Representative HRAM pressure topography plots of squeeze during standard (a) vs enhanced (b) instruction and verbal feedback, demonstrating increased pressure and prolongation of squeeze duration (black arrow) (reproduced from Heinrich et al. [6])
5.1.1 Sphincter Resting Pressure
Resting pressure is the result of the activity of the internal anal sphincter (IAS) and the external anal sphincter (EAS). Anal resting pressure is not uniform over the longitudinal extent of the anal canal. Conventional ARM catheters have a limited number of unidirectional sensors (up to eight) which often do not measure pressures over the entire length of the anal canal at the same time; moreover the measurement of resting pressure may be influenced by the ultraslow wave cycling activity [7].
5.1.2 Squeeze Pressure
The squeeze anal pressure measures voluntary contraction of the EAS. The squeeze pressure is lower in women than in men and lower in older than in younger people. Because ARM cannot assess contractile symmetry, it is not useful for identifying contraction of the puborectalis muscle, which only generates forces on the posterior side of the anorectal region; thus it is not able to assess if possible pressure changes are due to EAS or to a puborectalis muscle injury.
5.1.3 Straining Maneuver
During simulated evacuation, patients are asked to expel the manometric probe, typically with the balloon empty and less frequently with the balloon inflated with low air volumes. The assessment of pressure changes during simulated evacuation is limited by the type of recording catheter, the distension of the intrarectal balloon, the body position, the possible displacement of the catheter, and the degree of voluntary participation, because some people find it embarrassing to defecate in the laboratory without the necessary privacy. Finally, about 20% of asymptomatic healthy people undergoing ARM have manometric abnormalities characterizing a straining disorder [8].
5.1.4 Rectoanal Inhibitory Reflex (RAIR)
Rapid rectal distension by inflating the intrarectal balloon elicits an intrinsic reflex, mediated by the myenteric plexus, that relaxes the IAS. The absence of the intrinsic reflex during the rapid rectal distension is typical of the Hirschsprung disease so ARM proved to be a reliable and minimally invasive technique for the diagnosis of this disturbance.
In patients with acquired megarectum, RAIR may be absent because the rectal balloon does not adequately distend the rectum: in this case higher inflation volumes are able to elicit RAIR and therefore should be used in order to distinguish acquired megarectum from Hirschsprung disease.
ARM can also have a role to evaluate the persistance of sympotoms after surgery of Hirschsprung disease, although often it does not give enough information for understanding the cause of a possible persistence of obstructive symptoms [9].
5.1.5 Rectal Compliance and Sensation
Assessing rectal sensation involves the measurement of the volume able to evoke the so-called “first sensation” and subsequently urgency and maximum tolerable volume. The rectal balloons supplied with ARM catheters are usually relatively stiff and moreover their stiffness can vary over time in case of multiuse catheters which are cleaned and reused. For these reasons, rectal compliance and pressure thresholds for rectal sensation sometimes cannot be reliably measured with ARM. Particularly rectal compliance can be reliably assessed only using the barostat which is provided with a long infinitely compliant polyethylene bag [10].
5.1.5.1 Conventional ARM Versus High Resolution Anorectal Manometry
The introduction of 2D high resolution anorectal manometry (HRAM) system, acquiring measurements from at least ten closely spaced pressure sensors across the anal sphincter, removes the need for a pull-through procedure and provides visual feedback to the operator allowing maintenance of a stable catheter position. Both HRAM and 3D high resolution anorectal manometry (HDAM) offer a standardized technique during the examination, evaluating the same parameters for every patient. Unfortunately, we are still far from having a “Chicago classification” for HRAM/HDAM, due to the lack of reliable normal values able to give a real homogeneity to the anorectal manometric reports and making them easily comparable.
Jones et al. [11] reported that HRAM values are highly correlated with water-perfused manometry measurements. In 29 patients resting, squeeze, and relaxation pressures were simultaneously recorded showing the two methods were significantly correlated although anal sphincter pressures recorded by HRAM tended to be higher than those recorded with conventional water-perfused ARM. Furthermore, HRAM provided greater resolution of the intraluminal pressure.
