Cervical Incompetence and Cerclage - 5 - Cervical Assessment
In today’s post, let’s put the simplest case behind us first – the patient who appears to have cervical incompetence by past obstetrical history. In most instances, the most sensible option is to simply place a cerclage in early pregnancy usually around 13 weeks. This timing was originally recommended (prior to ultrasound) because most patients who were going to spontaneously miscarry a pregnancy will usually do so by the end of first trimester – including most babies with chromosomal abnormalities – fetal heart tones could be detected to confirm ‘viability’ and, it is exceedingly rare to have any significant cervical change prior to this time that would lead to miscarriage in and of itself.
In recent years, we have another reason that this is a good time to place a cerclage – the opportunity to perform combined first trimester screening for aneuploidy and to obtain a definitive diagnosis by chorionic villus sampling (CVS) if the screening result appears to place the patient ‘at risk’ for a chromosomally abnormal baby prior to placing a cerclage. The patient must be told that this screening test will not detect all chromosomally abnormal babies, and that babies can have other problems not detectable by ultrasound at this time, but it certainly offers significant reassurance.
If the past obstetrical history raises some doubt as to the diagnosis of cervical incompetence, or if the patient simply prefers, the alternative is to serially follow cervical length in the hope of detecting changes that would permit timely placement of a cerclage should the need arise. Under these circumstances, I usually begin cervical assessment by transvaginal ultrasound at about 16 weeks with the interval of testing determined by the findings at a given visit. Incidentally, even after a late first trimester cerclage has been placed, it has been my approach to serially follow these patients by transvaginal ultrasound through midtrimester. The advantage of having the cerclage in place is that it often provides a margin of safety that allows additional intervention to prevent extremely preterm delivery before advanced cervical changes prevent that opportunity.
With regard to the transvaginal ultrasound assessment of the cervix, one of the points I did not discuss in our last post was what constitutes “significant cervical change.” I do not think anyone would argue that membranes bulging at the level of the external cervical os or that a patient with ‘risk factors’ who starts with a 40mm cervix and presents two weeks later with an endocervical length of 10 mm with membranes ballooning to that point in the cervix constitute problems. Nor would anyone argue that the patient who has a very ‘suspicious history’ for cervical incompetence but maintains a cervical length of 40 mm without any loss of integrity at the internal os throughout midtrimester is not likely to be a problem. However, what about the ‘in between’ cases?
In the latter, there are lots of shades of gray and multiple scientific publications dating back 20 years or more to provide some guidance to suggest the diagnosis of cervical incompetence. Personally, I rely on three factors: change from baseline (33% or more decrease), overall length (usually < 26 mm and definitely < 16 mm), and distention of the endocervical canal by membranes (indicating loss of integrity at the internal cervical os). And, it is not at all unusual to have ‘abnormalities’ in all of these parameters in the woman with an incompetent cervix. Although it is controversial, I will frequently ‘challenge’ the integrity of the internal os by exerting slow, steady fundal pressure on the uterus while observing the cervix transvaginally by ultrasound in women in whom I am very suspicious of having cervical incompetence. If by doing so, the internal os opens and membranes then extend into the cervical canal, significantly shortening the cervix, I am much more likely to view this dynamic event as an abnormality consistent with cervical incompetence. On the other hand, if a patient has a cervical length of only 25 mm at 16 weeks, but retains integrity at the internal os and does not shorten with funneling when challenged, I am not at all adverse to simply following that patient over time…