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Keywords:

  • abnormal visceral sensitivity;
  • functional constipation;
  • pathophysiological mechanisms;
  • rectal hyposensitivity

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

Abstract  Rectal hyposensitivity (RH) relates to a diminished perception of rectal distension. It may occur due to afferent nerve dysfunction and/or secondary to abnormal structural or biomechanical properties of the rectum. The aim of this study was to determine the contribution of these underlying pathophysiological mechanisms by systematically evaluating rectal diameter, compliance and afferent nerve sensitivity in patients with RH, using methodology employed in clinical practice. The study population comprised 45 (33 women; median age 48, range 25–72 years) constipated patients (Rome II criteria) with RH and 20 with normal rectal sensitivity on balloon distension and 20 healthy volunteers. Rectal diameter was measured at minimum distending pressure during isobaric distension under fluoroscopic screening. Rectal compliance was assessed during phasic isobaric distension by measuring the slope of the pressure–volume curve. Electrical stimulation of the rectal mucosa was employed to determine afferent nerve function. Values were compared to normal ranges established in healthy volunteers. The upper limits of normal for rectal diameter, compliance and electrosensitivity were 6.3 cm, 17.9 mL mmHg−1 and 21.3 mA respectively. Among patients with RH, rectal diameter, but not compliance, was increased above the normal range (megarectum) in seven patients (16%), two of whom had elevated electrosensitivity thresholds. Rectal diameter and compliance were elevated in 23 patients (51%), nine of whom had elevated electrosensitivity thresholds. The remaining 15 patients (33%) with RH had normal rectal compliance and diameter, all of whom had elevated electrosensitivity thresholds. Two-third of the patients with RH on simple balloon distension have elevated rectal compliance and/or diameter, suggesting that impaired perception of rectal distension is due to inadequate stimulation of the rectal afferent pathway. However, a proportion of such patients also appear to have impaired nerve function. In the remaining one-third of the patients, rectal diameter and compliance are normal, while electrosensitivity thresholds are elevated, suggestive of true impaired afferent nerve function. Identification of these subgroups of patients with RH may have implications regarding their management.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

Rectal hyposensitivity (RH) relates to a diminished perception of rectal distension. RH is prevalent1 and its importance increasingly recognized, both in the aetiology of symptoms2 and outcome of interventions,3–5 in certain patients with disorders of hindgut function. In everyday clinical practice, it is recommended that rectal sensation be assessed using balloon distension using a hand-held syringe.6 Accordingly, RH may be diagnosed on the basis of elevated sensory threshold volumes required to elicit rectal sensations.7,8

Traditionally, it has been considered that diminished perception of rectal distension in such patients reflects impaired afferent nerve function,8 such as that which may occur following pelvic or sacral nerve injury.9–11 However, sensory assessment using balloon distension using a hand-held syringe may not accurately reflect the function of visceral afferents under certain circumstances, as sensory threshold volumes may also be influenced by structural and/or biomechanical properties of the rectum.8,12 A subgroup of patients with intractable constipation has persistent dilatation of the rectum, termed megarectum.13 In addition to increased diameter, such patients also have increased rectal compliance,14 and are reported to have impaired perception of rectal distension (i.e. RH).15 However, in the presence of increased rectal diameter and/or compliance, greater volumes will be required to distend and thus stimulate the rectum,16 and thus elevated sensory threshold volumes may conceivably reflect increased rectal size and inadequate stimulation, rather than dysfunction of the rectal afferent pathway itself.8,17

This distinction between what might be considered RH due to ‘primary’ afferent nerve dysfunction from that ‘secondary’ to abnormal structural or biomechanical properties of the rectum is of fundamental importance, but cannot be made using balloon distension with a hand-held syringe alone.8 Rectal compliance is most commonly used as a measure of biomechanical properties in clinical practice, and may be determined by constructing pressure–volume curves during phasic isobaric distension, using the barostat.18,19 By contrast, the assessment of rectal structure (diameter) is more controversial due to inherent methodological limitations of traditional techniques.20 However, a recently described technique, also utilizing the barostat, has enabled patients with megarectum to be more accurately identified during fluoroscopic screening by measuring rectal diameter at minimum distending pressure.20 Given the potential limitations of balloon distension using a hand-held syringe in the assessment of the rectal afferent pathway,8,12 electrical stimulation of the rectal mucosa is also employed to assess rectal sensory function, as this is not influenced by rectal biomechanical properties,21 as it relies solely on contact with the rectal mucosa and thus may provide a more accurate representation of rectal afferent fibre function in patients with RH.22

