Anal resting tone and the functional length of anal canal

During the first phase of anorectal manometry, the extraction of the probe manually in 1 cm steps (stationary pull-through technique) or at constant speed using an automatic extractor arm (motor pull-through technique) allows to evaluate the functional length of the anal canal and the anal resting tone.

The functional length of anal canal (high pressure zone, HPZ) is defined as a region (or length) over which resting pressures are ≥30% higher than rectal pressure [27].

We can calculate the mean resting anal pressure since it is the average of all the pressures detected in the HPZ and, the maximal resting anal pressure, defined as the difference between intrarectal pressure and the highest recorded anal sphincter pressure at rest, generally recorded 1–2 cm from the anal verge. Physiologically the anal resting tone is predominantly due to internal anal sphincter (IAS) activity (55–80%, most due to nerve-activity and the remainder purely myogenic) [28], expression of an involuntary function, and to a lesser extent external anal sphincter (30%) and hemorrhoidal pads (15%). Resting sphincter pressure varies according to age, sex, and techniques used. Usually, pressures are higher in men and younger subjects, but with considerable overlap [18, 29, 30].

According to perfused catheter anorectal manometry, the recorded anal canal is often asymmetric. In the proximal anal canal, anterior quadrant pressures are lower than the other quadrants at rest while distally, posterior quadrant pressures are reduced, and in the mid anal canal radial pressures are generally equal in all quadrants [26, 29, 31]. Furthermore, conventional anorectal manometry allows to obtain, through a specific software, the two-dimensional reconstruction of the pressure profile of the anal canal (vector volume) with a detailed evaluation of the pressure asymmetries caused by possible sphincter anatomic pathologies. However, these data are today better obtained through three-dimensional sphincter ultrasound [32].

Maximal squeeze pressure and maximal squeeze duration

During the second phase of anorectal manometry patients were asked to squeeze the anus as hard as possible, avoiding contracting the accessory muscles and, in particular, limiting gluteal muscle involvement. Moreover, the squeeze should be maintained for 30 s, to obtain a measure of fatigability of the external anal sphincter (EAS) [17]; during the squeeze maneuver, the maximal voluntary pressure is recorded at each station to detect appropriate external sphincter activity.

The maximal squeeze pressure is measured by evaluating the difference between the pressure increments during a maximal voluntary contraction and the basal resting tone at the same level of the anal canal [8, 17, 27].

The sphincter endurance is the time interval at which the patient is able to maintain a squeeze pressure above the resting pressure, in particular greater than or equal to 50% of the maximum squeeze recorded pressure [17, 27, 33].

Both of these measurements primarily reflect the strength and fatigability of the EAS [11, 19, 33, 34].

The assessment of rectal sensibility and rectal compliance

Testing rectal sensitivity is generally performed with a balloon distention, positioned in the rectum, filled (manually using a hand-held syringe or pump-assisted) with air or water. It is able to record intraballoon pressure expression of rectal pressure and distending volumes by means of incorporating water-perfused catheters or microtransducers. During the test, patient is instructed to report the first sensation that is the minimum rectal volume perceived by the patient, desire to defecate, urgency that is the volume associated with the initial urge to defecate, maximum tolerated volume that is the volume at which the patient experiences discomfort and an intense desire to defecate, and pain. These sensory thresholds are recorded (through the distending volume or less frequently the pressure) [3, 8, 35].

This assessment allows also to calculate rectal compliance from the derived pressure–volume curve: it is defined as the “volume response to an imposed pressure,” and represents the change in rectal pressure in response to changes in rectal volume (change in volume divided by change in pressure = ΔV/ΔP). In response to distention, the rectal wall is able to have an “adaptive relaxation” at the beginning due to its viscoelastic properties and this allows accommodation of significant increases in volume despite low intraluminal pressures, so that continence is guaranteed; continuing distention the rectum becomes more resistant to stretch until the elastic limit is reached and regular contractions start, causing an increase of intrarectal pressure [36, 37].

Despite large variation, in literature there is a high degree of reproducibility about recording sensory thresholds [38, 39], and many consensus statements and technical reviews have attested that this test is useful in the assessment of functional intestinal disorders [16, 18, 34].

Another test to get rectal sensitivity makes use of an electronic barostat. Briefly, the barostat maintains a constant pressure on the inside of a bag containing air by means of feedback. The feedback mechanism consists of a strain gauge connected to an injection/aspiration system by means of a relay. Both the strain gauge and the injection/aspiration system are independently connected by a double-lumen polyvinyl tube, one lumen is used for inflation, the other for monitoring pressure, to a non-elastic, oversized, polyethylene ultrathin bag and so very compliant to avoid any influence on internal pressure. A dial allows the selection of the desired pressure level. Pressure and volume within the bag are continuously recorded [40–42]. Measurement of rectal compliance and capacity using the barostat are more specific than those using balloon, considering that this last needs correction because of its intrinsic elasticity. Although barostat is less available, it is advisable to consider it in patients with alterations of rectal sensation already assessed by balloon distention and/or with a strong suspicion of abnormal rectal compliance or capacity [17, 33].

The assessment of attempted defecation.

In patients with symptoms of disordered evacuation, the manometric assessment of rectoanal coordination during defecatory maneuvers can help in the diagnosis.

During this part of anorectal testing, the patient is asked to strain or bear down, as during defecation, while pressures of anus and rectum are detected simultaneously; normally an increase in intrarectal pressure is detected, due to the Valsava maneuver, associated with a decrease in intraanal pressure, due to coordinated relaxation of the EAS; these mechanisms facilitate the process of defecation, allowing propulsive forces to drive stool easier through the anal canal with learned response under voluntary control [20, 21].

When defecation is impaired during ARM is possible to observe inadequate rectal propulsive force and/or inadequate relaxation or paradoxical anal contraction [20, 43–45].

Specifically, four patterns of pressure changes seen in the rectum and anus during attempting defecation have been described [33, 44, 46].

Type 1: increase of rectal propulsive pressure (rise in intraabdominal pressure with generation of an adequate pushing force) with paradoxical increase of anal pressure as well.

Type 2: inadequate rectal propulsive pressure (no increase in intrarectal pressure) with paradoxical anal contraction.

Type 3: adequate rectal propulsive pressure (increase in intrarectal pressure) with absent or incomplete anal relaxation (≤20%) (i.e., no decrease in anal sphincter pressure).

Type 4: inadequate rectal propulsive pressure and absent or incomplete anal sphincter relaxation (≤20%) (Fig. 3.3) [44, 46].

Unfortunately, some of that abnormal manometric patterns (for example an abnormal reduced rectoanal pressure gradient) during simulated evacuation are found in more than 50% of the asymptomatic subjects and, therefore, the diagnosis of functional defecation disorders cannot rely only on anorectal manometry [2, 8, 33, 44].

According to Rome III criteria, diagnosis of functional defecation disorders is possible in presence of (1) constipation symptoms, (2) the presence of inadequate rectal propulsive force and/or inadequate relaxation or paradoxical anal contraction at ARM (or electromyography), and (3) at least another positive test among balloon expulsion test or impaired rectal evacuation by imaging [47].

The new Rome IV diagnostic criteria for functional defecation disorders (Table 3.2) incorporates also IBS with constipation patients [48]. In addition, the diagnosis of dyssynergic defecation has been limited to the finding of paradoxical anal contraction at either ARM or pelvic floor electromyography.