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1
Intestinal Imaging Centre, Level 4V, St. Mark's Hospital, Northwick Park,
Watford Rd., Harrow, London, HA1 3UJ, United Kingdom.
2
Department of Radiology, Chelsea & Westminster Hospital, Fulham Rd.,
London, SW10 9NH, United Kingdom.
Received May 24, 1999;
accepted after revision August 11, 1999.
Supported by Lister Bestcare, Selby, United Kingdom.
Abstract
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SUBJECTS AND METHODS. Fifty healthy adult volunteers (25 men and 25 women; age range, 20-66 years; mean age, 34 years) were prospectively recruited and examined using dynamic MR imaging. All subjects were interviewed and established as healthy using a validated questionnaire. Axial, coronal, and sagittal MR imaging was performed at rest and during maximum pelvic strain using a static 1.0-T unit and a fast-field-echo sequence, providing 10 slices in 31 sec. Standardized measurements of pelvic configuration were taken, and rest and strain imaging were compared to determine the range of normal appearances.
RESULTS. Three women developed a cystocele during maximum pelvic strain, two of whom also showed grade 1 uterocervical prolapse, which was also seen in another woman. Three men showed posterior pelvic floor descent in excess of 3 cm during straining. No rectocele, enterocele, rectal prolapse, or perineal hernia was seen in any subject.
CONCLUSION. The normal range of pelvic organ descent in asymptomatic subjects seen on dynamic MR imaging included cystocele, uterocervical prolapse, and excessive anorectal junction descent. Although we encountered pelvic prolapse in seven volunteers, it was infrequent and low grade, suggesting that criteria for abnormality derived from proctography are generally applicable to MR imaging.
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MR Imaging
MR imaging was performed with a 1.0-T static unit (Gyroscan NT 1.0;
Philips, Hammersmith, United Kingdom). No preparation of the subject was
required. The subject lay supine on a waterproof pad placed on the MR table.
Maximum pelvic straining was practiced with the patient before examination;
patients were encouraged to bear down as if emptying their bowels. Images were
obtained in sagittal, axial, and coronal orientations using the body coil,
first with the patient at rest and then again during maximum pelvic strain. A
T1-weighted fast-field-echo sequence was used with the following parameters:
flip angle, 55°; TR/TE, 79/240 msec; field of view, 34 cm; slice
thickness, 8 mm; interslice gap, 2 mm; matrix size, 256 x 256; and four
excitations. This sequence gave 10 slices in 31 sec. The examinations were
downloaded onto a dedicated workstation (Easyvision; Philips) for radiologist
review.
Image Analysis
All examinations were analyzed by two radiologists in consensus, who
recorded standard measurements of pelvic floor anatomy and also noted the
presence or absence of any structural abnormality. The pubococcygeal line,
defined as the line that joined the inferior border of the pubic symphysis to
the last coccygeal joint [7,
11], was drawn onto the
midline sagittal resting image. The position of the bladder base, cervix, and
anorectal junction, defined as the junction of the rectal ampulla and anal
canal, were measured at 90° to the pubococcygeal line. The anorectal
angle, defined as the angle between the longitudinal axis of the anal canal
and the posterior rectal wall, and the levator plate angle, defined as the
angle between the levator plate and the pubococcygeal line, were measured
[7]. These values were then
compared with measurements taken from the corresponding sagittal straining
image, and the change on straining was calculated. A cystocele was diagnosed
if the bladder base descended below the inferior border of the symphysis pubis
[1,
5]. Uterocervical prolapse was
defined as cervical descent below the pubococcygeal line (grade 1), to the
introitus (grade 2), or to the exterior (grade 3). Anorectal junction descent
was defined as excessive if more than 2.5 cm below the pubococcygeal line at
rest or if more than 3 cm on maximum strain
[10,11,12].
The axial images were used to calculate the pelvic floor hiatal area and
perimeter at rest and during maximum strain
[7,
8], which were measured at the
level of the most inferior point of the symphysis pubis. All images at rest
and during maximum strain were also evaluated for the presence or absence of
any other structural abnormality as follows: enterocele was defined as small
bowel within the rectovaginal septum that reached or crossed the junction of
the upper one third and distal two thirds of the vagina
[13]. Rectocele was defined as
an anterior rectal wall bulge exceeding 2 cm
[10,
11]. Rectal prolapse and any
perineal hernia through the levator plate were also noted if present.
Statistical Analysis
Descriptive statistics were performed on all data at rest and during
maximum strain. The Student's t test was used to compare
parametrically distributed continuous data, and statistical significance was
assigned to a p value of less than 0.05. Calculations were performed
using Arcus Quickstat Biomedical 1.0 (Research Solutions, Cambridge, United
Kingdom).
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Mean values with standard deviations (SDs) for the bladder base, cervix, and anorectal junction at rest, during maximum strain, and the descent on straining are shown in Table 1. The bladder base lay above the pubococcygeal line in all subjects at rest but descended below this line during maximum strain in three women, two of whom were parous (one with two and one with three vaginal deliveries), resulting in a diagnosis of cystocele (Table 1, Figs. 1A,1B and 2A,2B). Although the mean bladder base was significantly higher in men than in women at rest and during maximum strain, no significant difference was seen between men and women with respect to the distance descended. No statistical difference was noted between any bladder base value when parous and nulliparous women were compared.
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The cervix descended below the pubococcygeal line in two (one nulliparous and one with three vaginal deliveries) of the three women with cystocele, and in a third (who did not have a cystocele), resulting in a diagnosis of grade 1 uterocervical prolapse (Figs. 3 and 4A,4B). Grade 2 or 3 uterocervical prolapse was not seen in any woman. No significant difference was seen when nulliparous and parous women were compared for any uterocervical measurement.