Ambartsumyan et al. studied 30 children with constipation showing that HDAM, compared to ARM, allowed to distinguish the individual contribution of each component of the intra-anal pressure [12]. In addition to these findings, HDAM could have the ability to better detect the normal asymmetry of pressures within the anal canal, with higher pressures in the posterior proximal and anterior distal regions of the sphincter.
A more recent study [13] performed in 14 patients showed that the ARM and HRAM were similar in misuring resting and squeezing pressures. It confeme that the measurement time for HRAM was significantly shorter than the one for conventional water-perfused ARM. Furthermore, some evidence support the hypothesis that pelvic floor abnormalities, not previously identified by conventional ARM, can be detected using HRAM.
5.1.5.2 HDAM Versus HRAM
HDAM utilizes a rigid probe made by 256 pressures sensors arranged in a 16 × 16 grid (i.e., 16 rows spaced 4 mm apart, each containing 16 circumferentially oriented sensors 2.1 mm apart) with an active area of measurement of 6.4 cm. This technology defines the anatomical anal morphology more precisely than HRAM. Manometric data undergo linear interpolation through dedicated software which displays 2D or 3D cylindrical topographical models of the anal canal which can be rotated and viewed from all sides.
HDAM images. Normal (a) and absent puborectalis tone (b) at rest. Normal squeeze (c) and focal defect at squeeze (d). Normal bear down (e) and paradoxical contraction (f) on bear down (reproduced from Raja et al. [14])
2 × 2 Table for concordance between 2D and 3D diagnosis of dyssynergia. The + sign indicates the presence of dyssynergia respectively with 2D and 3D analysis (reproduced from Raja et al. [14])
5.2 Clinical Meaning of HRAM/HDAM
Up to now, the principal indications of HRAM and HDAM are the same of conventional ARM: e.g., the diagnostic workup of FI, chronic constipation, and Hirschsprung disease. They may be also used to improve the results of the pelvic rehabilitation training, assessing patients before the therapy, and/or objectively evaluating them when the rehabilitation course is completed.
5.2.1 Fecal Incontinence
FI is defined as the recurrent uncontrolled passage of fecal material for at least 3 months and is reported to affect 5–10% of the general population, affecting the quality of life and often leading to surgery [15, 16].
There is general agreement that the anal sphincter mechanism is the most important barrier against leakage of rectal contents [17].
Recent studies showed that anal resting and squeeze pressures measured with ARM and HDAM were lower in incontinent patients than in healthy persons.
Mion et al. conducted a prospective multicenter study in three groups of subjects: healthy asymptomatic controls, patients with FI, and patients with chronic constipation (CC) to evaluate how HDAM could differentiate patients with FI or CC from asymptomatic subjects. To distinguish FI from asymptomatic women, the two most important discriminant variables were: squeeze pressure (AUC of ROC: 0.786) and maximal squeeze pressure (AUC of ROC: 0.777) [18]. Push maneuver results were similar in the three groups, except for the nadir anal pressure that was significantly lower in FI women. Rectal constant defecatory sensation and maximum tolerable volumes were significantly lower in the FI women, compared to asymptomatic and CC women.
HDAM analysis of 24 asymptomatic healthy subjects and 24 patients with FI symptoms was performed; the authors developed and evaluated a robust prediction model to distinguish patients with FI from controls using linear discriminant, quadratic discriminant, and logistic regression analyses. FI severity index scores correlated with low resting pressure (r = 0.34) and peak squeeze pressure of the anal canal (r = 0.28). The combination of pressure values, anal sphincter area, and reflective symmetry values differentiated FI patients and controls with good accuracy (AUC: 0.96) [13]. Since the anal canal pressure is not symmetric along its length and circumference [19] and HDAM is able to better detect the length and the asymmetry of anal canal pressure [20–22], it appears particularly suitable for studying FI patients.
