There are few comparisons of conventional ARM and VVM in either health [9] or disease [10]. Work comparing conventional ARM with VVM has been conducted in normal patients and those with passive fecal incontinence, full-­thickness rectal prolapse, and chronic anal fissure, measuring the mean resting vector volume (MRVV), mean squeeze vector volume (MSVV), HPZ lengths (at rest and during squeeze), maximal averaged pressure at rest (MR), maximal averaged pressure during sustained squeeze (MPS), and the percentage asymmetry as the percent deviation of the integrated cross-sections from a perfect circle. Sectorial pressures have been pooled for analysis, creating right, left, anterior, and posterior mean pressures. As expected, significant differences at rest and during squeeze are demonstrable for MRVV, MSVV, MPR, and MPS that mirror those obtained with conventional ARM, whereas, in general, MPR values tend to exceed mean resting anal pressure, and MPS values tend to be lower, on average, than mean squeeze pressure values. It may be that during automated catheter withdrawal, there is some voluntary sphincter contraction that triggers the MPR recording to be higher than the mean resting anal pressure value obtained with conventional ARM. This also may result from perfusate leakage. The lower value of MPS over mean squeeze pressure (in conventional ARM) may reflect a greater difficulty in sustaining an adequate squeeze contraction during the vector volume technique (Table 7.1) [11].

Although there is an expected sectorial ordering from patients with incontinence through to hypertonic anal fissure, there is no evidence of inherent sectorial differences for the different anorectal conditions, although there is a trend toward higher anterior sector pressures in patients with fissure. This sectorial variation has been identified in some other studies [12–14] and is shown in Fig. 7.2a, b. The correlation coefficients between conventional and vector volumetric variables for rest and squeeze confirm a strong correlation for HPZ length measurement with both techniques (see Table 7.1). There is, however, no correlation between sectorial asymmetry and demonstrable EAS defects [15] or in patients after internal anal sphincter (IAS) division for chronic anal fissure [16], although recent data has suggested that VVM may assist in defining those cases with an EAS defect in the last centimeter of the anal canal [17, 18]. It would seem, however, that when initial ultrasound is inconclusive in the diagnosis of a reparable sphincter defect, an ultrasonographically defined use of VVM would somewhat defeat its purpose [19, 20]. Further recent data have shown significant differences in all vector resting parameters, HPZ length at rest, and percentage asymmetry at rest, along with changes in most squeeze variables, after IAS division for topically resistant chronic anal fissure [16], with marked differences between postoperative continent and incontinent cohorts, particularly in resting HPZ length. This latter finding may suggest an overly zealous internal anal sphincterotomy that has been previously endosonographically recorded [21] and during which the extent of IAS division often can be far greater than intended. In continent postoperative patients, percentage resting asymmetry tends to increase (by about 6.7 %), whereas in incontinent postoperative cases it tends to fall (by about 3.1 %). The explanations for these changes in squeeze parameters are not understood, but it is conceivable that there is excessive voluntary sphincter fatigue after surgery between groups (even in continent cohorts), where impending leakage occurs because contents entering the anal canal after rectal motor activity are poorly discriminated. This latter phenomenon, known as “anorectal sampling,” permitting the distinction between flatus and feces, is discussed in the section in Chap. 6 on the rectoanal inhibitory reflex and is believed to represent one of the functions of the IAS [22]. It also may be that in some patients there is a constitutively deficient subcutaneous, overlapping segment of the EAS (as has been shown using endoanal magnetic resonance imaging preoperatively in some patients with fissure [23]) so that distal IAS division will render the distal anal canal unsupported and lead to incontinence and possible attendant weakness in postoperative squeeze function [24]. At this time, the role of VVM must still be regarded as experimental, although it has provided an interesting tool for the study of sectorial sphincter asymmetry. The equipment and software is expensive and not widely available, but as a manometric instrument, VVM has been validated sufficiently [25]. It is conceivable that, with its use in prospective trials of patients with fissure, it could identify those patients who are likely to function poorly after IAS division and assist in the decision making for sphincter-sparing surgical alternatives [26, 27]. Further specific parameter assessment may delineate subtle dysfunctions that may predictably respond to directed biofeedback therapies in some forms of incontinence after anorectal surgery and that may better identify patients more suited to neorectal reservoir reconstruction or who are precluded from perineal rectosigmoidectomy for rectal prolapse [28–30].