| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Letters to the Editor |
Department of Radiology, Mount Sinai Hospital, Box 1234, One Gustave L. Levy Place, New York, NY 10029
Editor:
In their article in the September 1997 issue of Radiology, Dr Laing and colleagues (1) found low sensitivity and specificity for the intradecidual sign in diagnosing an early intrauterine pregnancy. Since I and my colleagues described the intradecidual sign in 1986 (2), however, I have always found the sign to be very effective, and our original description of high sensitivity and specificity for the intradecidual sign in diagnosing an early intrauterine pregnancy still holds true. After many years of experience, by observing the work of technologists and residents, with the advent of equipment that shows increases in shades of gray and dynamic range, and with endovaginal technology, I can understand that problems sometimes arise in finding the intradecidual sign and obtaining a diagnostic image. I suggest the following tips for solving these problems:
1. Demonstration of the intradecidual sign requires demonstration and recognition of the uterine cavity line, which sometimes is very thin and subtle. It may be easier to identify by tracing it upward from the uterine cervix, since the uterine cavity line is almost always visible in the cervical area.
2. Modern ultrasonographic (US) equipment that shows a high number of shades of gray and wide dynamic range may cause a "flattening" appearance due to low contrast, and the uterine cavity line may become difficult to recognize (Fig 1). The contrast can be improved by changing the gray-scale map, turning down the dynamic range while turning up the contrast knob (on some machines), or selecting a certain pre- or postprocessing curve. In some cases, the uterine cavity line is better visualized with transabdominal scanning than with endovaginal scanning. In fact, I have found that the uterine cavity line is more clearly visualized with older equipment than with unadjusted newer equipment. In Dr Laing and colleagues' article, the uterine cavity line is not visible in six of 10 images. Without a uterine cavity line, an image cannot be diagnostic. A nondiagnostic image reviewed by any person still cannot be diagnostic.
|
4. When the intrauterine fluid collection (or sac) shows a small "beak" (2) that connects with or points toward the uterine cavity line, it is highly suggestive of a fluid collection within the uterine cavity (ie, possibly a pseudogestational sac). Of four pseudogestational sacs discussed in Dr Laing and colleagues' article, one shows fluid clearly within an elongated uterine cavity and was correctly diagnosed as a pseudogestational sac. But the three other cases all show such a beak sign (figs 6a, 6c, and 7b in the article by Dr Laing and colleagues [1]), and all were misinterpreted as intrauterine pregnancies. An exception to this is when a major focal myometrial contraction or a leiomyoma causes compression on a gestational sac; a beak sign may also be seen in this case. No evidence of a major focal myometrial contraction or leiomyoma is evident in Dr Laing and colleagues' figures 6a, 6c, and 7b.
An endometrial cyst may occasionally be confused with an intrauterine pregnancy. Endometrial cysts are usually located in or close to the basal layer of the endometrium (3), and the cyst walls are very thin. This is different from a gestational sac, which is usually closer to the uterine cavity line and has thick echogenic walls because of the trophoblastic tissue around it.
Dr Laing and colleagues stressed the importance of the double decidual sac sign in diagnosing an intrauterine pregnancy before the appearance of a yolk sac, and they considered the double decidual sac sign to be most useful at 4–6 weeks gestational age. The double decidual sac sign is based on the presumption that the uterine cavity is so displaced and curved by the gestational sac that two layers of decidua wrap around the gestational sac. A finding of double echogenic rings has been designated as the double decidual sac sign. In fact, in early pregnancy the uterine cavity line remains straight (Fig 2), even up to 7 weeks gestational age. The double echogenic rings actually do not represent two layers of decidua. The outer echogenic ring represents only the basal echogenic layer and not the entire layer of decidua. The inner echogenic ring represents the gestational sac, which is echogenic and thick walled because of choriotrophoblastic tissue.
|
But can we diagnose an intrauterine pregnancy by simply finding an echogenic thick-walled sac in the uterus? Not necessarily. Although such a finding is unlikely to represent an endometrial cyst, it can be seen in a pseudogestational sac (2) (fig 2a and 2b in ref 4). Hence, we still need an intradecidual sign to diagnose an intrauterine pregnancy.
