There are various methods of measuring the pressure exerted by the anal sphincters including water-perfused, solid-state and microballoon systems.

Water-perfused systems use a 4–5 mm diameter disposable catheter with multiple narrow channels (<0.5 mm) running longitudinally along its length. The flow of water is driven by pressure of 1,000 mBar. The rate of flow through the catheter is limited by a flow resistor to give a flow rate of 0.15–0.5 ml/min, depending on the number of channels used. The pressure exerted on the opening of the channel is transmitted via the column of water to a pressure transducer in the central control unit. Higher flow rates provide a greater response rate (the maximum increase in pressure per unit time) but deliver more water into the rectum which can be troublesome in long studies and in systems with a large number of channels.

Solid-state systems have pressure transducers within the catheter. They are generally more expensive than water-perfused systems. As the catheter is not disposable and contains the pressure transducers, they are potentially less robust. They have a greater response rate than water-perfused systems.

Microballoon systems transmit pressure within a small intra-anal balloon to a remote pressure transducer via a closed column of water. This gives an overall measure of anal canal pressure rather than at specific points. It cannot therefore provide a measure of radial symmetry.

The channel openings in water-perfused catheters and the pressure transducers in solid-state catheters can be placed in different configurations. Typically they are arranged radially with four to eight points of measurement. They can also be arranged in a spiral configuration allowing simultaneous pressure measurements at different levels of the anal canal.

When using a catheter with radially arranged pressure transducers, the pressure along the length of the canal is measured by either station pull through or continuous pull through. Station pull through involves inserting the catheter to 6 cm from the anal verge and withdrawing the catheter at 5–10 mm intervals and measuring for 1–5 min at each ‘station’. Continuous pull through involves withdrawing the catheter at a set speed by hand or by a mechanical puller. Continuous pull through technique results in higher pressures possibly through movement-induced recruitment of the external anal sphincter or simply movement artefact.

Pressures are typically measured during rest, squeeze, endurance squeeze, cough and distension of a rectal balloon to assess the rectoanal inhibitory reflex. In addition for patients with suspected animus, anal canal pressures can be measured during straining.

The pressure is calculated for all levels of the anal canal during the resting period. If a radially arranged catheter is used, the mean pressure is used. The highest intra-anal pressure is the resting pressure and is expressed as cmH₂O or mmHg relative to atmospheric pressure. The functional length of the anal canal is calculated by the point (either station or distance depending on whether station pull through or continuous pull through is used) that the pressure rises significantly above rectal pressure. There is significant longitudinal variation in anal canal pressure, and the region of the anal canal that has a pressure above 50 % of the maximum pressure is defined as the high-pressure zone. There is also significant radial variation in anal canal pressures. Proximally the pressure is higher posteriorly owing to the action of the puborectalis sling. In the mid anal canal and high-pressure zone, the pressures are at their most symmetrical. Distally the pressures are higher anteriorly resulting from the external sphincter tonal activity. Within the limitations specified above, the normal range for resting pressure is approximately 60–120 cmH₂O.

The smooth muscle of the internal anal sphincter (IAS) and the striated muscle of the external anal sphincter (EAS) overlap and surround the anal canal where the contraction of the individual muscles summate to create the positive intra-anal pressure. The IAS exhibits continuous tonic activity and is responsible for 55–85 % of the resting anal canal pressure [29–31]. Its contribution to resting tone is variable along the length of the anal canal with the proximal two thirds being more reliant on IAS tone to maintain adequate resting pressures. Structurally the IAS is the anal canal’s equivalent of the circular muscle in the rectum. The IAS is 1–1.5 cm long with its caudal margin demarcated anatomically by the intersphincteric groove, beyond which the EAS continues in isolation. The EAS has constant tonic activity contributing to the resting anal canal pressure [32]. The tonic activity within the EAS disappears in spinal injury patients without significant afferent input from the anal canal [33]. The tonic activity within the EAS therefore relies upon input from supraspinal centres. Histologically the EAS is composed of largely type 1 muscle fibres characteristic of muscles with constant tonic activity capable of sustained contraction utilising oxidative metabolism.

