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Letters to the Editor |
Intestinal Imaging Centre, St Mark’s Hospital, Northwick Park, Harrow HA1 3UJ, United Kingdom, e-mail: c.bartram@ic.ac.uk
Editor:
We enjoyed the recent article by Dr Rociu and colleagues in the August 1999 issue of Radiology (1) on comparing endoanal ultrasonography (US) and magnetic resonance (MR) imaging in the diagnosis of anal sphincter tears. There may be several factors that cause anal incontinence, and those related to sphincter damage can be classified as either internal or external sphincter disruption or atrophy or a combination of sphincter disruption and atrophy.
External sphincter injury is the most common sphincter disruption seen, since it is related to vaginal delivery (2). In contrast, isolated internal sphincter injury almost never follows childbirth and indicates a primary traumatic cause from within the anal canal, most commonly surgery. Sphincter atrophy may occur without sphincter trauma, and, although the cause of this remains uncertain, there may be a relationship to neurologic degeneration from denervation and aging (3). Only external sphincter tears are amenable to repair, and now that postanal repair is not performed, most incontinent patients will never undergo surgery. Because of this, any series with surgery as the standard will inevitably include a spectral bias toward external sphincter tears.
In our experience, the internal sphincter is exquisitely well seen at US but is more difficult to define at MR imaging, a difference that is readily appreciable from the figures in the article (1). Supporting this hypothesis, a prospective, blind, crossover study of 52 consecutive unselected patients with anal incontinence found that 12 errors related to internal sphincter integrity were made at MR imaging compared to only one at US (4).
Dr Rociu and colleagues used a 7-MHz transducer for endoanal US. The improvement with the 10-MHz crystal we use relates as much to near-field focusing as higher frequency. Probably of greater importance is examination in the prone position, which is substantially superior to the left lateral position for demonstrating tears related to obstetric trauma (5). The exact US anatomy of the external sphincter has long been controversial. Improved resolution has allowed a clear distinction between the longitudinal muscle and external sphincter.
Dr Rociu and colleagues have actually placed their marker for the external sphincter over the longitudinal muscle (figs 1, 7a), whereas the external sphincter is the less reflective band immediately peripheral to this layer. In figure 7a, which shows a US scan that is reported as normal, there is a break in this outer layer that corresponds exactly to the MR imaging defect in figure 7b. The distinction between external sphincter defects and scars is really just one of degree. If defects and scars of the external sphincter are considered together, US was correct in 15 of 17 surgically proved cases, and MR was correct in 16. This is not significantly different according to the Fisher exact test results. This study therefore demonstrated no statistically significant difference as to which technique was preferable to answer the main clinical question, namely, whether a surgically remedial tear in the external sphincter was present.
As yet, there is no endoanal US criterion for external sphincter atrophy, which remains a diagnosis made at MR imaging. There is no doubt that MR imaging provides more clearly defined images of the external sphincter, but, in our experience, fibrosis is more easily detected at US, and tears in the external sphincter at MR imaging are mostly based on seeing fat replacement in the torn muscle segment, as in figure 7b. The internal sphincter is very clearly seen at US, and it is easier to appreciate atrophy and small tears of this sphincter than at MR imaging.
In many ways, MR imaging and US are complementary, and we routinely use both modalities, tailoring them to the clinical question being asked. The strengths of the MR imaging endoanal coil, eloquently depicted in this study, can be used to further our understanding of endoanal US and vice versa. Comparison of techniques is commonly subject to bias, frequently because of particular familiarity and experience with one or another of the modalities being investigated—so much so that it is often this difference that is actually being tested (6).
REFERENCES
Department of Radiology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ Amsterdam, the Netherlands, e-mail: j.stoker@amc.uva.nl
We thank Drs Bartram and Halligan, both experts in the field, for their interest in our article (1). They comment on the use of a 7-MHz transducer in our study. At present, this transducer is widely used in endoanal US, and the results of our study are therefore of direct interest to those in the field. We agree that with the use of a 10-MHz transducer, the external sphincter may be more easily recognized, and this may influence the accuracy of endoanal US in the depiction of external sphincter defects and improve the reproducibility of the technique. This, however, needs to be substantiated in a comparative study of endoanal MR imaging and endoanal US.
In figure 7a, the marker for the external sphincter has not been placed over the longitudinal muscle, as Drs Bartram and Halligan presume, but is placed correctly over the external sphincter. Endoanal US is a real-time examination, and evaluation on the basis of one image can be difficult. In this patient, the linear structure lateral to the external sphincter was not a continuous structure in the superior or inferior direction. The correct position of the abbreviation "ES" (external sphincter) is also demonstrated in the corresponding endoanal MR image (fig 7b). In this figure, it is clear that the intersphincteric space is very small, and the external sphincter is close to the internal sphincter. Drs Bartram and Halligan have inferred from figure 7b that fat replacement is our major criterion for an external sphincter tear. However, we use multiple criteria, including disturbance of the architecture and changes in signal intensity, and would like to advise others also not to adhere to only the criterion of replacement by fat.
The major advantage of endoanal MR imaging is in the detection of external sphincter atrophy, which cannot be detected at endoanal US. In a recent study (2), we have demonstrated not only that the detection of an external sphincter defect at endoanal MR imaging is important, but also that external sphincter atrophy is likely to be another important finding for treatment planning. Patients with external sphincter atrophy at preoperative endoanal MR imaging had a substantially worse outcome than patients without external sphincter atrophy. An accurate visualization of the internal structure of the external sphincter is therefore mandatory, and, at present, this is possible only with endoanal MR imaging.
We agree with Drs Bartram and Halligan that experience and familiarity are important for any diagnostic technique, including both endoanal US and endoanal MR imaging, and this may influence the results of any study. Our experience is more toward endoanal MR imaging, while at St Mark’s Hospital the experience is more toward endoanal US. The findings in our study should therefore be compared with those in forthcoming studies on both techniques conducted at several centers.
REFERENCES
This article has been cited by other articles:
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  G. Engin Endosonographic imaging of anorectal diseases. J. Ultrasound Med., January 1, 2006; 25(1): 57 - 73. [Abstract] [Full Text] [PDF]
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