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The mean anorectal junction position lay on the pubococcygeal line in both men and women (Table 1) and was not more than 2.0 cm below this level in any subject at rest. However, anorectal junction descent was more than 3.0 cm in three men during maximum strain (Fig. 5A,5B). No overall statistical difference was noted between men and women (Table 1) or between parous and nulliparous women.
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Mean values at rest and during maximum strain for the anorectal angle, the levator plate angle, the pelvic floor hiatal area, and the hiatal perimeter in men and women are shown in Table 2. The anorectal angle during maximum pelvic strain was significantly more acute in men than in women, but no significant difference was seen either between sexes or between women of different parity for any other measurement. No rectocele, enterocele, rectal prolapse, or perineal hernia was seen in any subject when all images were examined for further structural abnormality. An unsuspected ovarian dermoid cyst was seen in one woman, and this diagnosis was subsequently confirmed surgically.
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Although evacuation proctography is rapid and easy to perform, the modifications necessary to image other organs may be time-consuming, invasive, and complex, and the musculature of the pelvic floor itself is not visualized. Furthermore, examination involves irradiation, and many patients will be women in their childbearing years, especially if the examination is being performed to investigate constipation. In an attempt to overcome these limitations, MR imaging has been applied to pelvic floor dynamics with promising results. Initial reports were necessarily compromised by slow acquisition times [15, 16], but advancing MR technology has enabled multislice imaging during a single straining effort. A study of constipated and anally incontinent women using this technique revealed unsuspected visceral prolapse at multiple sites [7]. However, these studies have recruited limited numbers of control subjects. Previous proctographic studies of asymptomatic control subjects have unexpectedly shown that many of these individuals show findings previously considered abnormal [10, 11]. MR criteria for abnormality have been based on proctographic examinations but it is likely that values will differ between the two techniques, primarily because the seated position achieved during proctography is available only to the few investigators with access to open-architecture magnets [17, 18]. Supporting this, a study of 10 women who underwent both proctography and dynamic MR imaging found good correlation but poor agreement between corresponding measurements [19]. Because of this, the range of normal values for dynamic MR imaging needs to be defined. We have attempted to do this in the largest dynamic MR study to date, to our knowledge.
All subjects examined in this study satisfied vigorous and validated
criteria for health. Despite this, we encountered some findings con-sidered
abnormal. We showed significant differences between men and women with respect
to the position of the bladder base, as would be expected, but a cystocele
developed in three women during maximum pelvic strain, two of whom also
developed grade 1 uterocervical prolapse; the latter was also seen in one
additional woman. However, no large cystocele was seen, nor was any higher
grade of uterocervical prolapse. Similarly, three men showed significant
pelvic floor descent (
3 cm) when judged by proctographic criteria, but
this also was low grade. These findings are broadly in agreement with those
encountered in proctographic studies of asymptomatic individuals in which most
subjects were "normal," but there is some crossover with findings
considered abnormal [10,
11]. Supporting this, no
rectocele, enterocele, rectal prolapse, or perineal hernia was seen in any
subject when all images were reviewed. The range of normal values for the
anorectal and levator plate angle has also been defined. Although the
anorectal angle during maximum pelvic strain was significantly more acute in
men, this result is likely to have occurred because of multiple statistical
testing. These measurements were included for completeness but are rarely of
major clinical significance and do not influence management, as would
diagnosis of a prolapse site. The pelvic floor hiatus is a relatively new
measurement that is already clinically relevant and is increased in women with
pelvic organ prolapse [20]. We
were unable to find any significant difference between the sexes for this
measurement, nor were we able to show any significant difference between
parous and nulliparous women for any parameter, possibly because of the
relatively few parous women recruited. The vigorous questionnaire excluded
many older and parous women from the study, predominantly because of minor
anal incontinence, a symptom strongly associated with previous vaginal
delivery [21].
Dynamic pelvic MR imaging is evolving, and an optimal technique remains to be defined. A major disadvantage of the technique may relate to the inability to ensure an adequate straining effort. In an attempt to minimize this effect, straining was practiced with the subject before the examination, and the volunteer was placed on an absorbent pad to minimize the fear of leakage. Despite this, some subjects raised their pelvic floor during straining, evidenced by ano-rectal junction ascent, which suggests pelvic floor contraction rather than relaxation. To eliminate this effect, some investigators fill the rectum and bladder and encourage evacuation in the magnet despite the supine position [9]. This approach will provide additional information relating to the rate and completeness of rectal evacuation, important parameters when constipation is being investigated [22]. A seated position is optimal but only achievable in an open magnet [17, 18].
If prolapse is shown, it is tempting to attribute symptoms to the prolapse, but the prolapse may merely be secondary to an underlying functional disorder. For example, constipated patients who strain excessively may develop cystocele, enterocele, rectocele, rectal prolapse, and pelvic floor descent [23], but surgery to correct these will not treat the underlying disorder [24]. The results of any dynamic pelvic examination, be it cystoproctography or MR imaging, need to be considered in the light of the patient's history and other tests so that therapy is appropriate.
In summary, we examined 50 healthy volunteers using dynamic pelvic MR imaging to define the range of normal appearances encountered during this relatively new test. Although we encountered some pelvic floor prolapse, it was infrequent and of low grade, suggesting the criteria for abnormality derived from proctographic examinations are generally applicable to MR imaging.
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