The pressure generated by the resting tone of the IAS maintains a positive anorectal pressure gradient. The extent to which the IAS is needed to generate the resting pressure is not known, as internal anal sphincterotomies can be performed for anal fissures with usually only minor disturbances in continence, providing a full-length division of the sphincter is avoided. The interactions between the vascular anal cushions and the IAS may well be vital in the fine control of continence. The IAS cannot close the anal canal completely and requires these anal cushions to fill a gap of at least 7–8 mm in diameter [34].

The tonic activity within the IAS is variable and slow, and ultraslow waves have been identified. Slow waves occur with a frequency of 10–20/min [35]. They are more frequent in the distal anal canal with the proposed function of moving anal canal contents proximally into the rectum. Ultraslow waves are less than 2/min and are not always present. They are likely to be a result of rectal stimulation inducing IAS relaxation.

The IAS has intrinsic and extrinsic autonomic innervation which modulates its activity. There is conflicting evidence on the response of the IAS to parasympathetic and sympathetic activity and which neurotransmitters and receptors mediate these responses. Lubowski et al. induced IAS relaxation by stimulating the presacral nerves and also administering alpha- and beta-blocking agents which indicates that sympathetic outflow acting via alpha-adrenoreceptors may cause an inhibitory response in the IAS [36]. However, Parks et al. found that in vitro the IAS has a contractile response to noradrenaline [37], and Frenkner et al. used spinal anaesthesia at different levels to demonstrate that there is a tonic excitatory sympathetic outflow to the IAS [38]. Nitric oxide and other non-adrenergic and non-cholinergic (NANC) neurotransmitters are also likely to be key inhibitory mediators [39, 40].

The most common cause of a low resting pressure is an IAS defect caused by obstetric or surgical trauma, with the defect usually demonstrable on endoanal ultrasound. Surgical trauma includes accidental injury during haemorrhoidectomy, anal stretch procedures, fistula surgery, sphincterotomies for anal fissures and damage during sphincter saving rectal resection surgery via a number of mechanisms including inadvertent dilatation. The reduced resting pressures result in passive faecal incontinence. External anal sphincter injuries usually accompany obstetric IAS injury and urgency, and urge incontinence may be the primary presenting symptom in this group. IAS injury can rarely occur in isolation from EAS injury in obstetric injury due to the shearing forces generated during labour. In addition to IAS defects, IAS atrophy can be identified on endoanal ultrasound.

Despite increasing understanding of the factors involved in the modulation of IAS tone, therapeutic pharmacological intervention to increase IAS tone is limited. Loperamide has been found to increase the volume required to induce IAS relaxation. This effect is inhibited by naloxone suggesting this response is mediated by opioid receptors. There is also evidence that loperamide increases resting pressures although the mechanism of action is not understood [41].

With sphincter preserving surgery becoming more common, attention has been directed at understanding the pathophysiology of continence disturbance that can complicate this type of surgery. The resting tone is commonly reduced, and this is thought to be caused by disturbance in the extrinsic autonomic nerve supply, direct trauma due to stretch and excision of the internal anal haemorrhoidal complex. Stapled rather than hand-sewn anastomosis more completely preserves IAS tone [42].

Elevated resting pressures are associated with the presence of anal fissures. The notion that impaired vascular supply caused by raised internal anal sphincter tone causes anal fissures is supported by the findings of Doppler laser flowmetry [43]. The posterior midline, the most common site of an anal fissure, has been shown to receive the lowest flow rate of the four segments. In addition, there is a direct relationship between resting anal pressure and anodermal blood flow, with decreased resting pressures resulting in increased blood flow. Glyceryl trinitrate 0.2 % cream reduces anal canal pressure by around a quarter [44], and internal sphincterotomy results in a 26–50 % reduction in anal canal pressures [45]. GTN cream also causes anodermal mucosal vasodilation. Topical 2 % diltiazem is as effective but is not complicated with headaches which are a common side effect of GTN cream [46].

