David Cohen, M.D.·Published 22 February 2008·22 min read
Key points
Fecal incontinence is a common problem in women after vaginal delivery.
Anorectal manometry is a quantitative method of assessing resistance to spontaneous evacuation provided by the anorectal mechanism and the sensory capabilities of the rectum to provide a feeling of imminent emptying.
Coordination of pressures and motor events between the rectum and anal canal is important to maintain an efficient mechanism of continence.
EAUS provides a morphologic assessment of the internal and external anal sphincters, the puborectalis muscle, and the rectovaginal septum.
Fecal incontinence is a common problem in women after vaginal delivery. Overt sphincter damage from a third or fourth degree tear occurs in approximately 0.6% to 3% of women undergoing vaginal delivery. Twenty-nine percent to 48% of these patients develop anal incontinence between 3 months and 3 years after primary sphincter repair. Between 6.8% and 35% of primiparal and 12% and 44% of multiparal women have occult sphincter damage recognized on endoanal ultrasonography (EAUS)(Table 1). From one-third to two-thirds of women with such sphincter damage recognized on EAUS have bowel symptoms such as urgency or anal incontinenence.
After vaginal delivery pudendal nerve conduction can also be impaired. Prolonged pudendal nerve terminal motor latency (PNTML) is thought to occur in 42% of women who undergo vaginal delivery. Subsequent recovery is noted in 60% of these patients within the first 2 months after delivery.
Muscular and neurologic damage may also coexist. Indeed 60% of incontinent women who have sustained an obstetric injury to the external anal sphincter have prolonged PNTML.
Both severity and the prevalence of fecal incontinence increase with age, suggesting that other factors may also be involved. A progressive denervation of the anal sphincter muscles may be responsible for the delayed onset of fecal incontinence. But no clear correlation can be demonstrated between the weakness of the external anal sphincter and PNTML in elderly incontinent patients. These findings suggest that in this population a contribution to the weakness of external anal sphincter may be a decrease in the activity of the anterior horn cells in the spinal cord or the reduction in upper motor neurons caused by old age. The reduction in strength of the connective tissue fascia that occurs with the decline of estrogen production at menopause might weaken the pelvic floor and exacerbate any tendency to pudendal neuropathy caused by obstetric factors. This plethora of contributors mandates accurate assessment of anorectal and pelvic floor function in women with fecal incontinence to correctly identify the cause of the incontinence and to plan the most appropriate treatment.
Anorectal manometry
Anorectal manometry is a quantitative method of assessing resistance to spontaneous evacuation provided by the anorectal mechanism and the sensory capabilities of the rectum to provide a feeling of imminent emptying. It provides useful information in determining the causes of fecal incontinence and allows therapeutic decisions to be based on objective parameters. It also provides a baseline for comparison after treatment.
Anal canal pressures can be measured in a number of ways. The available methods use different sensory devices that include water-filled perfusion catheters, water- or air-filled balloons, sleeve catheters, and pressure transducers. Manometry can be performed with the stationary technique, the manual or stationary pull through technique, and the continuous pull through technique.
Written and revised by
David Cohen, M.D.
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With the stationary technique the catheter is left in one position where the measurements are recorded. With the manual pull through technique the catheter tip is initially positioned 6 cm from the anal verge into the rectum and then gradually and progressively withdrawn at intervals of 1 cm. At each level the patient is asked to squeeze the sphincter muscles and hold for a few seconds and subsequently to strain to simulate evacuations while pressures are recorded. A 20- to 30-second interval is given between measurements to stabilize the anal pressure. Using multilumen catheters, the continuous pull through technique can assess anal canal pressures on a cross-sectional basis at each recording station from the anal verge, allowing examination of pressures in each quadrant of the anal canal. This measurement provides radial and longitudinal pressure profiles from which vector diagrams, when plotted together, can generate a three-dimensional picture of the anal canal, known as the vector volume; the vector volume analysis demonstrates the anal canal pressure symmetry.