Currently, the relative importance of each of the pathophysiological mechanisms in patients with RH is unknown.8 Therefore, the aim of the present study was to systematically assess rectal diameter, compliance, together with afferent nerve sensitivity in constipated patients with RH, using methodology that is widely available in routine clinical practice, so as to determine whether diminished perception of rectal distension, as detected by elevated sensory threshold volumes, was due to dysfunction and/or inadequate stimulation of the afferent pathway.

Materials and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

Subjects

Patients  The study population comprised 45 patients (33 women; median age 48, range 25–72 years) referred to a tertiary centre for physiological investigation of intractable constipation (Rome II criteria23), and found to have RH on balloon distension using a hand-held syringe. Rectal sensitivity was performed as part of standard anorectal physiological investigation by recording the volumes required to elicit a first constant sensation, a sustained desire to defaecate, and maximum toleration, during ramp distension of a latex balloon with air.24 RH was defined by elevation of one or more of the three rectal sensory threshold volumes beyond the normal range (mean + 2SD) for our unit,1 i.e. first sensation volume >160 mL, desire to defaecate volume >230 mL or maximum tolerable volume >315 mL in men, and first sensation volume >120 mL, desire to defaecate volume >210 mL or maximum tolerable volume >325 mL in women. In all such patients, the maximum tolerable volume, which has been suggested as the most reproducible threshold during sensory assessment,25 was elevated. The clinical and physiological features and the presence of potential risk factors that might lead to nerve injury (e.g. pelvic surgery/spinal trauma etc.2) in the constipated patients are shown in Table 1.

Table 1.   Clinical features and potential risk factors in constipated patients
 RH (n = 45)NS (= 20)
  1. RH, rectal hyposensitivity (i.e. elevation of one or more of the three rectal sensory threshold volumes during balloon distension beyond the normal range); NS, normal rectal sensation (i.e. sensory threshold volumes during balloon distension within the normal range); OD, obstructed defaecation; Values represent actual numbers (percentages); *= 0.004; = 0.03; = 0.0001.

Gender (F : M)33 : 1215 : 5
Clinical symptoms
 Duration of symptoms (years)11 (1–60.5)3 (0.5–60)*
 Infrequency alone1 (2%)0
 OD alone17 (38%)14 (70%)
 Infrequency and OD27 (60%)6 (30%)
 Sensation of ‘call to stool’14 (31%)19 (95%)
 Nulliparous7 (16%)5 (25%)
 Parity2 (0–7)2 (0–4)
 Previous pelvic surgery17 (38%)6 (30%)
 Previous spinal trauma3 (7%)0
 Diabetes mellitus1 (2%)1 (5%)
Physiological abnormalities
 Slow-transit alone8 (18%)2 (10%)
 Outlet obstruction alone14 (31%)8 (40%)
 Both slow-transit/outlet obstruction13 (29%)6 (30%)
 Neither slow-transit/outlet obstruction10 (22%)4 (20%)

So that any observed differences could be attributed to the physiological abnormality under investigation (i.e. RH), rather than the presenting symptom (i.e. constipation), comparison was made with data relating to rectal compliance and diameter obtained in constipated patients found to have normal rectal sensitivity on balloon distension using a hand-held syringe (normal sensation, NS).18,20 In addition, rectal electrosensitivity was also evaluated in a further 20 constipated patients with NS (15 women; median age 45, range 22–65 years). All patients had undergone appropriate assessment to exclude organic disease before referral. There was no history of overt neurological disease in any of the constipated patients. None of the patients were taking medication that could influence rectal sensitivity.