Although the EAS contributes to the resting pressure, the specific function of the EAS can be assessed during the squeeze, cough and rectal balloon inflation manoeuvres. The pressure increment above resting pressures during these manoeuvres is a direct representation of EAS function. The normal range is approximately above 50 cmH₂O.

The external anal sphincter and the striated muscles of the pelvic floor form an integrated functional unit which contribute to the resting anal pressure but also contract to maintain a positive anorectal pressure in response to raised rectal pressure generated by raised intra-abdominal pressure or rectal filling.

The external anal sphincter (EAS), levator ani and puborectalis form a continuous funnel of striated muscle which comes to surround the internal anal sphincter and the anal canal. Puborectalis has been found to be separated from the external anal sphincter by a distinct fascia although its histological resemblance to the EAS suggests it serves a similar function. The external anal sphincter itself can be subdivided into deep, superficial and subcutaneous sections.

The most common cause of external anal sphincter dysfunction is obstetric injury. Grade 3a involves less than 50 % and grade 3b involves greater than 50 % of the EAS. The injury causes a tear in the anterior portion of the muscles which is typically repaired primarily using an end-to-end or overlapping technique. Persisting sphincter defects cause dysfunction due to the mechanical disadvantage of an absent continuous muscular ring.

There is much interest into the pathophysiology of incontinence in patients without a structural defect of the external anal sphincter, previously termed idiopathic faecal incontinence. The concept of the dysfunction being neuropathic in nature was first described by Sir Alan Parks [47]. On histological examination of the external anal sphincter and puborectalis in patients with idiopathic faecal incontinence, they found marked myopathic changes consistent with denervation and reinnervation in addition to loss of myelinated axons and endoneural fibrosis of the intramuscular nerve fascicles. The changes were found to be most marked in the EAS and relatively spared in levator ani and puborectalis. The findings of these histological studies are consistent with other methods of diagnosing neuropathy including elongation of motor unit potential duration and increased nerve fibre density in single-fibre electromyography studies [48]. The EAS is innervated by branches of the pudendal nerve, whereas the relatively spared puborectalis is innervated directly by the S3 and S4 nerve roots. Pudendal nerve damage can occur by stretching during childbirth, stretching due to pelvic floor descent or entrapment. Entrapment can occur when the nerve emerges from between coccygeus and piriformis, at the greater or lesser sciatic notches or in the pudendal (Alcock’s) canal. Pudendal nerve damage therefore seems likely to be a contributory factor in patients with incontinence without structural sphincter defects. As such idiopathic faecal incontinence is now commonly termed neuropathic faecal incontinence in contrast to myopathic faecal incontinence which occurs with sphincter defects. In fact there is considerable overlap in these two types with the majority of patients with obstetric EAS defects having evidence of pudendal neuropathy with increased muscle fibre density on single-fibre EMG studies [49]. This reflects their common obstetric aetiology.

The ability of the external sphincter to generate an elevated pressure over a prolonged period is an important characteristic. It allows a positive anorectal pressure gradient to be maintained following rectal filling to allow time for the rectal pressure to normalise as it undergoes the process of receptive relaxation. The predominance of type 1 skeletal muscle fibres makes this task achievable. To assess the endurance squeeze pressure, measurements are taken during a 5-s- to 1-min-long squeeze. In health it is normal to be able to generate an increased pressure for at least 5 s. The fatigability of the external sphincter is qualitatively abnormal if a raised pressure is generated for less than 5 s.

Marcello et al. developed a quantitative measure of fatigability [50]. By calculating the rate of fatigue rate (using linear regression on the mean pressure over 1 s periods throughout the endurance squeeze), a measurement can be derived which represents the time it would take for the pressure to return to resting pressure. For reference the equation to derive the fatigue rate index (FRI) is:

As with maximum squeeze pressures, there are significant differences in FRI between incontinent and healthy volunteers, but there is still a large degree of overlap [51].

