Anorectal Physiology Testing

The shared diagnostic toolkit used to evaluate Rectal Prolapse (Procidentia), Solitary Rectal Ulcer Syndrome (SRUS), Rectocele, Constipation, Fecal Incontinence, and pelvic floor disorders generally. Substantially expanded by ASCRS Ch57 - Common Tests for the Pelvic Floor (Thorsen AJ, Roth L), a dedicated testing chapter that supersedes the brief technique descriptions previously drawn only from Sabiston Ch52 - Pelvic Floor Disorders & Constipation and ASCRS Ch03 - Anorectal Physiology. The chapter also refers readers to the ASCRS Consensus Statement of Definitions for Anorectal Physiology Testing and Pelvic Floor Terminology for fuller detail.

Anorectal manometry

Catheter technology

Anorectal manometry has been performed since Gowers first measured anal resting tone in 1877 (also the first to describe the RAIR — see below). Modern systems consist of a pressure-sensing probe, an amplifier/recorder, a display, and analysis software.

• Water-perfused catheters — thin tubing with side holes 0.5–2.0 cm apart, perfused via a pneumohydraulic pump (0.2–0.4 mL/min at 10 psi); pressure is inferred indirectly from resistance to outflow. Low cost and simple, but cannot be used upright and are susceptible to anatomic/technique-induced artifact. Conventional (few-sensor) probes require pull-through or rotation maneuvers to sample the whole anal canal, preventing simultaneous whole-canal measurement.
• Solid-state catheters — a linear array of miniature strain-gauge transducers. Higher frequency-response than water-perfused systems (better captures fast striated-muscle pressure transients), simpler to calibrate, less cumbersome, but fragile and more expensive.
• Air-charged catheters — an air column from a balloon to an external transducer gives gravity-independent circumferential readings; low-cost and disposable (avoiding high-level disinfection and its personnel-time cost). Shown to reproduce water-perfused and solid-state results across intrauterine, intraurethral, esophageal, and anorectal applications.
• High-resolution anorectal manometry (HRAM) (2008) and high-definition 3D manometry/ topography (HDAM) — HRAM uses 2 rectal-balloon sensors plus 10 anal-canal sensors at 6-mm intervals; HDAM uses a larger (10.75 mm) catheter with 256 sensors in 16 rows of 16, interpolated to 1-mm spacing. Both require thermal-drift compensation. The added sensor density allows a stationary exam (no pull-through needed) and color topographic display via a data-reduction tool ("eSleeve"), in principle allowing shorter, more comfortable exams — though studies have not shown HRAM to be more comfortable than a similarly-sized water-perfused catheter (HDAM was felt somewhat more comfortable). Practical note from the chapter's own authors: despite the technological advance, both continue to use conventional air-charged manometry in their own labs — HRAM/HDAM systems and their fragile reusable catheters cost more, current CPT codes don't reimburse differently for HRAM/HDAM vs. conventional studies, and the authors have not found topography data to change clinical decision-making in pelvic floor disorders.

Technique

Bowel prep is optional (an enema is used only if DRE reveals a stool-full rectum); patients should empty their bowels ≥30 minutes before testing but need not fast. Positioned left lateral, knees/hips at 90°, after a DRE. The probe is lubricated (non-anesthetizing agent) and placed with the most distal sensor external to the anal verge; any attached rectal balloon should sit 3–5 cm above the anal canal to avoid impingement. A minimum 6-cm recording length is required, with a 3–5 minute rest period before recording to let reflex sphincter tone from insertion abate.

Two techniques exist for water-perfused/air-charged catheters: stationary (fixed position) or stationary pull-through (resting/squeeze pressure measured at 6, 5, 4, 3, 2, 1 cm from the anal verge, extracted in 1-cm increments) vs. continuous pull-through (constant 1 cm/second, motorized). Stationary pull-through is now preferred — continuous pull-through provokes a reflex sphincter contraction from probe stimulation, artifactually raising pressures. HRAM/HDAM's multi-sensor design allows a stationary exam after a single placement.

