Chapter 16:       Summary

 

In this thesis I have endeavoured to validate the following assertions:

 

·        CPC are a benign anatomical variant and do not represent a brain malformation;

·        CPC occur in about 1% of the normal pregnant population;

·        CPC are in general associated with a 3 - 4 fold increase in the risk for T18 to 1/601 which is lower than the loss rate for amniocentesis and therefore not high enough to warrant amniocentesis;

·        other aneuploidies are associated much less frequently: the change in risk is not clinically significant and is probably due to chance: this includes Down syndrome;

·        CPC characteristics vary greatly in both normal and aneuploid fetuses;

·        cyst size greater than 10mm increases the risk of T18 to 1/325, which is still lower than that for amniocentesis;

·        bilaterality increases the risk of T18 to 1/1542, which is still lower than that for amniocentesis;

·        cyst persistence beyond 23 weeks increases the risk of T18 to 1/764, which is still lower than that for amniocentesis;

·        isolated cysts increases the risk of T18 to 1/652, which is still lower than that for amniocentesis;

·        the presence of other abnormalities increases the risk of T18 to 1/95 which is greater than that for amniocentesis (but which will vary according to the type and number of abnormalities).

·        the 18-20 week ultrasound only appears to confer measurable benefit if it is done well: training, equipment and available time must be optimised if routine scanning is to be cost-effective;

·        the use of a pro-forma, such as the Geelong Hospital’s “2nd & 3rd Trimester  Obstetrical Ultrasound Examination” may assist sonographers in performing a thorough survey;

·        careful and thorough attention to the extremities, the heart and the head of the fetus is advised whenever CPC, or other “soft signs” are found on antenatal ultrasound;

·        the use of a special pro-forma, such as the “Ultrasound Chromosomal Markers”, may assist sonographers in the exclusion of T18 or other aneuploidy;

·        if an adequate scan cannot be obtained due to technical factors, a rescan might be warranted when the fetus is larger, and when other signs of T18 such as IUGR, enlarged cisterna magna and cardiac defects are more easily detectable;

·        maternal serum tests may assist in the discrimination of aneuploidy, but their full role is yet to be determined when used in conjunction with the 18-20 weeks scan;

·        amniocentesis for isolated cysts will result in the loss of over 23 normal fetuses for each fetus with T18;

·        detection of T18 in fetuses with CPC at 18-20 wks will have only a small affect on the social and medical costs of T18;

·        fetuses with T18 will often present in the 3rd trimester with fetal distress, IUGR and more easily identified structural abnormalities;

·        the awareness of the presence of T18 can affect management in the complicated late pregnancy and in care of the newborn: rapid karyotyping may be useful in the compromised fetus with any soft signs.

 

 

The offering of prenatal karyotyping remains controversial, but the low incidence and high spontaneous mortality of T18 should put much of this controversy in perspective.  Advocates of amniocentesis need to be aware of that ultrasound skills are continually improving for the detection of abnormalities and to consider the cost-benefit analysis of detecting a disorder with the poor prognosis of T18. 

 

Without doubt, due to the potential loss of many normal fetuses and the great expense of the protocol, isolated CPC should not indicate karyotyping.  Amniocentesis should be offered only when the risks are higher than the loss rate for amniocentesis by that physician at the centre where the procedure is to be performed.  The mother should be informed of the physical risks of the procedure to a normal fetus, the likelihood of spontaneous death and still-birth in T18, and many would consider that she should express a willingness to terminate the pregnancy if the result is abnormal.  The psychological effects of abortion in the late second trimester should not be underestimated89, 156,232. 

16.1     Conclusion

In order for an older paradigm to be replaced it must be discredited and the new one accepted by the community.  The ultrasound community tried to find significance in the physical nature of the CPC, but the characteristics of size, bilaterality and persistence, have been shown to offer no assistance.  The new paradigm is that the CPC remains a soft sign for T18, irrespective of its characteristics.  CPC are a sign, like a speed limit sign on a roadway: a big sign, many signs, signs on both sides of the road – the speed limit is the same.  And just as the speed limit remains in force after the sign is no longer visible, the risk of T18 remains present despite cyst regression.  We should look beyond the sign to the areas around.  We should assiduously exclude other abnormalities, and consider a rescan  when technical difficulties limit complete demonstration of the fetal anatomy.  The presence of a CPC should not be ignored, as its presence can provide useful information later in pregnancy, should complications arise. 

 

The improving resolution of ultrasound equipment has resulted in quandaries for sonographers as finer detail reveals normal variants in anatomy that previously were invisible and unknown.  No doubt in the future other controversies will arise, similar to CPC.  We must not be afraid of these controversies, but we must analyse them carefully, while keeping an eye on the bigger picture of health and society.  We must use reason tempered by common-sense as well as scientific methodology, and must not allow ourselves to be caught in the groove of an untrue or unhelpful paradigm.