Approximately 50% of patients with functional constipation have difficulty in defecating (1) and may have the functional defecatory disorder (FDD) (2). FDD significantly affects productivity, mental health, and quality of life (QOL) (3).

Intact rectal sensation and motility are critical to normal bowel movement and defecation. The presence of sufficient stool and intact sensation will trigger the perception of rectal fullness through rectal afferent pathways (4). Rectal hyposensitivity (RH) refers to a blunted sensation of mechanical distension, which is indicated by the elevation of sensory thresholds beyond the normal range (5). As sensation and motility are inextricably linked, alteration in one domain can affect the other. RH, rectal motor dysfunction, and altered recto-anal reflex activity are particularly associated with FDD (6).

Patients with RH commonly present with constipation (48%) (7), and about 18%–68% of constipated patients have RH (8). It is reported that RH is more common in patients with functional disorders (i.e., dyssynergic defecation) rather than structural diseases (i.e., rectocele and intussusception) (9). Our team has found that RH is associated with defecation symptoms and specifies an eventual diagnosis of FDD over delayed gut transit (10).

Rectal hyposensitivity is associated with constipation, but its clinical importance remains unclear. In addition, little is known about the characteristics of FDD patients with RH and the related factors of RH in these individuals. Given the above deficiencies, we carried out this study to explore the influence of RH on constipation symptoms, mental state as well as QOL, and related factors of RH in an FDD population.

Rectal hyposensitivity was reported in almost 25% of adult patients with chronic idiopathic constipation (21, 22). RH was associated strongly with pelvic floor dysfunction other than abnormal motility. A recently published study revealed that patients with three abnormally elevated sensory thresholds suffered almost two times as frequent defecation disorder as patients with normal rectal sensation (44.3% vs. 23.2%) (23). However, in a study that enrolled 107 patients with FC (37.4% had RH), no significant difference in RH was observed between the non-FDD and FDD groups (24). The impaired rectal sensation may be negatively associated with abnormal rectal/anal pressure and paradoxical pelvic contraction. Biofeedback therapy (BFT) is the first-line treatment for FDD but patients with RH poorly respond to it (10). Our study might help physicians identify patients with both FDD and RH timely in order to manage them more individually.

We detected RH in 50/311 (15.11%) and borderline RH in 37/331 (11.18%) of patients with FDD. More than a quarter of patients with FDD had one or more abnormally elevated sensory thresholds. This finding is consistent with an observational study where 163 of 667 constipated patients (24.4%) had one or more elevated thresholds (5). It is also suggested that there is a smaller proportion of constipated patients with ≥2 elevated sensory thresholds (13–17%) (23, 25). However, we did not find that patients with FDD suffered more RH than generalized constipated patients.

The underlying mechanism of how RH causes anorectal disorders is still unknown. The intact rectal sensation is fundamental to recto-anal and pelvic floor coordination (26). Some scholars hypothesize that individuals with RH have altered recto-anal reflexes and/or sensorimotor response, and the balloon volumes for inducing their rectoanal inhibitory reflex and contractile reflex were higher (27). Another study showed that patients with RH have reduced rectal wall contractility in response to distension, which likely contributes to failed defecation (28). However, we only found that the anal relaxation rate was lowest in the RH group but no difference in anorectal pressure or presence of pelvic floor disorder was observed among the three groups. As to comparisons of three rectal sensory thresholds, the presence of abnormally elevated FCSV, DDV, and MTV were all higher in patients with RH compared to those with borderline RH and non-RH, especially for DDV which was elevated in all patients with RH. We speculated that DDV might be a useful indicator for impaired rectal sensation. Only weak correlations were seen between FCSV and anal resting pressure, maximum squeeze pressure, and residual pressures, which is of limited clinical significance.

It is demonstrated that an increasing number of elevated sensory thresholds was associated with a more severe constipation phenotype (23). In our study, FDD patients with RH had more severe defecation symptoms. Meanwhile, the BSFS score was lower in these patients, indicating that they suffered from the lumpy or hard stool. The hard stool is closely related to RH in patients with FDD. Thus, more patients with RH needed manual maneuvers to help defecate. The conscious withdrawal of attention from rectal sensations or habitual suppression of the desire to defecate may contribute to impaired call to stool, which could cause rectal impaction and secondary dilatation of the rectum, leading to RH (29–31). The longer stool stays in the colon and rectum, the harder it may become, which could explain the lumpy or hard stool which patients with RH frequently have. Thus, these patients with FDD experience more severe defecation symptoms and need to use digital assistance or enema. Defecation symptom severity was correlated to QOL in these patients, which needs more attention.