Resting pressure

Reflects internal sphincter tone (up to 80% of the total) plus external sphincter and hemorrhoidal plexus contribution. Recorded ~3 minutes after insertion (allowing insertion-reflex contraction to abate), typically over 20–30 seconds; mean resting pressure is the time-averaged value, maximum resting pressure the single highest reading. The high-pressure zone (HPZ) is the length of anal canal with pressure greater than half the maximum resting pressure. Resting pressures are lower in women than men and decline further with age in women. Hypotonic resting pressure is associated with passive fecal incontinence; hypertonic resting pressure with anal fissure or constipation.

Squeeze pressure

Measures voluntary external sphincter contraction, typically over 30 seconds of maximal effort (a 5-second "as if avoiding flatus/stool" squeeze can also assess endurance). Like resting pressure, lower in women than men and lower in older vs. younger women. HDAM can isolate the puborectalis's specific contribution to squeeze pressure but hasn't been shown to better identify puborectalis/external sphincter injury than simpler manometry.

Cough reflex

Assesses the anal canal's response to a sudden rise in intraabdominal pressure (a single cough, repeated 30 seconds later) — normally provoking reflex external sphincter contraction via the sacral reflex arc. Preserved with spinal lesions above the sacral level; absent with cauda equina lesions or pudendal neuropathy — a bedside localizing test.

Push / simulated evacuation and dyssynergic defecation

Evacuation requires an adequate propulsive rise in rectal pressure with simultaneous anal relaxation (3 pushes, 15 seconds each, 30 seconds apart; no rectal balloon inflation needed). Dyssynergic defecation reflects failure to coordinate abdominal, rectoanal, and pelvic floor muscles — not simply paradoxical anal contraction as originally thought — and is classified into four reproducible types:

Type	Push (intraabdominal pressure rise)	Anal sphincter response
I	Adequate	Paradoxical contraction
II	Inadequate	Paradoxical increase
III	Adequate	Absent/incomplete (<20%) relaxation
IV	Inadequate	Absent/deficient relaxation

Important caveat: manometry alone cannot diagnose dyssynergic defecation — up to 90% of asymptomatic controls and non-constipated proctalgia patients can show dyssynergic patterns on testing, likely reflecting the nonphysiologic left-lateral test position and the absence of real rectal contents during simulated defecation. The defecation index (maximal intrarectal pressure ÷ anal sphincter residual pressure, normal ≥1.2) is a simple quantitative measure of rectoanal coordination during simulated evacuation that partially addresses this limitation.

London Protocol and classification (2014–2018)

A 29-member international consensus group (IAPWG: gastroenterologists, GI physiologists, colorectal surgeons) developed a four-part classification of anorectal disorders (analogous to the Chicago classification for esophageal motility): (a) disorders of the rectoanal inhibitory reflex, (b) disorders of anal tone/contractility, (c) disorders of rectoanal coordination, and (d) disorders of rectal sensation. Major findings (not seen in healthy controls): rectoanal areflexia (absent RAIR), anal hypotension/hypocontractility, rectal hyposensitivity, and rectal hypersensitivity. Minor/inconclusive findings (can occur in health as well as disease): anal hypertension and dyssynergia — this distinction is the protocol's main practical contribution, since it explicitly downgrades dyssynergic-pattern findings (given the above caveat) from automatically diagnostic to merely supportive.

Rectoanal Inhibitory Reflex (RAIR)

First described by Gowers: rectal wall distension triggers reflexive external sphincter contraction followed by internal sphincter relaxation, mediated by the myenteric plexus. Elicited by inflating a rectal balloon with ≥30 mL air (ramp or phasic insufflation); may fail to elicit with low distending volumes in a large-capacity rectum (repeat with larger volumes if megarectum is suspected) or with very low resting pressures. Absent in Hirschsprung's disease, acquired myenteric neuropathies, and after rectal reconstruction.