Controls  Normal ranges for rectal diameter, compliance and electrosensitivity were established in 20 healthy volunteers (HV) [12 women; median age 34, range 23–62 years (< 0.001 vs RH, Kruskal–Wallis anova with Dunns comparison)]. These subjects were recruited by advertisement, and comprehensively screened using a gastrointestinal symptom questionnaire and all proved to have a normal pattern of defaecation. None had a history of gastrointestinal symptoms or disease, were taking medication, nor had concomitant medical disease. Ethical approval was obtained from the East London and City Health Authority Research Ethics Committee, and written informed consent was gained after explanation of the study protocol.

Methods

Anorectal physiological investigation  Comprehensive assessment of anorectal function was performed in all patients, including radiopaque marker studies of colonic transit and evacuation proctography. Slow colonic transit was diagnosed when ≥20% of administered radiopaque markers remained on a plain abdominal radiograph at 100 h and outlet obstruction was diagnosed in the presence of slow, prolonged evacuation (>180 s) or early termination of evacuation, with a significant proportion of neostool retained (>25%).4 Consequently, on the basis of these studies, patients with constipation were classified as having (i) slow colonic transit, (ii) outlet obstruction, (iii) slow colonic transit and outlet obstruction, or (iv) normal colonic transit and evacuation.2

Conduct of the study  All subjects underwent detailed evaluation of rectal diameter, compliance and mucosal electrosensitivity. In 31 patients and 14 control subjects, each of these three parameters was evaluated during the same session, in the order presented below, but on a different day to standard anorectal physiological investigation. In the remaining 14 patients and six control subjects, rectal electrosensitivity testing was performed during anorectal physiology testing, while rectal compliance and diameter were assessed on a separate occasion, a median of 8 (range 1–19) days later. Subjects presented at 09:00 hours on the day of the experiment, following a 12 h fast. All bowel medication was withdrawn 24 h prior to the start of the study. Without sedation or bowel preparation, a rigid sigmoidoscopic examination of the anorectum was performed to ensure an empty rectum, and through which a catheter was inserted to lie within the rectum with the distal border of the barostat bag 5 cm from the anal verge. In the presence of residual faeces, a 500 mL tap water enema at 37 °C was performed until the effluent was clear. The catheter was then connected to the barostat, and the bag inflated and deflated with 100 mL of air to unfold it.

Electromechanical barostat recording assembly  A barostat assembly was used to evaluate rectal diameter and compliance. This was identical to that previously described,18 and consisted of an infinitely compliant 15 cm long polyethylene bag (Synectics Medical Ltd., Enfield, Middlesex, UK) secured proximally and distally to the end of a closed-tip double lumen polyvinyl tube (Synectics Medical Ltd.) linked to an electronic barostat device (visceral stimulator; Synectics Medical Ltd.). The maximum capacity of the bag was 1100 mL, as it was anticipated a priori that a proportion of patients with RH might have megarectum. The volume of air inside the barostat bag was determined electronically from the known excursion of the piston within the cylinder system.

Assessment of rectal diameter  Evaluation of rectal diameter was performed as previously described.20 Continuous stepwise isobaric distension was performed using a barostat with 1 mmHg increments of pressure every 60 s until the minimum distending pressure (MDP) was reached. This was defined as the threshold at which respiratory variations were clearly noted from the volume recording, signifying that there was sufficient air within the intra-rectal bag to prevent its collapse.26 An image was captured during fluoroscopic screening (Siemens, Berkshire, UK), and stored and analysed using magicview 300 image tool software (Siemens), calibrated to the known dimensions of the rectal catheter. Subsequently, rectal diameter was recorded by measuring the widest transverse diameter at 90° to the rectal lumen17,27 between the anorectal junction and the pelvic brim.20

Assessment of rectal compliance  The barostat assembly and conduct of the study were identical to that described above. Phasic isobaric distensions were performed to assess rectal pressure–volume relationships. Rapid distensions from 0 mmHg to a maximum pressure limit of 50 mmHg were performed at 2 mmHg increments. Each distension lasted 60 s and was separated by a 60 s period of bag deflation at 0 mmHg. Pressure–volume curves were constructed and rectal compliance (ΔVP, mL mmHg−1) was measured over the steep linear section of the curve.18,19,28