Several methods of analysis have been introduced in an attempt to quantify and express anal canal manometric asymmetry. The total radial asymmetry measures the mean asymmetry of the anal canal from the sum of the deviance of minimum vector volumes from the mean of all vector volumes at each level within the anal canal. The vector symmetry index (VSI) is calculated dividing the anal canal into eight segments, the volume of each being the sum of the vector volumes within each segment. Reconstruction of these manometric data generates a three-dimensional wire frame and the VSI represents the ratio of minimum segment volume to the maximum segment volume. A VSI of 1.0 represents perfect symmetry; varying degrees of asymmetry lie between 0 and 1.0. Physiologic radial asymmetry of the anal canal has been demonstrated with anterior quadrant pressures being lower than those laterally and posteriorly. Nevertheless, symmetry of the anal canal pressures can be distorted or reduced in incontinent patients, so they have been used as parameters for evaluating anorectal pathology. But it has been demonstrated that vector volume analysis shows a lower squeeze pressure profile than does the stationary pull through technique. This difference might be from the stabilization period that is obtained at each single point of the anal canal with the stationary pull through technique, or perhaps it reflects the patent's inability to perform sustained squeeze during the catheter withdrawal process. So the vector volume manometry theoretically provides a less reliable pressure measurement than does the stationary pull through technique.
Only 13.2% of patients with isolated decreased electromyographic activity have a sphincter defect localized by vector volume analysis and only 11% of patients with a sphincter defect demonstrated on EAUS have identical findings on vector volume analysis. Both standard and vector manometry findings have shown no correlation with morphologic parameters as demonstrated on EAUS and endoanal magnetic resonance imaging (MRI). But a more recent study found a good correlation among anal manometry parameters, VSI, EAUS, and clinical findings. Total asymmetry failed to show this correlation. It was concluded in this study that anal vector manometry using VSI analysis of anal squeeze pressure with its high sensitivity is complementary to EAUS because it provides information regarding anal sphincter function and verification of anal sphincter disruption.
Resting pressure is determined mostly by the internal anal sphincter, and the external sphincter is mainly responsible for squeeze pressure. When normal subjects contract their anal sphincters, the anal canal pressure rises between 150% and 250%. Resting is slightly lower in women than in men; and squeeze pressure is significantly lower in women, anal canal pressures also reduce with age. Squeeze pressure is more affected than is resting pressure with age, and the decrease is more marked in women than in men.
Resting and squeeze pressures are both decreased after vaginal delivery. Squeeze pressure is more frequently reduced and is more pronounced after forceps delivery (Table 2). Resting pressure is also reduced after proctectomy with coloanal anastomosis and by pelvic irradiation.
Cinedefecography is a radiographic method of evaluating the dynamics of evacuation. This technique involves the filling of the rectum with 50 mL of liquid barium, followed by the introduction of 100 to 200 mL of barium paste with a consistency similar to stool. The patient is seated on a translucent commode and the anorectum laterally visualized by fluoroscopic technique. Lateral films are obtained at rest, and during squeeze and maximum strain after which the mechanism of evacuation is recorded by video. Parameters evaluated during resting, squeeze, and straining phases include the anorectal angle, puborectalis length, and perineal descent. The changes in these parameters during the different phases are also analyzed. The examination is not routinely indicated in patients with incontinence because these patients often cannot retain the contrast, and it seldom contributes to the diagnosis in these patients. But in selected cases it provides information about the presence of associated factors such as intussusception, rectal prolapse, and increased perineal descent.
Magnetic resonance imaging
In general MRI has a high sensitivity to soft tissue abnormalities, but conventional MRI with external body coil does not provide adequate definition of the anal sphincters. Dedicated endoanal coils have been developed to increase the resolution in the assessment of the anal canal. Endoluminal MRI, with its high intrinsic contrast resolution and the high spatial resolution of an endoanal coil, has provided detailed definition of the anal canal anatomy. The examination is normally undertaken with the patient in the supine position with a cylindrical rigid receiver coil placed in the anal canal. The investigation does not require any preparation and, except in patients with extensive perianal fistula or anal stenosis, is well tolerated by the patients. With endoluminal MRI, all layers of the anal sphincter can be easily identified. Lesions of the external anal sphincter such as sphincter defects and scar tissue are demonstrated with an accuracy of 90% to 95%.