Rectal sensation (balloon insufflation)

A rectal balloon placed 3–5 cm above the anal canal is inflated (continuous 1–2 cc/s ramp or phasic) to determine three volume thresholds: first sensation, first urge to defecate, and maximum tolerated volume — with concurrent pressure recording allowing calculation of rectal compliance and capacity. Sensation can be normal, hyposensitive (elevated thresholds), or hypersensitive (reduced thresholds) in both chronic constipation and fecal incontinence.

• Hyposensitivity: seen in 18–66% of chronic constipation patients; identifies a subset that responds poorly to both biofeedback and colectomy.
• Hypersensitivity: seen with urge fecal incontinence, radiation proctitis (see Radiation Colitis (Proctitis)), ulcerative colitis, IBS-D (see Irritable Bowel Syndrome (IBS)), and low anterior resection syndrome — see Low Anterior Resection Syndrome (LARS).
• Sensory thresholds can improve alongside symptomatic improvement from behavioral, pharmacologic, or surgical therapy (including neuromodulation for obstructed defecation).

Balloon Expulsion Test (BET)

A screening test for obstructed defecation: a 4-cm balloon filled with 50 mL warm water is placed in the rectum, and the patient attempts to expel it privately on a commode. Inability to pass the balloon, or passage taking >1 minute, suggests a functional defecation disorder. Highly specific but only ~50% sensitive for obstructed defecation, and does not identify the mechanism of disordered evacuation.

Neurophysiologic testing

• Needle EMG — maps external anal sphincter defects and denervation-reinnervation via needle electrodes in the external sphincter/puborectalis. Painful and poorly tolerated, with diminished clinical utility now that endoanal ultrasound/MRI can image sphincter defects directly.
• Surface EMG — a better-tolerated, more global assessment (intraluminal anal-plug electrode preferred over surface pads for accuracy). Correlates well with manometric squeeze pressure and is useful for detecting paradoxical contraction/failure of relaxation in defecatory dysfunction; also used therapeutically to retrain external anal function via biofeedback in fecal incontinence and obstructed defecation.
• Pudendal nerve terminal motor latency (PNTML) — measures neuromuscular integrity between the pudendal nerve and external anal sphincter via bipolar electrode stimulation at each ischial spine; normal value 2.0 ± 0.2 ms (per ASCRS Ch03 - Anorectal Physiology). Relevant to injury from third-trimester pregnancy, second-stage labor, and forceps delivery. Substantial methodologic criticism: highly operator-dependent, poor sensitivity/specificity, can be falsely normal if only some nerve fibers are damaged, variable normal limits, and inconsistent correlation with outcomes of interventions like sphincteroplasty. Up to 23% of patients with idiopathic fecal incontinence show a cauda equina conduction delay on more proximal testing (see Spinal motor latency, below).
• Spinal motor latency — transcutaneous lumbar stimulation assesses conduction along the cauda equina/sacral plexus proximal to the pudendal nerve origin, using needle EMG at the puborectalis, external sphincter, and urethral sphincter; comparing latencies across levels localizes the injury level. Magnetic stimulation-evoked motor potentials (translumbar/transsacral MEPs) are an alternative found to be better tolerated than PNTML and more often abnormal in fecal incontinence (87% prolonged MEPs vs. 63% prolonged PNTML in one study) — though the clinical significance of prolonged spinal latencies for guiding fecal incontinence therapy remains unsettled. ASCRS Ch61 - Fecal Incontinence Evaluation and Treatment corroborates the PNTML criticism from the fecal-incontinence-treatment side: a prolonged PNTML does not independently predict treatment success, and the technique has slowly faded from routine clinical use as a result.

