Background
The optimal imaging modality of obstetric anal sphincter injuries needs to take into
consideration convenience, availability, and ability to assess the sphincter morphologic
condition. Endoanal ultrasound imaging currently is regarded as the reference standard,
but it is not widely available in obstetric units. Exoanal alternatives exist, such
as 3-dimensional introital or transperineal ultrasound imaging, which are already
readily available in most obstetrics and gynecology units.
Objective
The primary objective was to evaluate the diagnostic accuracy of 3-dimensional introital
and 3-dimensional transperineal ultrasound imaging compared with 3-dimensional endoanal
ultrasound imaging as the reference standard for the detection of anal sphincter defects
in women who sustained obstetric anal sphincter injuries. The secondary objective
was to correlate a diagnosis of anal sphincter defect on imaging to symptoms of anal
incontinence, and to assess patient discomfort that is experienced for each imaging
modality.
Study Design
A cross-sectional study was conducted of 250 women who sustained obstetric anal sphincter
injuries, all of whom underwent 3-dimensional introital, transperineal, and endoanal
ultrasound imaging. Introital and transperineal ultrasound imaging were assessed with
tomographic ultrasound imaging. All of the women completed a validated modified St
Mark’s Score and Visual Analogue Score for discomfort. Optimal cut-off values for
a significant defect on tomographic ultrasound imaging were defined as those with
the greatest sensitivity and specificity based on receiver operating characteristic
curves with endoanal ultrasound imaging as the reference standard. Diagnostic test
characteristics of introital and transperineal ultrasound imaging were calculated
with the use of these optimal cut-offs.
Results
Optimal cut-off for a significant external anal sphincter defect was ≥3 of 7 slices;
sensitivity and specificity were 0.65 and 0.75 on introital imaging and 0.70 and 0.69
on transperineal ultrasound imaging. Optimal cut-off for a significant internal anal
sphincter defect was ≥2 of 5 slices; sensitivity and specificity were 0.59 and 0.84
on introital imaging and 0.43 and 0.97 on transperineal ultrasound imaging. The area
under the curve for the diagnosis of external and internal anal sphincter defects
ranged from 0.70–0.74 (P<.001) for introital and transperineal imaging. Positive predictive value for external
and internal sphincter defects ranged from 0.37–0.63, and negative predictive value
ranged from 0.85–0.93 for introital and transperineal ultrasound imaging. Endoanal
ultrasound imaging was the only modality for a defect to correlate with symptoms;
mean modified St. Mark’s score for a defect sphincter was 2.4 (standard deviation,
4.1) and for an intact sphincter was 0.9 (standard deviation, 2.7; P<.01). Introital and transperineal ultrasound imaging were associated with less discomfort
than endoanal ultrasound imaging.
Conclusion
Endoanal ultrasound imaging remains the most accurate diagnostic imaging modality.
With low positive predictive values, introital and transperineal ultrasound imaging
are not suitable for the identification of sphincter defects; however, high negative
predictive values show a good ability to detect an intact sphincter. The optimal cut-off
number of slices on tomographic ultrasound imaging for external and internal anal
sphincters allows for standardization of a significant defect. In women with a history
of obstetric anal sphincter injuries, introital and transperineal ultrasound imagings
are suitable to screen for an intact sphincter if endoanal ultrasound imaging is not
available. When defects are found, women should then have endoanal ultrasound imaging
to verify the diagnosis.
Key words
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Article Info
Publication History
Published online: April 11, 2019
Accepted:
April 8,
2019
Received in revised form:
March 18,
2019
Received:
December 19,
2018
Footnotes
Supported by the Mayday Charity Fund .
The funding agent had no involvement in study design, collection of data, writing of the report or decision to submit the article.
Clinical Trial Registration: Registered in clinicaltrials.gov; NCT 02655900; https://clinicaltrials.gov/ct2/show/NCT02655900?term=02655900&rank=1.
The authors report no conflict of interest.
Cite this article as: Taithongchai A, van Gruting IMA, Volløyhaug I, et al. Comparing the diagnostic accuracy of 3 ultrasound modalities for diagnosing obstetric anal sphincter injuries. Am J Obstet Gynecol 2019;221:134.e1-9.
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