Finally, in a recent study on healthy women and women with FI, the use of a newly developed parameter, the HRAM contractile integral, increased the sensitivity of detection of anal hypocontractility, from 32% to 55%, compared with ARM measurements of squeeze [23].
5.2.2 Chronic Constipation
CC is a polysymptomatic, multifactorial disorder affecting 15–20% of the general population. It is characterized by symptoms of difficult, infrequent, or incomplete defecation. Lumpy or hard stools, sensation of anorectal obstruction/blockage, and manual maneuvers to facilitate the defecation are frequently reported [24].
A consistent number of patients with CC and irritable bowel syndrome with constipation also report symptoms suggestive of a functional defecation disorder (FDD) [25, 26], which is characterized by a paradoxical contraction or an inadequate relaxation of the pelvic floor muscles and/or inadequate propulsive forces during attempted defecation [27]. From a clinical point of view, FDD is frequently associated with excessive straining, feeling of incomplete evacuation, and digital facilitation of bowel movements [28]. However, symptoms do not consistently identify patients with FDD [29, 30]. Thus, the criteria for FDD must rely on both symptoms and physiological testing. Indeed, to diagnose FDD the Rome IV criteria require features of impaired evacuation in at least two of the following tests: anorectal manometry, rectal balloon expulsion test, barium or magnetic resonance (MR), defecography, and anal surface electromyography [27].
Manometric criteria for FDD include impaired anal relaxation, failure to increase rectal pressure, and a negative rectoanal gradient (i.e., rectal pressure lower than anal pressure) during simulated evacuation. However, Mion et al. [18] observed that many asymptomatic healthy people have a negative rectoanal gradient during evacuation, perhaps due to the left lateral position of the subjects during the procedure. Moreover, unlike normal defecation, during anorectal manometry the urge to defecate induced by rectal distention is not preceded by a normal predefecatory motor pattern associated with anal relaxation. Furthermore, patients may not completely understand the instructions provided during the test or may not be keen to accomplish the task [6, 31, 32].
In type I the patient can generate adequate propulsive forces (rise in intrarectal pressure ≥40 mmHg) along with paradoxical increase in anal sphincter pressure.
In type II the patient is unable to generate adequate propulsive forces; additionally there is paradoxical anal contraction.
In type III the patient can generate adequate propulsive forces, but there is either absent relaxation or inadequate (≤20%) relaxation of anal sphincter.
In type IV the patient is unable to generate adequate propulsive forces together with an absent or inadequate (≤20%) relaxation of anal sphincter.
The four types (I–IV) of dyssynergic defecation patterns described in the text are shown using conventional manometry (lines) and HRAM (color topographic plots) (reproduced from Lee et al. [29])
The three defecatory subtypes based on principal components analysis: (a) high anal, (b) hybrid, and (c) low rectal phenotype (reproduced from Ratuapli et al. [30])
However, several questions exist about the use and the ability of anorectal manometry to diagnose FDD and identify clinical phenotypes: indeed the utility of a negative rectoanal pressure gradient as a marker of FDD is unclear because the gradient values overlap considerably among healthy subjects and constipated patients with and without FDD [33–35].
Another interesting matter of debate is the potential use of HRAM/HDAM in the differential diagnosis between functional and structural abnormalities. A total of 188 consecutive patients with obstructive defecation underwent a full investigation consisting in HRAM and defeco-MR. Compared with patients with dyssynergia on MR imaging, patients with structural pathology, such as rectocele and rectal prolapse, had lower resting and squeeze pressures but a higher rectoanal pressure gradient on HRAM. HRAM diagnostic accuracy for dyssynergia was 82% compared with 77% MR. Interobserver agreement was substantial for HRAM diagnoses. If the data will be confirmed by other studies, these manometric patterns could play a predictive role in identifying patients needing a defecographic study [36].
5.2.3 Hirschsprung Disease
Hirschsprung disease is characterized by the absence of ganglion cells in the myenteric and submucosal plexus on rectal biopsy.