A comparative study of endoluminal ultrasonography and endoluminal MRI in 22 patients with fecal incontinence has demonstrated that endoluminal MRI is superior to endoanal utrasonography in detecting defects of the external anal sphincter (accuracy = 91% versus 73%). In that study, EAUS was performed with a 7-MHz transducer. It is likely that with the use of a near-field focusing 10-MHz transducer, the definition and accuracy of the investigation will increase. Conversely, it seems that the internal anal sphincter, which is well seen by EAUS, is more difficult to define by endoanal MRI. A prospective study of 52 consecutive unselected patients with anal incontinence found that 12 errors related to internal sphincter integrity were made at endoanal MRI compared with only one at EAUS. The same study found endoanal MRI and EAUS equivalent in diagnosing external anal sphincter injury. Although endoluminal MRI can easily identify atrophy of the external anal sphincter, as yet there are no EAUS criteria for external sphincter atrophy. External sphincter atrophy, as identified on endoanal MRI, might also represent a predictor of the negative outcomes of anterior anal repair.
Anal sensation
The sensory function of the anorectum is involved in discrimination between flatus, liquid, and solid stool. It is unclear whether the integrity of sensory function in the anal canal plays a role in the maintenance of continence. But it is likely that minor degrees of sensory impairment are not by themselves causative of incontinence in patients with otherwise normal anorectal function. In patients with poor sphincter function though, the sensory abnormality might make patients unaware of the seepage of fecal material through the anal canal and determine a worsening of symptoms.
Anal sensation was originally measured using pinprick and metal plates at different temperatures. Such techniques are crude and provide only qualitative data. Mucosal electrosensitivity and thermal sensitivity are the two currently used tests to assess anal sensation. The mucosal electrosensitivity technique requires a current generator connected to a probe with the active electrode and a reference electrode. The reference electrode is placed on the patient's thigh, while the probe is lubricated with a conductive gel and introduced into the anal canal so that the electrode lies in the upper anal canal. The current is then increased in 1-mA increments until the subject feels a tingling or prickling sensation. The voltage at which sensation is first felt is recorded. The procedure is repeated three to five times at the same level and the mean of these readings is documented as the threshold of sensation for that particular level. The probe is withdrawn and the same process repeated for the mid- and lower anal canal.
Thermal sensitivity assesses the thermal threshold of the anal canal. A 1-cm probe introduced into the anal canal through which water of different temperature can be pumped provides cold and hot stimuli. The temperature of the thermode can be maintained at 37 °C and then rapidly increased or decreased by 4.5 °C. A small thermocouple placed on the outside of the probe continuously records the temperature of the thermode-mucosal interface. The thermal threshold is assessed for four temperature ranges: normal to hot, hot to normal, normal to cold, and cold to normal. The assessment is performed in the upper, mid-, and lower anal canal and a median value of the four thresholds represents the minimum detectable temperature change for that level. The anal canal is very sensitive to thermal stimuli and this sensitivity is diminished in patients with idiopathic fecal incontinence.
In conclusion, fecal incontinence is a complex problem, often of multifactorial origin. Although the use of all tests currently available for assessment of this condition is generally not required, complete functional and anatomic assessments of the anorectum, anal sphincters, and pelvic floor are mandatory in all women with fecal incontinence to correctly identify the cause and type of incontinence and allow correct treatment. Determining the most appropriate tests will largely depend on the patient's history and symptoms and can vary for each patient.
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Presented at the American College of Surgeons 86th Annual Clinical Congress, Chicago, IL, October 2000. Presentation sponsored by the Advisory Council for Gynecology and Obstetrics and the Advisory Council for Colon and Rectal Surgery.
Pasquale Giordano MD and Steven D. Wexner MD, FACS, FRCS, FRCS(ED)
Department of Colorectal Surgery, Cleveland Clinic Florida, Weston, FL USA