Assessment of rectal mucosal electrosensitivity  A 1 cm long bipolar urethral ring electrode (Dantec Ltd., Bristol, UK) mounted on a 14G Foley catheter was positioned within the mid rectum. To allow accurate placement, and avoidance of faecal matter, the stimulating electrode was inserted under vision during proctoscopic examination of the anorectum to lie in direct contact with the rectal mucosa at 10 cm from the anal verge in the 12 o’ clock position. The electrode was then connected to a constant current stimulator (Dantec Ltd.). A stimulus of 500 μs duration at 10 Hz frequency was then applied with increasing Amperage from 0 to a maximum of 60 mA at a rate of 1 mAs−1, controlled using an adjustable rheostat.22 Subjects were asked to report the onset of any sensation, which was recorded as the electrosensitivity threshold. The stimulus was switched off and then on again without the subject’s knowledge, and the subject was asked to report changes in the stimulus. The process was repeated three times in this way, and the lowest of the readings accepted as the definitive electrosensitivity threshold.22

Data and statistical analysis  Data are expressed as mean (standard deviation), unless otherwise stated. Due to the normal distribution of data, mean values for rectal diameter, compliance and afferent nerve sensitivity in each of the groups were compared using a one-way analysis of variance with Bonferroni multiple comparison test. In addition, normal ranges for each of these parameters were derived by calculating the mean ± 2SD of values obtained in the HV. Individual values for patients with constipation were then compared to the normal range, to identify those patients with elevation of rectal diameter, compliance and mucosal electrosensitivity (i.e. megarectum, excessive rectal laxity and impaired afferent nerve function respectively). The chi-square and Fisher’s exact tests were used to analyse contingency tables where appropriate. Statistical analyses were performed using a commercially available statistical software package (Prism® 3.0, GraphPad Software, Inc., San Diego, California, USA). A P value of <0.05 was considered to be statistically significant.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

No adverse events occurred during the study. The rectal catheter was inserted with ease in all cases, although rectal irrigation was necessary in 14 patients with RH (31%) and in one patient with normal sensation (NS) (5%). All subjects completed the study protocol.

Rectal diameter

Overall, the mean rectal diameter at MDP was significantly greater in constipated patients with RH compared to the other two groups (< 0.001), but similar in constipated patients with NS and HV (Table 2). The upper limit of normal for rectal diameter at MDP was 6.3 cm.20 Accordingly, rectal diameter was normal in all patients with NS but only in 15 patients (33%) with RH (Fig. 1). The remaining 30 patients (67%) with RH had an increased diameter at MDP (i.e. >6.3 cm).

Table 2.   Rectal parameters in the study groups
ParameterHealthy volunteersNSRH
  1. RH, rectal hyposensitivity; NS, normal rectal sensation; *< 0.001 vs NS and healthy volunteers (one-way anova); Values represent mean (standard deviation).

Rectal diameter MDP (cm)4.4 (0.9)4.4 (1.1)6.5 (1.3)*
Rectal compliance (mL mmHg−1)12.8 (2.6)10.9 (3.8)18.9 (5.8)*
Afferent nerve sensitivity (mA)12.3 (4.5)14.8 (3.4)27.9 (14.1)*
image

Figure 1.  Rectal diameter at minimum distension in constipated patients with RH and NS. The mean rectal diameter (horizontal lines) was significantly elevated in patients with RH (P < 0.001). Rectal diameter was normal in all patients with NS and in 15 patients (33%) with RH, being increased (megarectum) in the remaining 30 (67%). The shaded area represents the normal range. Key: RH, rectal hyposensitivity; NS, normal rectal sensation.

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Rectal compliance

The pressure–volume relationships in each of the study groups are shown in Fig. 2A. Overall, mean rectal compliance was significantly greater in constipated patients with RH compared to the other two groups (< 0.001), but similar in constipated patients with NS and HV. The upper limit of normal for rectal compliance was 17.9 mL mm Hg−1.18 Accordingly, rectal compliance was normal in only 22 patients (49%) with RH (Fig. 2B). The remaining 23 patients (51%) with RH had increased rectal compliance (i.e. excessive laxity) (Fig. 2B).

image

Figure 2.  (A) Pressure–volume relationship during phasic isobaric distension. The mean rectal compliance, taken as the slope of the steep linear part of the curve, was significantly greater in patients with RH than those with normal sensation (NS), and healthy volunteers (HV) (< 0.001). However, there were no differences in the right–left shift of the curves between the groups. (B) Rectal compliance in constipated patients with RH and NS. The mean rectal compliance (horizontal lines) was significantly elevated in patients with RH (< 0.001). However, rectal compliance was normal in 22 patients (49%) with RH. The shaded area represents the normal range. Key: RH, rectal hyposensitivity; NS, normal rectal sensation.

