CHAPTER 21Cervical Failure to Dilate
The uterine cervix, with the vulva and the vestibular fold, forms the natural barrier that separates the uterus from the external environment, thus protecting it from contamination. Some authors consider the cervix to include the most caudal part of the uterus, but because it is the main topic of this chapter we will refer to it as a single structure. The uterine cervix is a complex organ that works under neural, hormonal, and myogenic influences and is amazingly able to dilate during parturition. A brief review of the anatomy is necessary before we explore the different scenarios that the clinician may experience when the cervix fails to dilate. Some reported anomalies that interfere with the ability of the cervix to relax and dilate will also be discussed.
ANATOMY
The cervix averages 5 to 7 cm in length and 3 to 4 cm in width.1,2 The cervix communicates cranially with the body of the uterus and caudally with the vagina, into which the caudal cervical portion projects. The cervix has a thick layer of smooth muscle and is rich in elastic fibers. Its degree of tone depends on the presence of progesterone, and its relaxation is determined by circulating estrogens during estrus. A distinctive feature of the mare’s cervix is the complete absence of cervical rings, such as those found in cows. The cervix of the mare is arranged in longitudinal mucosal folds that are continuous with the endometrial folds of the uterine body and horns.3 Each dorsal and ventral fold may continue onto the floor of the vagina as a prominent frenulum.4 Among its different functions are the production of a large amount of mucus that may act as a lubricant for breeding purposes (especially natural breeding) or as a sealant during pregnancy, and the occlusion of the uterine lumen so that it is impermeable to foreign material and bacteria. The other important function is the dilation and expansion to its maximal diameter to allow the passage of the foal during parturition.
CONDITIONS IN WHICH THE CERVIX FAILS TO DILATE
The cervix does not normally dilate during two physiologic conditions in the mare. The first occurs when the mare is in the diestrus period of the estrous cycle, which falls between day 1 (day 0 = ovulation) and day 14 or 15, before the development of the follicular wave that will result in the next estrus period. The second physiologic condition in which the mare does not relax the cervix occurs during pregnancy. From the moment the mare recognizes her pregnancy status (maternal recognition between day 14 and 16), until hours before parturition when cervical dilation occurs due to hormonal changes to permit the delivery of the foal, the cervix is very tight. The hormonal changes that cause cervical relaxation include a decrease in progesterone and an increase in estrogens promoted by the cortisol released by the fetal adrenal glands.5 It is worth noting that this closing of the cervix is due to the progesterone produced by the primary corpora lutea during diestrus and the progesterone/progestagens that are produced by the secondary corpora lutea and placenta when pregnant.
Adhesions
Adhesions usually occur in response to trauma or injury of various kinds. Any cervical injury can result in fibrous adhesion formation.6
Adhesion formation can be viewed as a variant of the normal physiologic healing process. Usually adhesions start with a fibrin matrix that occurs during coagulation. Over a period of a few days (in the rat model) a variety of cellular elements become encased in the fibrin matrix, which is gradually replaced by vascular granulation tissue containing macrophages, fibroblasts, and giant cells. By 4 days postinjury, most of the fibrin is gone and more fibroblasts and collagen are present. During days 5 through 10, fibroblasts align within the adhesion. At 2 weeks, the relatively few cells present are predominantly fibroblasts. At 1 to 2 months, the collagen fibrils organize into discrete bundles. Eventually, the adhesion matures into a fibrous band, often holding small calcifications. Extensive adhesions often contain blood vessels.7
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