Increased intra-anal pressure is observed during manoeuvres that increase intra-abdominal and therefore intrarectal pressure in order to maintain a positive anorectal pressure gradient. Commonly used manoeuvres are the cough and the Valsalva manoeuvre. Intra-anal pressure increase during a cough is normally above around 50 cmH₂O.

This measures an important role of the external anal sphincter in the maintenance of continence. The pressure within the rectum is not constant and rises considerably during episodes of raised intra-abdominal pressure and during rectal filling. The rise in the rectal pressure has to be responded to in order to prevent incontinent episodes. This led early theorists to postulate the existence of some sort of valve mechanism which enables the pressure within the rectum to exceed that of the anal canal without incontinence. The ‘flap valve’ was born from the concept of the anorectal angle that is generated by pelvic floor muscles, specifically puborectalis. It was postulated that the anterior wall of the rectum is forced down on the proximal anal canal to seal the anal canal preventing leakage of rectal contents. However, the external anal sphincter has been shown to contract in response to increased abdominal pressure maintaining a positive anorectal pressure gradient [52]. If a positive anorectal pressure gradient is not maintained, there is leakage of stools. Imaging the rectum and anus during the Valsalva manoeuvre confirms that the anterior rectal wall does not come into contact with the upper anal canal [53]. All of this makes the ‘flap theory’ an unlikely mechanism in the maintenance of continence during raises in the intra-abdominal pressure. The role of puborectalis and the anorectal angle is, however, central to the continence mechanism. Incontinence consistently results from division of puborectalis. The contraction of the external anal sphincter in response to raised intra-abdominal pressure is likely to be a learned response rather than a spinal or intramural reflex.

Balloon expulsion and anal canal pressure measurements during straining are relatively easy investigations to perform on patients with evacuatory difficulty. In order to understand the limitations of these tests, the sequence of visceral and somatic afferent and efferent responses involved in normal defaecation needs to be appreciated.

Normal defaecation is initiated with the sensation of urge or fullness following delivery of colonic contents into the rectum. The act of defaecation can be deferred with voluntary contraction of the pelvic floor and the external anal sphincter. Deferral can cause proximal movement of the stool back into the sigmoid colon or just allows time for the rectal smooth muscle to undergo receptive relaxation reducing rectal pressure.

Once the decision has been made to evacuate the rectum, the process of reversing the normally positive anorectal pressure gradient has to take place. Behavioural mechanisms such as sitting allow the anorectal angle to widen facilitating the passage of solid stool. Contraction of the abdominal wall muscles raises the intra-abdominal pressure which is transmitted to the rectum which deforms the anterior wall making it concave. The anal canal shortens in length, becomes funnel shaped and opens and with further increase in the intra-abdominal pressure allows the rectum to empty fully. There are various visceral elements to the defaecatory mechanism. The rectum has large motor complexes causing contraction, and the sigmoid and left colon have been shown to empty by an average of 32 % [54].

Defaecation is therefore dependent on the integration of anorectal sensation and visceral motor activity of the colon and rectum in addition to the somatic control of the abdominal wall and pelvic floor.

Our knowledge of normal defaecation is based largely on evacuation proctography, and the limitations of this investigation reflect the limitations in the understanding of normal defaecation. There is a significant psychological aspect in the investigation of defaecation with the inevitable embarrassment of being observed whilst voiding. In addition, the normal urge to defaecate is often absent, as are the physiological changes of the rectum and colon that accompany defaecation. Thus all laboratory investigations in defaecation assess the voluntary aspect of defaecation in the absence of any upstream colonic activity.

Anatomical abnormalities such as rectoceles and rectal intussusception are a frequent finding in proctography in patients with an evacuatory problem. Understanding of these anatomical pathologies is essential to correctly interpret physiological measures of evacuation such as balloon expulsion.