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Relationship between rectal compliance and diameter  All 15 patients with RH, in whom rectal diameter was normal, had normal rectal compliance. Conversely, all patients with RH and increased compliance (n = 23) had increased rectal diameter. However, rectal compliance was normal in the remaining seven patients with increased rectal diameter. Therefore, on the basis of rectal diameter and compliance, further subgroups of patients with RH were identified: (i) increased diameter alone, n = 7; (ii) increased diameter and compliance, n = 23; and (iii) normal diameter and compliance, n = 15.

Rectal mucosal electrosensitivity

Overall, the mean electrosensitivity thresholds were significantly different in the groups studied, being elevated in patients with RH compared to the other two groups (< 0.001), but similar in patients with NS and HV (Table 2). Subgroup analysis (Fig. 3) revealed that the mean electrosensitivity threshold was significantly elevated in patients with RH and normal rectal diameter and compliance [38.1 (10.1) mA] compared to all other groups (< 0.001), and in patients with RH and increased rectal diameter and compliance [24.3 (13.3) mA] compared to patients with NS and HV (< 0.01 and < 0.001 respectively). However, the mean thresholds were similar in patients with RH and increased rectal diameter alone [18.3 (9.3) mA] and patients with RH and increased rectal diameter and compliance, NS and HV (> 0.05). For individual patients, electrosensitivity thresholds were normal (3.3–21.3 mA) in all patients with NS and in 19 patients with RH (Fig. 3). Electrosensitivity thresholds were raised in the remaining 26 patients with RH.

image

Figure 3.  Rectal mucosal electrosensitivity thresholds in subgroups of patients with RH. The mean thresholds (horizontal lines) were significantly elevated in patients with RH and normal diameter and compliance compared to all other groups, and in patients with RH and increased diameter and compliance compared to patients with NS. No RH with normal diameter and compliance had normal electrosensitivity thresholds. The shaded area represents the normal range. Key: NS, normal rectal sensation; RH-incr diameter & compliance, rectal hyposensitivity with increased compliance and capacity; RH-incr diameter, rectal hyposensitivity with increased diameter alone; RH-normal diam&comp, rectal hyposensitivity with normal diameter and compliance.

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Relationship between rectal mucosal electrosensitivity and wall properties Elevated electrosensitivity thresholds (n = 26): In those patients with abnormal rectal structure and/or biomechanical function, nine of 23 (39%) of those with increased rectal diameter and compliance and two of seven (29%) of those with increased rectal diameter alone had elevated rectal electrosensitivity thresholds, suggestive of co-existent afferent fibre dysfunction. All 15 patients with normal rectal diameter and compliance had elevated electrosensitivity thresholds (Fig. 3).

Normal electrosensitivity thresholds (n = 19): Fourteen of 23 patients with increased rectal diameter and compliance (61%) and five of seven patients (71%) with increased rectal diameter alone had normal rectal electrosensitivity thresholds, suggestive of normal afferent fibre function (Fig. 3).

A summary of the results of the assessment of rectal diameter, compliance and electrosensitivity in patients with RH on balloon distension using a hand-held syringe is shown in Fig. 4. Accordingly, patients with RH could be divided into those with:

image

Figure 4.  Rectal diameter, compliance and electrosensitivity in patients with RH. Summary and relative importance of pathophysiological mechanisms in patients with RH with stratification on the basis of rectal wall properties and electrosensitivity thresholds. Key: Incr, increased.

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  • 1
    Normal rectal diameter/compliance with abnormal electrosensitivity.
  • 2
    Increased rectal diameter alone.
  • 3
    Increased rectal diameter with associated abnormal electrosensitivity.
  • 4
    Increased rectal diameter and compliance alone.
  • 5
    Increased rectal diameter and compliance with abnormal electrosensitivity.