Ultrasound and dynamic imaging

Endoanal ultrasound

The gold standard for anal sphincter pathology in fecal incontinence workup, having replaced EMG mapping as the modality of choice for identifying surgically repairable sphincter defects; also used for fistula characterization (rectovaginal, cryptoglandular, Crohn's) and postoperative sphincter/ fistula-repair assessment. A rotating 3D endocavitary probe (typically 3–20 MHz) collects a 360° image over a 6-cm axis without moving the probe; patients receive two cleansing enemas beforehand. Interpretation: the anal canal has three sonographic zones — upper (hyperechoic puborectalis horseshoe), mid (intact hypoechoic internal sphincter ring + hyperechoic external sphincter ring), and distal (external sphincter only, internal sphincter absent). Anterior sphincter defects from childbirth are visualized here. Quantified normal anatomy: internal sphincter thickness increases with age (4.5±0.7 mm under 30 years vs. 5.9±1 mm over 50 — a 33% increase); external sphincter is thicker posteriorly (24.7±4.6 mm) than anteriorly (6.6±1.7 mm) in nulliparous women; mean anal canal length in females is 3 cm; mean nulliparous perineal body length is 17.4±2.7 mm, with <10 mm associated with an anterior sphincter defect.

Static MRI

Endoanal-coil MRI of the sphincter complex has fallen out of favor (expensive vs. ultrasound); a prospective comparison for surgical-candidate selection found both endoanal MRI and anal ultrasound sensitive for anterior external sphincter defects. Largely superseded by MR defecography for dynamic questions.

Defecography (evacuation proctography)

Fluoroscopic imaging of defecation; cost-effective, standard equipment, evaluates the physiological seated position. Used mainly for obstructed defecation and posterior-compartment pelvic organ prolapse (rectal prolapse, enterocele, cystocele). Barium paste simulates stool; oral/vaginal/bladder contrast can additionally assess enterocele and other pelvic organs. Mean radiation dose 0.5–5.0 mSv.

• Normal evacuation (5 criteria, from the original 1984 study): increased anorectal angulation, obliteration of the puborectalis impression, wide anal canal opening, total rectal evacuation, and normal pelvic floor resistance. The anorectal angle (anal canal axis vs. posterior rectal wall) is measured but inherently variable; accepted normal ranges are 90–110° at rest and 110–180° during evacuation (mean values 91.96° rest → 136.76° evacuation in the original study, though later studies show a wide normal range). The pubococcygeal line (pubis to coccyx tip) is the standard pelvic floor reference for defining organ descent/prolapse.
• Rectocele: anterior rectal wall bulge into the vagina from rectovaginal septum weakness. <2–3 cm is normal/insignificant (seen in up to 93% of asymptomatic women); depth is measured from the anterior anal canal border to the rectocele's most anterior point. Should be assessed post-evacuation for barium trapping; large rectoceles are only pathologic when associated with outlet-dysfunction symptoms. See Rectocele for the clinical/surgical-repair side.
• Intussusception/prolapse: unilateral or circumferential rectal infolding on straining — intrarectal, intra-anal, or external (complete rectal prolapse). Oxford grading (5 grades): I–II don't descend into the anal canal, III–IV descend into the anal canal without protruding, V protrudes from the anus. Increasing intussusception grade is associated with worsening fecal incontinence but not with constipation — a finding not yet cross-referenced onto Rectal Prolapse (Procidentia).
• Enterocele/sigmoidocele: small bowel or sigmoid colon herniation into the pouch of Douglas, most common after hysterectomy; best seen with oral contrast during straining. Normally, bowel should not descend below the upper rectum.
• Anismus: failure of normal pelvic floor muscle relaxation during defecation — 90% of patients unable to evacuate two-thirds of a 120-cc contrast enema within 30 seconds had anismus confirmed on further physiologic testing in one study. Overlaps with Constipation's Rome IV anismus/ dyssynergic-defecation criteria.
• Pelvic floor descent: normally <3 cm during evacuation (measured as anorectal junction descent); chronic straining/constipation can produce abnormal descent >3.5 cm, leading to hypermobility/stretch injury of muscles and nerves.

MR defecography

Similar concept to fluoroscopic defecography but without radiation — vagina and rectum filled with ultrasound gel, imaged supine during defecation-simulating maneuvers. Allows comprehensive structural and functional pelvic floor evaluation, but the supine position (patients don't normally defecate supine) may limit validity for some symptoms. A 2016 study of 50 women found no statistically significant difference between MR and conventional defecography for posterior-compartment dysfunction (peritoneocele, rectocele, rectal prolapse, anismus) — both are reasonable choices based on local availability/expertise, with combined use suggested in difficult cases.

