This is the first study to report normal HRAM values in healthy Vietnamese adults, and predominately includes anorectal pressure and rectal sensation values. In addition, we also measured other values, including functional anal canal length, HPZ length, defecation index, and the threshold volume to elicit RAIR. All HRAM values were compared by sex, age group, and BMI, in which anorectal pressure and rectal sensation values were significantly different between men and women. We found that a significant proportion of healthy people had dyssynergic patterns during simulated defecation while in a prone position. This study provided evidence of normal HRAM values along with the proportion of dyssynergic patterns in healthy adults which may be considered baseline data with which to compare clinical measurements in order to enhance diagnosis and treatment for anorectal diseases in Vietnam, and to allow comparison with future studies.
Our results also show similarities in the patterns and range of values when compared with those of previous studies regarding normal HRAM values in healthy people [8–11, 16]. However, besides the values for anorectal pressure and rectal sensation, previous studies reported different parameters. Although these studies focused on normal HRAM values, they had specific goals in their reporting and methodology, and so the manner and structure of HRAM indicators presented differed from ours. There are currently no guidelines for the selection of indicators when conducting HRAM, so difficulties in comparing results between studies may arise. As such, we suggest that a standard international HRAM measurement reporting scheme should be developed [3, 11, 17].
Except for the study conducted by Carrington et al. [11], previous studies did not report functional anal canal length values [8–10, 16]. The mean value of functional anal canal length was slightly higher in our study than that reported by Carrington et al. [11]. We also did not find a significant difference in this value between males and females. We did find a significant difference in the functional anal canal length between participants with BMI < 25 and those with BMI ≥ 25, suggesting that BMI variation could affect the functional anal canal length. In the study conducted by Paul et al., functional anal canal length was used to evaluate anal sphincter dysfunction in patients with fecal incontinence or constipation, using baseline values collected from a healthy volunteer control group [18]. However, functional anal canal length did not help diagnosis of fecal incontinence or constipation in that study. Similarly, in the present study HPZ values were relatively similar to other studies [9, 10, 19]. We also found no difference in HPZ values according to sex and age group. Previously, it has been shown that patients with increased HPZ lengths may suffer from defecatory disorders [19]. We found a statistically significant difference in HPZ values according to BMI, such that participants with BMI ≥ 25 had increased HPZ values when compared to those of participants with BMI < 25. BMI may as such also affect HPZ values, and we therefore suggest that further studies be conducted to better determine how BMI and other factors influence normal HPZ values.
Anorectal pressure values in our study are similar to those reported in previous studies, but there were certain differences. Resting pressure in our study was similar to that reported by Carrington et al. [11], although our value was higher those reported by Li et al. [9], Lee et al. [8], and Jones et al. [16]. Similar to previous reports, we found no significant difference in anal resting pressure between males and females [9, 11]. In a study conducted in healthy Korean people, Lee et al. reported that the maximum anal resting pressure was higher in men than in women [8], although that result may be influenced by the relatively small sample size of that study. Maximum anal squeeze pressure results from our study were lower than that of Carrington et al. [11] and Li et al. [9]. However, mean maximum anal squeeze pressure in our study was higher than that of Jones et al. [16]. Differences in values may relate to variation among participants, measuring instruments, or measurement protocol. Furthermore, the interpersonal interaction between clinical technicians and research participants may also affect measurement results [20]. As in previous studies [8, 11], we also found a statistically significant difference in maximum anal squeeze pressure between men and women. In addition, we report a number of other values not previously mentioned, including maximum anal cough pressure, maximum anal strain pressure, maximum rectal cough pressure, and maximum rectal strain pressure. These values also tended to be higher in men than in women. We suggest that these are also important values in the evaluation of anorectal functions, and that these values can be used as a reference for the diagnosis and treatment of anorectal disorders in the future.
The defecation index is simple and easy to calculate and is very effective in assessing disorders that are related to rectoanal coordination. This index is often referred to in studies involving certain anorectal diseases [21, 22]. If we consider a defecation index < 1.3 as indicative of an anorectal disorder, approximately 50% of participants in our study displayed such a condition. However, the participants in our study were healthy people, and this is something that should be taken into account when using the defecation index in diagnosis, as its use may give false-positive results. The mean defecation index in our study is similar to that reported by Lee et al. [8]. We also did not find significant differences in defecation index according to gender, age group, or BMI group.
Dyssynergic patterns were defined by Rao et al. to help diagnose patients with chronic constipation [23]. However, most previous studies of HRAM in healthy people did not assess dyssynergic patterns [8–11, 16]. In one study that used HDAM-3D, Enrique et al. reported that a dyssynergic defecation pattern occurred in more than 60% of health study participants [24]. In our study, approximately 50% of study participants displayed a dyssynergic pattern, which is consistent with the previously-reported value. We also compared the presence of a dyssynergic pattern by sex, age group, and BMI group, but we did not detect any statistically significant differences. Study that includes a larger sample size should be conducted to learn more about these factors.
RAIR is also considered an important indicator in HRAM as it can serve as a proxy signal that associates with various anorectal disorders [25–28]. In our study, RAIR was present in all patients, but the threshold volume to elicit RAIR was different. Although no previous studies have reported threshold volumes that elicited RAIR using HRAM [25–28], we suggest that this is also an appropriate indicator to use as a basis for comparisons of diagnoses. We found that the threshold volume to elicit RAIR was significantly different between the participant age groups, indicating that age might be a factor relating to changes in the structure of the anorectal area. This result is in line with a study that used high-definition anorectal manometry (HDAM-3D) [24], in which Coss-Adame et al. reported that the threshold volume that elicited RAIR was 16.1 ± 1.4 mL in a sample of 78 healthy participants, with no significant difference found between males and females. Using the conventional ARM method, it was suggested that RAIR may be absent in adult patients with megacolon, chronic constipation, or chronic intestinal pseudo-obstruction [29–31]. In children, however, the absence of RAIR is considered to be a highly sensitive and highly specific indicator of Hirschsprung disease [32].
With regards to rectal sensation, the results of our study are similar to those reported by Li et al. [9]. We found no significant difference in rectal sensation between men and women. However, as opposed to Li et al., we detected a significant difference in the volumes at first sensation and the threshold of desiring to defecate according to age group [9]. However, in an study using HDAM-3D, Coss-Adame et al. reported associations between sex and values of the threshold of the desire to defecate, the urgency to defecate, and the maximal tolerable volume [24]. In addition, we observed a significant difference in the volume at first sensation according to BMI groups. We suggest that age and BMI may influence the threshold of rectal sensation, although this conflicts with Lee et al., who also reported an association between BMI and the threshold of the first sensation, but one that was negatively correlated [8]. As the mechanisms involved in rectal sensation are still unknown, possible explanations of differences between study outcomes are unclear. A study that includes a larger sample size is needed to better understand the factors related to rectal sensation.
We are aware of some limitations of this study. The sample size was relatively small, and data were stratified according to gender, age, and BMI. The small sample size in a sub-group might thus affect statistical values during comparisons. In this study, we mainly focused on the comparison between males and females, so it may be possible that we missed characteristics that are unique to females, such as any history of maternity-related issues. This is a single-center study, so care must be taken when applying these results to other health centers. Sampling may have been insufficient to cover all possible age groups, and it is possible that patient age may affect HRAM values. In addition, this is a cross-sectional study, so care must be taken when interpreting the results in a causal relationship.