Imaging pelvic floor disorders
Endoanal ultrasound showing a large anterior tear (between arrows) involving the external and internal anal sphincters. This was due to obstetric trauma
Radiology can provide a global view of pelvic floor function and sphincter integrity (1), but often a more focused approach is required to place patients in treatment defined categories. Symptomatology may be imprecise, and a broad clinical classification into incontinence, constipation, difficult defaecation, and prolapse provides a useful starting point to review which examinations are appropriate in what circumstances.
A main concern with incontinence is to exclude direct sphincter damage that might be suitable for surgical repair. Endoanal ultrasound is the examination of choice to demonstrate either obstetric or post surgical sphincter trauma. It may also show abnormal thinning of an intact internal sphincter that may be associated with passive faecal incontinence(2). External sphincter atrophy remains a difficult entity to image, requiring specialized MRI endocoils as it is not directly visible on endosonography.
Constipation is often associated with difficult defaecation. A simple study of colonic transit is to use radio-opaque markers. Our technique is to give the patient 3 marker sets of geometrically distinguishable shapes (20 in each) on days 1 to 3, and the number remaining on a plain abdominal film taken on day 6(3) indicates if colonic transit is slow or not. A transit study is useful as a baseline assessment in patients who feel they are "constipated". Problems with defaecation are difficult to describe and may be part of straining and constipation. Evacuation proctography is a fluoroscopic examination of rectal emptying. It is not true defaecation as there is no colonic contraction component and only the distal rectum is emptied, but it is valuable to reveal several conditions that may well place the patient in a defined treatment category:
1. Anismus - the inability to relax the pelvic floor voluntarily during evacuation, leading to slow and incomplete rectal emptying with poor opening of the anal canal on evacuation proctography(4). This is a controversial subject and opinions differ as to the significance of anismus, but it may be defined radiologically.
2. Rectocoele - anterior bulging of the rectal wall during evacuation is common in women. The depth of the rectocoele may be measured from a vertical from the anterior anal canal to the most anterior part of the rectocoele. Rectocoeles >2cm in depth are considered significant, and >4cm large. However, the size of a rectocoele does affect the ability to evacuate. Rectocoeles may well be symptomatic only when there is trapping within the rectocoele. At the end of evacuation when the rectum has emptied, the rectocoele remains filled. Digitation, pressing on the posterior wall of the vagina is usually needed to empty the rectocoele.
3. Pelvic floor descent - this is a very simple assessment of the position of the pelvic floor at rest and at the start of evacuation. The position at rest is more important. Normally the anorectal junction is just at, or a little above, the level of ischial spines. Due to the restricted field of view during proctography, the pubococcygeal line is not seen and the level of the ischial spines used instead. Descent at rest or inability to elevate the pelvic floor voluntarily implies weakness, and has been found to be a significant finding in the investigation of incontinence that probably reflects striated muscle atrophy(5).
4. Rectal prolapse - classified as intra-rectal, intra-anal and external. There is no clear imaging diagnosis for intra-rectal intussusception. A grading system for patterns of rectal wall infolding may be used (6). Expansion of the anal canal at the end of evacuation is clear evidence of an intra-anal intussusception. The rectal infolding is best appreciated imaging in the AP plane, not in the normal lateral view. An intra-anal intussusception may progress into external rectal prolapse. The extent of the prolapse reflects the lack of tethering of the rectum. Full rectal prolapse occurs often rapidly at the end of evacuation and is always preceded by intra-anal intussusception. It may reduce spontaneously so that patients may be unaware of the prolapse.
Rectal evacuation may be imaged using MRI, but is technically more difficult both for the patient and to obtain good images actually during evacuation. With modern fluoroscopic units the radiation dose is low. MR becomes important when a more global view of the pelvis is required, i.e. to image vaginal prolapse and cystocoele as well. Dynamic rest and straining MRI examinations are easy to do but of limited value as the pelvic floor is not stressed by evacuation. Evacuation studies are possible using ultrasound gel as a marker in the rectum, and indicated if all three compartments of the pelvic floor require examination(7).
Reference List
(1) Bartram CI, DeLancey JO. Imaging Pelvic Floor Disorders. Springer-Verlag, Berlin, 2003.
(2) Vaizey CJ, Kamm MA, Bartram CI. Primary degeneration of the internal anal sphincter as a cause of passive faecal incontinence. Lancet 1997; 349(9052):612-615.
(3) Evans RC, Kamm MA, Hinton JM, Lennard-Jones JE. The normal range and a simple diagram for recording whole gut transit time. Int J Colorect Dis 1992; 7:15-17.
(4) Halligan S, Bartram CI, Park HJ, Kamm MA. Proctographic features of anismus. Radiology 1995; 197(3):679-682.
(5) Bharucha AE, Fletcher JG, Harper CM, Hough D, Daube JR, Stevens C et al. Relationship between symptoms and disordered continence mechanisms in women with idiopathic faecal incontinence. Gut 2005; 54(4):546-555.
(6) Shorvon PJ, McHugh S, Diamant NE, Somers S, Stevenson GW. Defecography in normal volunteers: results and implications. Gut 1989; 30(12):1737-1749.
(7) Kelvin FM, Maglinte DD, Hale DS, Benson JT. Female pelvic organ prolapse: a comparison of triphasic dynamic MR imaging and triphasic fluoroscopic cystocolpoproctography. AJR Am J Roentgenol 2000; 174(1):81-88.