In summary, the intradecidual sign is a well-established sign of an intrauterine pregnancy. When an echogenic, thick-walled sac is seen clearly within a thickened endometrium, it is highly specific for an intrauterine pregnancy. I believe that using a relatively high contrast setting to bring out the uterine cavity line, good experience, effort in scanning, and correct interpretation of the findings all play important roles in obtaining a diagnostic image.
References
Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, 75 Francis Street, Boston, MA 02115
We appreciate Dr Yeh's interest in our study (1) and are well aware of his previous article (2) in which he described the intradecidual sign with transabdominal US. This seems to be a useful sign, as it is based on a reasonable anatomic relationship; that is, the gestational sac is within the decidua and not within the uterine cavity. We undertook our study to validate the usefulness of the intradecidual sign when tested by multiple observers and to confirm its effectiveness with transvaginal US. We anticipated that with the use of transvaginal US, the intradecidual sign would be reliable for diagnosing an intrauterine pregnancy prior to visualizing the yolk sac. Unfortunately, our results were disappointing, which led us to conclude that this sign is not sufficiently reliable to distinguish an early intrauterine pregnancy from a pseudogestational sac.
Much of the problem in applying the intradecidual sign seems to be in identifying the thin echogenic line that represents the potential uterine cavity. Obviously, one needs to identify this line clearly to determine the relationship of the gestational sac to the uterine cavity. Dr Yeh provides useful points about how to identify the uterine cavity line, such as trying to follow it from the cervix, changing the contrast in the image, and scanning in different planes. Although our report was a retrospective, blinded review of the images, approximately one-third of the findings were obtained by two very experienced sonologists, who at the time of the examination attempted specifically to obtain images that demonstrated the relationship of the intrauterine fluid collection to the uterine cavity. Despite their knowledge, experience, and effort in demonstrating this sign, it was often not possible to identify the uterine cavity.
In our introductory and discussion comments, we mentioned briefly the double decidual sac sign and stressed its usefulness with a transabdominal approach. We agree with Dr Yeh that this is not a useful sign in transvaginal scanning because the yolk sac (which is more reliable for diagnosing an early intrauterine pregnancy) is visible normally before the double decidual sac sign is present. Therefore, we concur that, when using a vaginal approach, the double decidual sac sign is not helpful for identifying early intrauterine pregnancies.
Dr Yeh asks a very appropriate question, one that is at the core of this issue. We paraphrase, but the crucial question is before a yolk sac or embryo is identified, can one diagnose an intrauterine pregnancy with complete reliability if a spherical or slightly ovoid intrauterine fluid collection has a thick, echogenic periphery, regardless of the intradecidual sign? It is tempting to do so. It is interesting that 91 of 93 (98%) of the consecutive cases of a small intrauterine fluid collection in our series were intrauterine pregnancies. In our experience, most pseudogestational sacs tend to be elongated, sometimes with a beaked appearance, as Dr Yeh describes. However, with the use of these criteria, an important minority of pseudogestational sacs may be difficult to distinguish confidently from true gestational sacs. Although one will be wrong infrequently, we agree with Dr Yeh that one cannot diagnose an intrauterine pregnancy with 100% accuracy on the basis of the presence of a thick, echogenic periphery.
We disagree with Dr Yeh's statement that the intradecidual sign is a well-established sign of an intrauterine pregnancy. We are aware of Dr Yeh's study and the good results he reported, but we are unaware of any other study that either confirms or refutes the reliability of this sign. When we undertook our study, we were optimistic that we would confirm its reliability; unfortunately, our results proved otherwise. We do not believe that the anatomic basis of the sign is at fault, but we suspect that in some patients it is difficult, if not impossible, to identify the uterine cavity line confidently. Until a well-designed, completely prospective study confirms the reliability of the intradecidual sign with transvaginal US, sonographers and sonologists should be aware of potential limitations of applying this sign in clinical practice.
References
This article has been cited by other articles:
|
|
 |
|
  G. Chiang, D. Levine, M. Swire, A. McNamara, and T. Mehta The Intradecidual Sign: Is It Reliable for Diagnosis of Early Intrauterine Pregnancy? Am. J. Roentgenol., September 1, 2004; 183(3): 725 - 731. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| RADIOLOGY | RADIOGRAPHICS | RSNA JOURNALS ONLINE |