Clinical features/risk factors in the subgroups of patients with RH  The duration of symptoms was significantly longer and presence of the sensation of a ‘call to stool’ was significantly reduced in patients with RH compared to those with NS. Having identified the above subgroups of patients with RH on the basis of underlying pathophysiology (Fig. 4), the clinical and physiological features and presence of potential risk factors were compared in patients in each of these subgroups. The prevalence of clinical symptoms, the duration of symptoms and the proportions of patients with potential risk factors for the development of nerve injury (e.g. pelvic surgery/spinal trauma etc.) and physiological abnormalities were similar in each of the subgroups of patients (Table 3).

Table 3.   Clinical and physiological features and potential risk factors in subgroups of patients with RH
 RH-diam/ comp (n = 23) RH-diam (n = 7)RH-norm diam&comp (n = 15)
  1. OD, obstructed defaecation; NS, normal rectal sensation; RH-diam/comp, rectal hyposensitivity with increased compliance and capacity; RH-diam, rectal hyposensitivity with increased capacity; RH-norm diam&comp, rectal hyposensitivity with normal rectal diameter and compliance; Values represent actual numbers (percentages). Bold values indicate the number of years that symptoms have been present.

Clinical symptoms
 Duration of symptoms (years)11 (1–58)9 (1–56)9 (1.5–60.5)
 Infrequency alone1 (4%)00
 OD10 (43%)2 (29%)5 (33%)
 Infrequency and OD12 (52%)5 (71%)10 (66%)
 Sensation of ‘call to stool’6 (26%)2 (29%)6 (40%)
 Nulliparous3 (13%)2 (29%)2 (13%)
 Parity2 (0–7)2 (0–4)2 (0–3)
 Previous pelvic surgery7 (30%)2 (29%)8 (53%)
 Previous spinal trauma2 (9%)01 (7%)
 Diabetes mellitus1 (4%)00
Physiological abnormalities
 Slow-transit alone4 (17%)1 (14%)3 (20%)
 Outlet obstruction alone6 (26%)3 (43%)5 (33%)
 Slow-transit and outlet obstruction8 (35%)1 (14%)4 (27%)
 Normal transit/evacuation5 (22%)2 (29%)3 (20%)

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

Balloon distension using a hand-held syringe is recommended for the assessment of rectal sensation in routine clinical practice,6 and thus patients with RH were identified in the current study using this technique. Recently, the barostat has been used to assess rectal sensitivity during isobaric distension, with the suggestion that measurement of sensory threshold pressures during isobaric distension may obviate the limitations of the use of threshold volumes measured during balloon distension, as identified in the present study (see below). However, changes in pressure are not the direct stimulus for rectal perception, as rectal mechanoreceptors are stimulated by forces (strains) and deformations (stresses) acting in the rectal wall. Therefore, isobaric distension is not guaranteed to adequately control for differing geometrical/biomechanical properties.29 Furthermore, the measurement of threshold pressures in the evaluation of rectal sensitivity is less practical in the clinical setting. Consequently, the barostat has no clearly defined place in clinical practice and is thus not currently recommended by consensus groups.29

As clinical symptoms may not be a reliable indicator for the identification of homogenous subgroups,30 patients were evaluated using further tests of physiological function. Importantly, only those tests in widespread clinical use were employed for this purpose, rather than those confined at the present time to the research setting. Accordingly, the present study has demonstrated heterogeneity in terms of the underlying pathophysiological mechanisms leading to the development of diminished perception of rectal distension.

Rectal diameter

The function of the rectal reservoir is dependent on its size, biomechanical properties and sensitivity. However, the impact of rectal size is rarely evaluated in clinical practice,31 despite the fact that certain patients with hindgut dysfunction have megarectum.15 Traditionally, the diagnosis of megarectum has been made during anorectal manometry14 and double-contrast barium enema.32 However, methodological limitations of these techniques mean that they over-, and underestimate the prevalence of megarectum respectively.20 Consequently, a novel technique that was not susceptible to such inaccuracies20 was thus used to evaluate rectal diameter in patients with RH in the current study.