A rectocele is a bulge in the rectovaginal septum during defaecation. Evacuation proctography in normal individuals has shown up to 81 % of women have rectoceles with 48 % being greater than 1 cm in depth and 5 % being greater than 2 cm in depth [55, 56]. The size of the rectocele is a function of the stiffness of the rectovaginal septum and the maximum rectovaginal pressure gradient generated during straining. If there is significant external anal sphincter contraction during attempted defaecation (anismus), then rectal pressure will be allowed to increase further than in someone who is able to fully relax their anal canal. Rectocele size is therefore not an absolute figure but represents the distance that a tissue moves in response to a force. However, it is generally considered that a rectocele of less than 2 cm is unlikely to cause symptoms and that a rectocele of greater than 3.5 cm is large. The functional significance of a rectocele is probably related to size; however, the concept of ‘trapping’ has been introduced to further ascertain its functional significance. Trapping is defined as retention of greater than 10 % of the contents of the rectocele following evacuation of the rectal contents. The contents of the rectocele normally disappear following defaecation in isolation in a normal toilet, questioning the utility of the concept of trapping [57].

Although the size and extent of trapping are useful indicators for the functional significance of a rectocele, they are no substitute for a good history including feeling of a bulge in the vagina and eliciting a history of vaginal digitation to assist defaecation, the latter providing compelling evidence for a functionally significant rectocele [58].

Rectal intussusception can be graded depending on whether it is circumferential, the length of the infolding and the position of the leading edge being either intrarectal or intra-anal. Greater than 50 % of normal subjects have a grade 4 intussusception, defined as a greater-than-3 mm circumferential intrarectal intussusception [56]. This implies that a degree of intussusception is part of normal defaecation. Intra-anal intussusception is thought to be pathological and probably forms a continuum with rectal prolapse. Chronic intra-anal intussusception and rectal prolapse are often associated with thickening of the internal anal sphincter on endoanal ultrasound [59].

Rectal prolapse is idiopathic in adults although chronic excessive straining, eating disorders, connective tissue disorders and joint hypermobility syndromes are all risk factors. Clinical features include a sensation of a lump on staining, either spontaneously reducing or requiring digital reduction. Rectal prolapse should always be excluded in elderly patients with passive incontinence especially in the context of chronic constipation. Significant proportion are missed on clinical examination, and it is important that the patient is examined on a toilet or commode and the patient is given enough time alone to strain to the same extent they would normally do in order to defaecate.

Solitary rectal ulcer syndrome is defined by the presence of a rectal ulcer up to 12 cm from the anal verge, and histologically it is consistent with ischaemia or trauma [60].

The pathophysiology of solitary rectal ulcer syndrome is unclear but may be linked to rectal intussusception, with anal digitation possibly being a contributory factor in some cases. Certainly a pathologically thickened internal sphincter on endoanal ultrasound is almost always present.

One of the initial physiological studies of patients with evacuatory difficulty was Preston et al.’s study. They included patients with slow-transit constipation and compared them to normal subjects using electromyography of puborectalis and balloon expulsion [61]. They found that puborectalis EMG activity decreased during straining in normal individuals but increased in patients with slow-transit constipation. They termed this anismus, likening the condition to spasm of the pelvic floor muscles in vaginismus. This finding has been replicated in other studies using EMG, manometry and evacuation proctography. Delayed initiation of evacuation and incomplete and prolonged evacuation are the most accurate indicators of anismus in evacuation proctography [62]. However, further studies have shown that the pelvic floor and the EAS do not relax in up to 50 % of individuals with normal defaecation [63]. The functional significance of absence of relaxation or contraction of puborectalis during defaecation is unknown. It may be that it is normal to have a degree of contraction during normal defaecation, or it may be that this phenomenon only occurs under experimental conditions. It has been shown using high-resolution anal manometry that patients with increased intra-anal pressures on straining whilst in the left lateral position can have decreased intra-anal pressure on straining whilst in the seated position [64]. There is also debate on whether puborectalis contraction is in response to the absence of stool passing through the anal canal rather than a cause of the constipation. Certainly deferring defaecation has been shown to slow colonic transit in normal individuals, establishing colonic transit and rectal evacuation as inextricably linked [65].