on the Ageing Society FY2021. Cabinet Office Japan; 2021. [cited 2023 Mar 2]. Available from: https://www8.cao.go.jp/kourei/english/annualreport/2021/pdf/2021.pdf.
- 3.Camilleri M, Ford AC, Mawe GM, et al. Chronic constipation. Nat Rev Dis Primers. 2017 Dec; 3: 17095. [DOI] [PubMed] [Google Scholar]
- 4.Manabe N, Kamada T, Hata J, et al. New ultrasonographic evaluation of stool and/or gas distribution for treatment of chronic constipation. Int J Colorectal Dis. 2018 Jan; 33(3): 345-8. [DOI] [PubMed] [Google Scholar]
- 5.Manabe N, Kamada T, Kusunoki H, et al. Usefulness of ultrasonographic evaluation of stool and/or gas distribution for the treatment strategy of chronic constipation. JGH Open. 2019 Mar; 3(4): 310-5. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Matsumoto M, Misawa N, Tsuda M, et al. Expert consensus document: diagnosis for chronic constipation with faecal retention in the rectum using ultrasonography. Diagnostics (Basel). 2022 Jan; 12(2): 300. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Tanaka S, Yabunaka K, Matsumoto M, et al. Fecal distribution changes using colorectal ultrasonography in older people with physical and cognitive impairment living in long-term care facilities: a longitudinal observational study. Healthcare (Basel). 2018 May; 6(2): 55. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8.Mearin F, Lacy BE, Chang L, et al. Bowel disorders. Gastroenterology. 2016 Feb; S0016-5085(16): 00222-5. [DOI] [PubMed] [Google Scholar]
- 9.Mizukami T, Sugimoto S, Masaoka T, et al. Colonic dysmotility and morphological abnormality frequently detected in Japanese patients with irritable bowel syndrome. Intest Res. 2017 Apr; 15(2): 236-43. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Inoh Y, Kanoshima K, Ohkuma K, et al. Assessment of colonic contents in patients with chronic constipation using MRI. J Clin Biochem Nutr. 2018 May; 62(3): 277-80. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11.Park SY, Khemani D, Nelson AD, et al. Rectal gas volume measured by computerized tomography identifies evacuation disorders in patients with constipation. Clin Gastroenterol Hepatol. 2017 Apr; 15(4): 543-52.e4. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12.Ohkubo H, Takatsu T, Yoshihara T, et al. Difference in defecation desire between patients with and without chronic constipation: a large-scale internet survey. Clin Transl Gastroenterol. 2020 Sep; 11(9): e00230. [DOI] [PMC free article] [PubMed] [Google Scholar]