We found that patients with RH were older and age was positively correlated to three rectal sensory thresholds, suggesting the decreased rectal sensation might be related to aging. Age-related impairment in the mechanoreceptors of the rectal wall and the pelvic afferent nerves might play a role in this relationship (32). A previous study by our team had a similar finding in a general functional constipation population (10). In addition, the proportion of male patients was high in the RH group. It is known that female patients are prone to constipation but most of them suffer slow transit constipation compared with male patients. A previous study found that constipated male patients were significantly more likely to suffer from defecation disorder than female patients (33). Additionally, our team has found that male patients tended to have much more paradoxical anal sphincter contraction and impaired anal sphincter relaxation (34). We speculated that male patients might have higher stress and various pressure than female patients and they are inclined to suppress the stool calling, which could lead to RH and FDD. Based on the analysis of a large patient cohort, older age and male sex were associated with higher rectal sensory thresholds (35), which agrees with our findings. However, the pathophysiological mechanism is still unknown and warranted to be explored in future studies.

The visceral sensation may be influenced by personality profile, autonomic nervous system function, and psychological phenotype (36, 37). However, little evidence has yet to be found directly in patients with RH. In our study, depression symptom was positively related to MTV, though the link is weak. But anxiety symptom was not related to any sensory threshold. The concept of the brain-gut axis is well recognized and peptide hormones (neuropeptide Y, peptide YY, glucagon-like peptide 1, etc.) released from the gut play a critical part in the interaction between the brain and digestive system (38, 39). Our results revealed that GAD-7 and PHQ-9 scores were positively related to defecation symptom severity. Patients with irritable bowel disease (IBS) (40) mostly have acute rectal feelings and psychological distress may aggravate IBS symptoms (41). It is established that anxiety can enhance visceral feelings (42, 43) but the effect of depression on sensation is controversial. According to our findings, we speculate that depression rather than anxiety plays an important role in blunt rectal sensation. However, logistic regression revealed that anxiety or depression was not related to the occurrence of RH. The association between mental state and RH in patients with FDD has been rarely studied and needs to be explored in future research.

We acknowledged that there are some limitations regarding our study. First, our study focused on patients with FDD in a single tertiary center, which unavoidably ended up with a highly selected population so the results could not be generalized to a wider primary care population. Second, volumetric balloon distension instead of barostat was used to test patients’ rectal sensory thresholds. Constipated patients with RH usually have persistent dilatation of the rectum and greater volumes will be required to stimulate the rectum (44). Thus, constipated patients with RH who have elevated volume thresholds might not have impaired rectal sensation actually. Recording pressure thresholds with barostat rather than volume thresholds is of more physiological significance (45). Nevertheless, in routine clinical practice, volumetric balloon distension is well accepted and often used (12, 46). In the future, testing rectal pressure thresholds with barostat may be a better and more rigorous method to identify RH. Finally, the link between constipation and RH is well established, but the cause–effect relationship in observational studies is still unclear. RH could lead to harder stool and more difficult defecation and long duration or severe constipation may result in a dilated rectum and abnormal rectal wall compliance which impairs rectal sensation and vice versa. Advanced prospective researches and cohort studies are in need.

This study has summarized the characteristics of FDD patients with RH by investigating symptomology, mental state, QOL, and functional tests. It is shown that patients with RH are older, more male patients, and vulnerable to suffering more severe defecation symptoms. Elevated DDV is most frequently seen in FDD patients with RH. Although abnormal motility and sensation may interact with each other and induce defecation disorder, we did not find specific links between them. Older age, gender (male), and lumpy or hard stool are related factors of RH in FDD and depression is associated with elevated MTV. The above findings may help physicians identify high-risk patients more efficiently. Thus, FDD patients with RH could get much better management in time.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Written informed consent was not obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

YJ and YT designed the study. YJ, YW, and MW collected and analyzed the data. YJ wrote the manuscript. YJ, YT, and LL revised the manuscript. All authors contributed to the article and approved the submitted version.

This study is supported by National Natural Science Foundation of China (No. 81870378 and 82170556) for YT.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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