Overall, an increase in mean rectal diameter in constipated patients with RH was observed in comparison to those with NS and HV. It is thus feasible that increased rectal diameter could result in misinterpretation of afferent nerve sensitivity using volume-based distension techniques, as greater volumes will be required to initiate distension of the rectal wall.16 However, more meaningful interpretation of results is possible when individual values are compared with the normal range.33 Accordingly, rectal diameter was found to be normal in 33% of constipated patients with RH, and thus in these patients, the increased sensory threshold volumes on balloon distension using a hand-held syringe did not relate to changes in rectal diameter. Nevertheless, this study has shown that rectal diameter is increased in a larger subgroup of patients with RH (67%), i.e. they have megarectum. Consequently, elevated sensory threshold volumes in such patients may merely reflect rectal dilatation and thus inadequate stimulation, rather than dysfunction of the afferent pathway. Rectal diameter was normal in all constipated patients with normal rectal sensation. This was to be expected, as perception of rectal distension would not have been normal if rectal diameter was increased.

Rectal compliance

Measurement of compliance is most commonly employed to assess rectal biomechanical properties in current clinical practice. Overall, an increase in mean rectal compliance was noted in constipated patients with RH compared to those with NS and HV. However, individual data revealed that one subgroup of patients with RH had normal (49%), and the other increased compliance (51%). Nevertheless, in those patients with increased compliance, i.e. excessive laxity of the rectum, the elevated sensory thresholds observed during balloon distension using a hand-held syringe may reflect abnormal rectal wall biomechanics. The observation of normal rectal compliance in the remainder supports the assumption that elevated sensory threshold volumes in this subgroup of patients reflect impaired afferent nerve function.

In the present study, all patients with elevated rectal compliance had increased rectal diameter, in keeping with previous results, and the clinical assumption that patients with increased rectal compliance have megarectum.7,14,34 However, the converse was not true, in that an increased rectal diameter did not necessarily indicate elevated compliance, as 35% of patients with RH and increased rectal diameter (i.e. megarectum) had normal compliance. Accordingly, two sub-groups of patients with megarectum may be identified on the basis of rectal compliance. Those with excessive rectal compliance may be considered as having a ‘pathophysiological’ megarectum, as the loss of resistance to distension may account for the occurrence of rectal dilatation in such patients.35 By contrast, the remainder have normal rectal compliance, and may be considered to have an ‘anatomical’ megarectum.35 Rectal compliance was normal in all subjects with normal rectal diameter, i.e. all constipated patients with RH with normal rectal diameter and HV.

Rectal electrosensitivity

Rectal sensitivity was also assessed using electrical stimulation, in an attempt to evaluate the rectal afferent fibre function by a method that is not influenced by abnormal rectal wall properties, as it relies solely on direct contact with the rectal mucosa.21 Electrostimulation induces non-specific activation of afferent pathways, does not activate mucosal receptors,36,37 and thus measures the excitability of the nerves.38 Overall, mean electrosensitivity thresholds were higher in constipated patients with RH, compared to those with NS, and HV, suggestive of impaired afferent fibre function. Comparison of individual values with the normal range revealed that 42% of patients with RH on balloon distension using a hand-held syringe had normal electrosensitivity thresholds, suggestive of normal afferent nerve function. All of these patients had abnormal rectal compliance and/or diameter. This suggests that balloon distension using a hand-held syringe is indeed unreliable in the presence of altered rectal properties, and that elevated volumes may not indicate impaired afferent nerve function, but simply inadequate stimulation of the rectum. However, electrosensitivity thresholds were normal in all constipated patients with NS, and in all patients with RH and normal rectal wall properties, and thus the concordance between these different tests of sensory function suggests that balloon distension using a hand-held syringe is a reliable method of assessing actual afferent nerve function in patients with normal rectal properties.

