Incompetent cervix is characterized by painless cervical dilatation in the second trimester or perhaps early in the third trimester, with prolapse and ballooning of membranes into the vagina, followed by rupture of membranes and expulsion of an immature fetus. Unless effectively treated, this sequence tends to repeat in each pregnancy. Although the cause of cervical incompetence is obscure, previous trauma to the cervix (dilatation and curettage, conization, cauterization, or amputation) appears to be factor in many cases. In other instances, abnormal cervical development, including that following exposure to diethylstilbestrol in utero, plays a role.
The diagnosis of incompetent cervix is largely made based on a history of one or more prior midtrimester losses. There is little doubt, however, that ultrasound, especially transvaginal, is a useful adjunct for the diagnosis of cervical shortening or funneling of the internal os and in the early detection of cervical incompetence. Generally, cervical lengths less than 25 mm or less between 16 and 18 weeks’ gestation have been shown to predict preterm delivery in women with prior midtrimester losses.
The treatment of cervical incompetence is surgical, consisting of reinforcement of the weak cervix by some type of purse-string suture. Bleeding, uterine contractions, or ruptured membranes are usually contraindications to surgery. Cerclage should generally be delayed until after 14 weeks so that early abortions due to other factors will have naturally occurred. There is no consensus as to how late in pregnancy the procedure should be performed. The more advanced the pregnancy, the more likely surgical intervention will stimulate preterm labor or membrane rupture. For these reasons, some clinicians prefer bed rest rather than cerclage some time after midpregnancy. We usually do not perform cerclage after 24 to 26 weeks. Sonography to confirm a living fetus and exclude major fetal anomalies is done prior to cerclage. Obvious cervical infection should be treated, and cultures for gonorrhea, chlamydia, and group B streptococci are recommended. For at least a week before and after surgery, there should be no sexual intercourse.
Cerclage procedures done in the late midtrimester after cervical dilatation and effacement have already occurred, and are often called “emergency” or “rescue” procedures. Bulging membranes are associated with significantly increased failure rates and infection is always a threat. Amnio reduction (replacing the membranes up into the lower uterine segment or amniocentesis to remove fluid and relieve membrane prolapse) at the time of emergency cerclage may improve pregnancy prolongation. If there is a question as to whether cerclage should be performed, the woman is placed at decreased physical activity. Proscription of intercourse is recommended, and frequent cervical examinations should be conducted to assess cervical effacement and dilatation. Weekly ultrasonic surveillance of the lower uterine segment between 14 and 27 weeks may prove useful in some women. Unfortunately, rapid effacement and dilatation develop even with such precautions.
Two types of operations are commonly used during pregnancy. One is a simple procedure recommended by McDonald, the second is the more complicated Shirodkar operation (see Figures 36-1 and 36-2). Following the Shirodkar operation, the suture can be left in place if it remains covered by mucosa, and cesarean delivery performed near term. Conversely, the Shirodkar suture may be removed and vaginal delivery permitted.
FIGURE 36-1 McDonald cerclage procedure for incompetent cervix. A. Start of the cerclage procedure with a suture of number 2 monofilament being placed in the body of cervix very near the level of the internal os. B., C. Continuation of suture placement in the body of the cervix so as to encircle the os. D. The suture is tightened around the cervical canal sufficiently to reduce the diameter of the canal to 5 to 10 mm, and then the suture is tied. The effect of the suture placement on the cervical canal is apparent. Placement somewhat higher may be of value, especially if the first is not in close proximity to the internal os. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
FIGURE 36-2 Modified Shirodkar cerclage for incompetent cervix. A. A transverse incision is made in the mucosa overlying the anterior cervix, and the bladder is pushed cephalad. B. A 5-mm Mersilene tape on a Mayo needle is passed anteriorly to posteriorly. C. The tape is then directed posteriorly to anteriorly on the other side of the cervix. Allis clamps placed so as to bunch the cervical tissue to diminish the distance the needle must ravel submucosally facilitate placement of the tape. D. The tape is snugly tied anteriorly, after ensuring that all slack has been taken up. The cervical mucosa is then closed with a continuous chromic suture to bury the anterior Mersilene knot. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)
Success rates approaching 85 to 90 percent are achieved with both the McDonald and Shirodkar techniques. Thus, there appears to be little reason for performing the more complicated Shirodkar procedure. The Shirodkar procedure is often reserved for previous McDonald cerclage failures and structural cervical abnormalities. Success rates are higher when cervical dilatation is minimal and membrane prolapse absent.
Placing a suture, via an abdominal incision, at the level of the uterine isthmus has been recommended in some instances, especially in cases of anatomical defects of the cervix or failed transvaginal cerclage. The procedure requires laparotomy for placement of the suture and another laparotomy for its removal, for delivery of the fetus, or both. The potential for trauma and other complications initially and subsequently is much greater with this procedure than with the vaginal procedures.
Complications, especially infection, have been identified to be much less frequent when cerclage was performed by 18 weeks. When performed much after 20 weeks, there is a higher incidence of membrane rupture and associated intrauterine infection. With clinical infection, the suture should be cut, and labor induced.
There is no evidence that prophylactic antibiotics prevent infection, or that β-mimetic drugs are of any adjunctive value. In the event that the operation fails and signs of imminent abortion or delivery develop, it is urgent that the suture be released at once; failure to do so may result in grave sequelae. Rupture of the uterus or cervix may be the consequence of vigorous uterine contractions with the ligature in place.
Membrane rupture during suture placement or within the first 48 hours of surgery is considered by some to be an indication to remove the cerclage. Other clinicians permit observation with removal and delivery reserved for those women who develop infection. There are insufficient data upon which to base any firm recommendation, and the optimal management of such patients remains controversial.
For further reading in Williams Obstetrics, 23rd ed.,
see Chapter 9, “Abortion.”