The equipment required for this test consists of four components: a probe, a pressure recording device, record-displaying device and a data storage facility (Fig. 11.1). Two types of probes are used: a solid-state probe with strain gauge transducers and a water-perfused probe. Technique details and equipment configurations significantly vary and examples have been published by various groups [5, 6]. Manometry is used to assess the resting and squeeze anal pressures, the cough reflex and the rectoanal inhibitory reflex (RAIR) (Fig. 11.2). Rectoanal pressure changes during simulated defecation can be also assessed in certain cases.

Bowel preparation is not usually required but if digital rectal examination reveals faecal loading a preparatory rectal enema should be given. The patient is placed in the left lateral position with 90° flexion at the hips and knees. The procedure is preceded by digital rectal examination and the lubricated manometry probe is then gently inserted into the rectum. After a brief rest period to allow the sphincter tone to return to basal levels, measurements are taken during station pull through; the probe is gradually withdrawn in 0.5 or 1 cm steps and the recordings are taken when the transducer is stationery to avoid artefacts induced by reflex sphincter contraction during a rapid pull through procedure.

Resting anal pressure is defined as the difference between the intrarectal pressure and the maximum anal pressure at rest [5]. Pressures should be measured by averaging all four quadrants to account for sphincter asymmetry. Squeeze pressure is obtained by asking the patient to contract the sphincter for at least 30 seconds and to average the pressure measurement over this period [5]. The time interval in seconds during which the subject can maintain a squeeze pressure at or above 50 % of the maximum squeeze pressure represents the duration of sustained squeeze. The reflex increase in anal pressure during abrupt change in intra-abdominal pressure is tested by asking the patient to cough (cough reflex test) [7]. This protective reflex is thought to be mediated through a spinal reflex arc.

Studies have demonstrated high reproducibility of anal pressure measurements in the same subject on separate days [6]. Anal pressures are influenced by age and gender [7–9]; men have higher mean resting and squeeze pressures [10, 11] and pressures decline after the age of 60 years [10, 12]. There is relative lack of age- and sex-stratified normative data, and it is suggested that individual centres establish their own [5, 8]. Moreover, consideration of the different methods used to obtain the pressure measurements is essential when comparing the results of different studies.

The resting anal pressure predominantly represents the internal anal sphincter function and the voluntary squeeze pressure predominantly represents the external sphincter function. Normally, the high pressure zone which is the part of the anal canal where the resting pressure is >50 % of the maximum resting pressure is longer in males than females [13, 14]. In a normal individual the rise in pressure on maximal squeeze should be at least 50–100 % of the resting pressure [15].

Anal pressures are usually reduced in faecal incontinence [16–18]; however, in cases where aetiology does not involve sphincter malfunction (e.g. diarrhoea, disturbances of rectal sensations or compliance), anal pressures can be within normal range. Moreover, in a significant proportion of asymptomatic normal individuals, pressures were found to be low [19].

Differential reduction in anal pressures could reflect the site of deficit; a relatively maintained squeeze pressure with low resting pressure reflects a deficit in IAS function and vice versa. Reduced resting pressure is associated with passive leakage, whilst reduced squeeze pressure is associated with inability to delay rectal evacuation [20]. The loss of the normal cough reflex together with the voluntary squeeze function denotes a cauda equina lesion or a lesion in the sacral plexus, whereas in patients with spinal cord lesions above the conus medullaris, this reflex is present but the voluntary squeeze may be absent [21, 22].

Treatment of faecal incontinence cannot be monitored by changes in anal pressures. In some studies, anal pressures increased after biofeedback therapy; however, the magnitude of change was small and did not correlate with symptom improvement [23]. Similarly, changes in anal pressures following successful treatment with sacral nerve stimulation have been inconsistent [24].

This involves the use of micromanometers mounted on a catheter to simultaneously record rectal and anal canal pressures in a continuous manner [25, 26]. Although it provides information on the functioning sphincter in a more physiological situation in comparison with the static manometry, its use is primarily employed in the research setting.

This technique utilises a special device to automatically withdraw an eight-channel manometry catheter from the anal canal during rest and squeeze; and with the use of computer software, a three-dimensional reconstruction of the anal canal is then produced [27]. Vectrometry can be used to differentiate between global sphincter atrophy or weakness and traumatic sphincter injury by demonstrating global weakness or significant asymmetry indicating a localised sphincter abnormality respectively [27, 28]. However, poor correlation between the asymmetry on the vectogram and the electromyographic or ultrasound localisation of sphincter defects was demonstrated in some studies [29] and endoanal ultrasound remains the gold standard for identifying anal sphincter defects [30]. Nevertheless, a recent review suggests that vectrometry results correlate well with imaging and that clinical utility can be improved with the standardisation of technique and equipment [31].

This new approach utilises high-resolution techniques in combination with novel rapid data-interpretation software to allow the interpolation of manometric recordings into highly detailed topographical plots of the anorectal intraluminal pressure events relative to time [32]. Results of this technique seem to be well collaborated with conventional measurements and further studies are being conducted to establish its place in routine assessment.

This is assessed using a bipolar electrode catheter and pulse generator similar to those used for anal sensory testing. However, the stimulus used is modified to the frequency of 10 Hz and the duration of 500 μs. The threshold is determined by gradually increasing the stimulus (up to a maximum of 50 mA) until the subject reports perception of lower abdominal discomfort or colicky sensation [46, 55].

Normal electric sensory thresholds in the rectum range widely between 2.9 and 53.0 mA and is usually higher in females in comparison with males [56]. Patients with neurogenic incontinence have impaired sensation to electrical stimulation [49]. In a study of 68 subjects, Meagher et al. demonstrated circumferential variation of rectal electric sensory threshold suggesting that it is the pelvic floor rather than rectal mucosa which is stimulated with this testing modality [57].

Rectal distension thresholds can be tested either by manually distending a latex rectal balloon or by distending a polyethylene rectal balloon using a barostat device [30, 55]. The use of the barostat permits more controlled distensions and therefore provides more physiological and less rater-dependent assessment. Moreover, it enables the measurement of changes in both pressure and volume on distension, therefore permitting assessment of pressure-volume relationships and rectal compliance [58].

The distension thresholds assessed clinically include [7, 9, 59–61] (a) first sensation (FS) (first awareness of the intrarectal balloon), (b) desire to defecate (DD), (c) urgency to defecate (U) and (d) maximally tolerated volume (MTV) (the volume which can be tolerated in total before pain occurs).

Sensory thresholds to rectal distension in asymptomatic subjects vary significantly between different centres [7, 9, 59–61]. The pattern of balloon distension [62] as well as age of the subject [9] affects rectal perception. The large inter- and intra-subject variation in values and the wide normal range impact the clinical value of these measurements [63]. Moreover, thresholds for rectal distension may be elevated, normal or reduced in faecal incontinence [16, 17]. Nevertheless, evidence of loss or significant reduction of rectal perception to balloon distension (hindgut denervation) and conversely the evidence of exaggerated rectal sensation (rectal hypersensitivity and/or reduced rectal capacity) are important findings when assessing faecal incontinence. Impaired rectal sensation can be associated with symptoms of incontinence [64, 65], and there is evidence that sensory retraining can improve symptoms [65, 66]. On another hand, some patients with urge faecal incontinence have significantly reduced sensory thresholds to rectal distension; this rectal hypersensitivity has been found to be associated with reduced rectal compliance and exaggerated rectosigmoid motor activity highlighting a specific entity of incontinent patients [17, 67].