Clinical features in subgroups of patients with RH

In the present study, the nature and duration of clinical symptoms, the presence of potential risk factors for the development of RH and the associated anorectal physiological abnormalities were not predictive of underlying pathophysiological mechanism, although it is possible that the relatively small numbers present in each of the subgroups may explain the apparent lack of association. Nevertheless, this observation is in keeping with the notion that clinical features are not reliable in the stratification of patients with functional bowel disorders,30 and that formal objective, detailed physiological investigation is required to identify subgroups of patients with RH. Indeed, even the presence or absence of the sensation of the ‘call to stool’ was not accurate in the diagnosis of RH or NS. The relationships between each of the subgroups of RH, and whether they represent part of a spectrum of severity could not be addressed in the present study, and thus remain unclear. It is conceivable, for example, that megarectum develops secondary to a disorder of rectal sensitivity in certain cases,35 given that a proportion of patients have evidence of concomitant afferent nerve dysfunction.

Limitations

By its very design, the protocol of the present study involved several intubations and distensions of the rectum. The direct effect on rectal sensitivity has not been formally studied, although it has previously been shown that assessment of visceral sensitivity is only reproducible after a conditioning distension, following which comparable values for sensitivity are obtained during repeated distension.39 It should also be noted that the study protocol was neither randomized, nor its reproducibility formally examined, and thus it remains unknown whether an ‘order effect’ may influence the stratification of individual patients into the different subgroups. However, reproducible results have been demonstrated for each of the individual tests employed; thus there is no reason to suspect that this is not the case when they are combined with others in the present protocol.

Recently, there has been increasing recognition of the complexity of rectal properties.40 Accordingly, sophisticated techniques such as magnetic resonance imaging,41,42 impedance planimetry43 and endoluminal ultrasound imaging44 have been introduced for assessment. In addition, detailed neurophysiological testing and functional magnetic resonance imaging have allowed comprehensive assessment of visceral afferent pathways from the rectum to the central nervous system.45,46 However, such techniques are complex, have not been completely validated or applied to large numbers of subjects and are not widely available, being currently confined to the research setting. Thus, it would have been inappropriate to proceed directly to such techniques before employing tests that may be widely available in physiology laboratories and that can be utilized in everyday clinical practice, particularly as the basic pathophysiology of RH has not previously been explored.

Pathophysiological mechanisms

The observations in the present study have revealed that RH on balloon distension using a hand-held syringe may result from different mechanisms, principally, rectal afferent pathway dysfunction and/or abnormal anatomical or biomechanical properties of the rectum. In one-third of the patients with diminished perception to balloon distension, rectal wall properties (diameter and compliance) were normal and electrosensitivity thresholds elevated, supporting the assumption that impaired perception of rectal distension in such patients is likely to reflect ‘true’ impairment of the rectal afferent pathway. This may be due to abnormalities of transduction and/or transformation of stimuli by rectal mechanoreceptors, by aberrant impulse conduction or defective processing of information in the central nervous system (e.g. secondary to psychosocial disorders47).8 Additional studies are required to investigate this further.

The remaining two-third of the patients had abnormal rectal wall properties (compliance and/or diameter). Complementary tests of rectal sensory function revealed that a proportion of such patients had normal mucosal sensitivity, suggesting that impaired perception of rectal distension reflects inadequate stimulation, rather than a disorder of the afferent pathway. It remains to be determined whether these abnormal rectal wall properties reflect underlying structural defects of the rectal wall, and this also warrants evaluation in future studies. Finally, the remaining proportion of patients with abnormal rectal wall properties also had elevated electrosensitivity thresholds, suggesting that impaired perception of rectal distension is due to a combination of inadequate stimulation and dysfunction of the rectal afferent pathway.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

Balloon distension using a hand-held syringe appears satisfactory for the initial assessment of rectal sensory function, and will identify patients with elevated sensory threshold volumes, or rectal hyposensitivity (i.e. the test is sensitive). However, an abnormal balloon distension test only provides an accurate assessment of rectal afferent nerve function in patients with normal rectal properties (i.e. the test is not specific). Therefore, it is recommended that all patients with RH should undergo more detailed assessment of rectal diameter, compliance and afferent nerve sensitivity, allowing placement of an individual into one of five defined subgroups:

  • 1
    Apparent isolated disorder of the afferent pathway.
  • 2
    Increased rectal compliance and capacity (pathophysiological megarectum) ± afferent pathway disorder.
  • 3
    Increased rectal capacity (anatomical megarectum) ± afferent pathway disorder.

Whether identification of such subgroups is important in the management of patients with functional hindgut disorders remains to be determined but warrants further formal investigation.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References