Dynamic (transperineal/transvaginal/endoanal) ultrasound

Addresses defecography/MR defecography's radiation, cost, and rural-accessibility limitations, and avoids the embarrassment of defecating contrast material. Uses three probes in sequence (transperineal — convex probe from mons pubis to anal margin, visualizing the puborectalis and its pubic attachment; transvaginal — biplane 360° rotational probe for anterior/posterior compartments; endoanal — as described above), each imaged at rest, maximal Valsalva, and pelvic floor contraction. Comparative data: effective at diagnosing peritoneocele, enterocele, and perineal muscle alterations in experienced hands, with more anterior/central compartment information than defecography, though some studies show lower prolapse-detection rates than defecography; a blinded 68-patient comparison against MR defecography for rectocele/enterocele/intussusception/cystocele found dynamic ultrasound had a high negative predictive value, suggesting it can serve as a screening tool before reserving MR defecography for cases it flags as abnormal; 3D transperineal ultrasound also showed good agreement with 2D endoanal ultrasound for sphincter defects and prolapse.

Echodefecography

A 3D dynamic ultrasound technique (distinct from the above transperineal/transvaginal/endoanal battery) using rectal ultrasound gel to assess obstructed defecation: Scan 1 (rest, no gel) visualizes sphincter anatomic integrity; Scan 2 (rest/straining/rest, no gel) evaluates muscle relaxation during defecation effort to demonstrate anismus; Scan 3 (rest/straining/rest, with rectal gel) is repeated up to 3 times. A prospective multicenter trial of 86 women found echodefecography detects the same anorectal dysfunctions as defecography, is minimally invasive, well tolerated, and radiation-free.

Colonic transit time

A test of colonic inertia (unchanged from prior sourcing — not covered by Ch57, which focuses on anorectal/pelvic floor testing specifically). The patient ingests 24 radio-opaque markers (Sitzmarks) and avoids laxatives/mechanical measures; marker progression through three colonic regions (right, left, rectosigmoid) is tracked on plain films every other day through day 7. Healthy subjects expel 80% of markers by day 5; slow-transit constipation/colonic inertia patients retain a significant fraction through the study period.

Related pages

• Anorectal Physiology — the normal physiology this testing battery measures.
• Fecal Incontinence — the disorder most anorectal testing is used to evaluate; PNTML and neurophysiologic testing detail above is directly relevant to that page's neurologic incontinence section.
• Constipation — the anismus/dyssynergic defecation content above overlaps substantially with that page's Rome IV criteria.
• Rectocele, Rectal Prolapse (Procidentia), Solitary Rectal Ulcer Syndrome (SRUS) — posterior-compartment prolapse disorders diagnosed via the defecography/dynamic imaging content above.
• Irritable Bowel Syndrome (IBS) — IBS-D-associated rectal hypersensitivity noted above.
• Colorectal Trauma — manometry's role predicting functional outcome before colostomy reversal after traumatic anal sphincter injury.

Open items / gaps

• All Ch57 content is textbook-level/secondary. No individual study from this chapter has been independently ingested; given the chapter is itself a testing-technique survey rather than a therapeutic-outcomes chapter, no entries were added to Research Priorities.md — flagged instead as a candidate only if a specific test's diagnostic accuracy becomes clinically decision-relevant.
• The dyssynergic-defecation false-positive rate (up to 90% of asymptomatic controls/proctalgia patients) is a significant caveat on manometry's diagnostic value that is easy to overlook when only reading the four-type classification table above.
• The Oxford intussusception-grading/fecal-incontinence association has not yet been cross-referenced onto Rectal Prolapse (Procidentia) — worth doing in a future pass.
• No primary literature yet establishing these tests' sensitivity/specificity for specific diagnoses beyond what's quantified above (BET's ~50% sensitivity, endoanal ultrasound's ~100% sensitivity/specificity for sphincter defects) — presented here as the textbook's own technique descriptions and single-study figures rather than a systematic evidence synthesis.