The absence of the RAIR is known to be a pathognomonic feature of the disease. The absence of RAIR can be explained by the abnormality of the polysynaptic interneurons in the IAS and of the nitrergic inhibitory neurons [37].
The diagnosis is based on the combination of clinical symptoms and results from barium enema, anorectal manometry, and rectal suction biopsy with staining for calretinin or acetylcholinesterase [36–38].
Anorectal manometry has been proved to be a reliable and minimally invasive diagnostic technique: it is a simple screening test in patients with a clinical suspicion of Hirsprung disease. Its most important aim is the differential diagnosis between acquired megacolon and Hirschsprung disease, especially in the ultra-short form of the latter condition.
In infants and children, an absent RAIR has a sensitivity of 91% and a specificity of 94% for the diagnosis of Hirschsprung disease [39]. These figures are slightly but not significantly lower than rectal suction biopsy. When RAIR is present, it excludes an Hirschsprung disease diagnosis.
HRAM is an effective and safe method for the diagnosis in newborns as demonstrated by Tang, who reported a sensitivity of 89% and a specificity of 83% [40].
Wu et al. performed ARM in a group of 24 infants (eight with Hirschsprung disease and 16 without) and HRAM in a group of 21 infants (nine with Hirschsprung disease and 12 without). The authors assessed RAIR adequacy by calculating the sphincter relaxation integral (ASRI) during the HRAM study at pressure cutoff <10, <15, and <20 mmHg (ASRI10, ASRI15, and ASRI20) and investigated their diagnostic utility. They concluded that ASRI10 may be an indicative cutoff for the adequacy of RAIR in infants [41].
Many children with Hirschsprung disease have good surgical results; however, unfortunately, some patients continue to have persistent bowel dysfunction such as constipation and intestinal motility disturbance. The postoperative anorectal manometric evaluation of the patients after surgery provides detailed information about the function of anal canal and rectum. Demirbag et al. evaluated with ARM 18 children after surgery and found an absent RAIR in 14 (77.7%) and an abnormal RAIR in 4 (22.2%). They concluded that the majority of the patients have impaired anorectal motility after surgery but the manometric evaluation did not provide enough information in understanding the causes of symptoms. It is hoped that the new HRAM/HDAM techniques will help to solve this important issue [9].
5.2.4 Pelvic Floor Rehabilitation
Pelvic floor retraining is frequently recommended for defecation disorders. However, the lack of patient’s selections and the lack of homogeneity of rehabilitation methods and protocols jeopardize the results causing difficulty in evaluation outcomes [42].
Jodorkovsky et al. retrospectively reviewed 203 patients, who had previously undergone HRAM, in whom manometric results were used for recommending biofeedback as treatment strategy. Biofeedback was ultimately recommended in 119 (58%) patients (80 with CC, 27 with FI, 9 with a combination of CC and FI, and 3 with rectal pain), of whom only 51 actually received therapy. 38 out of 51 underwent at least five sessions of biofeedback, with real life outcome success reported in 66% [43].
Soubra et al. performed HRAM on 25 patients awaiting biofeedback for dyssynergic defecation previously diagnosed through ARM. HRAM pressures tended to be higher than conventional ARM. Although there was high consensus regarding diagnosis of dyssynergia, there was low correlation regarding pattern types. For these reasons, the authors concluded that new diagnostic pressure criteria should be adopted in centers converting to HRAM [44].
5.3 HRAM/HDAM: Potentialities and Perspectives
HRAM and HDAM offer the possibility to have a standardized technique for performing the exam. Moreover, new parameters have been recently studied and developed both in HRAM and HDAM and are being considered for a future introduction in clinical practice.
Without any doubt, the most important gain over conventional ARM is the better capability in studying and understanding the functional anatomy of the sphincter since the distribution of the pressures in the anal canal and the possible asymmetry on the axial and on the circumferential plane are clearly shown [45].
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