SeÁn A. Finan, Angus O. McKinnon
Prepartum Complications of Pregnancy
SeÁn A. Finan, Angus O. McKinnon
Complications in late gestation pose a diagnostic challenge for the practitioner because a set of differential diagnoses must be considered in addition to those that would be considered for the same problem in a nonpregnant animal. Problems arising in broodmares nearing term can be distressing for owners because some conditions threaten the viability of the pregnancy, and occasionally difficult decisions have to be made when the best interests of the fetus and the mare are at odds.
Mummification and Maceration
Fetal mummification is a rare event in the mare but can occur following the death of the fetus in mid to late gestation. The uterine environment remains sterile, and fetal fluids are absorbed without decomposition of the fetus. The fetus becomes dehydrated and compacted, and the skin and fetal membranes take on a leathery appearance. Fetal mummification is most commonly associated with twin pregnancies in which one fetus dies from placental insufficiency. Twin reduction techniques performed in midgestation, such as ultrasound-guided intracardiac injection and craniocervical dislocation, have the ability to produce a mummified fetus. Some cases of singleton mummified fetuses have been associated with use of exogenous progestogens, and it has been postulated that the progestogens may prevent normal expulsion of a dead fetus.
Diagnosis of this condition can be difficult. Clinical signs are varied and include premature lactation and prolonged pregnancy. Most commonly, fetal mummification is diagnosed on delivery of a mummified fetus or as an incidental finding during routine examination. Rectal palpation may reveal the uterus contracted around the fetus so that bony segments are palpable. Ultrasonography and hysteroscopy have been used to aid in diagnosis.
Fetal maceration is also rare in the mare and occurs when a fetus dies and is not expelled but undergoes bacterial contamination and autolysis. If fetal death occurs at more than 3 months’ gestation, ossification may not allow resorption of the fetal skeleton. Despite the expected endotoxin production of gram-negative organisms, systemic disease is rare in mares with a macerated fetus. The cause of fetal death is often unknown, but conditions such as ascending placentitis, twin pregnancies, and anomalous conditions may result in the fetus not being expelled because of uterine inertia, fetal malposition, and incomplete cervical dilatation.
Clinical signs may include a vaginal discharge and palpation of a crepitant uterine horn. Rectal and transabdominal ultrasonography, as well as hysteroscopy, can aid in diagnosis.
Treatment of both conditions is aimed at cervical dilation and manual removal of the fetus or fetal remnants. Administration of prostaglandins several days before attempted removal of the fetus may aid in cervical relaxation. Application of prostaglandin E2 gel topically to the cervix or topical application of the injectable form of butylscopolamine around the cervix immediately before attempted removal may aid cervical relaxation.
Uterine lavage and both intrauterine and systemic administration of antimicrobials selected by uterine culture and sensitivity results are indicated after removal of the fetus. The prognosis for future breeding can be determined by endometrial biopsy.
Uterine hydrops conditions result when excessive fluid accumulates in the allantois (hydrops allantois) or amnion (hydrops amnion) of mares in late gestation. Although hydrops allantois is reported more frequently, both conditions are rare. These conditions can be seen in primiparous mares but are more typically seen in multiparous mares in late gestation. Approximately 30 L of fetal fluids is expected in normal mares at term, varying in composition from 8 to 18 L of allantoic fluid and 3 to 7 L of amniotic fluid. Extreme volumes of fetal fluid accumulate in mares with hydrops, with up to 220 L of fetal fluid described in one case of hydrops allantois. The cause of these conditions is obscure, but they have been associated with congenital abnormalities of the foal in cases of hydrops amnion and torsion of the umbilical chord; placentitis and placental abnormalities have been implicated in cases of hydrops allantois.
Mares typically present in late pregnancy with dramatic abdominal enlargement that has developed over a few days to 2 weeks. The mare may be inappetent, have reduced fecal output, have labored breathing as a result of the pressure exerted by the uterine fluid on the diaphragm, and have ventral edema. Some mares may be reluctant to move or have difficulty walking, whereas others may become recumbent. Spontaneous abortion occurs in some cases.
The history and speed of onset, together with examination findings, can yield a tentative diagnosis of hydrops. A rectal examination usually reveals a grossly distended uterus, which may protrude into the pelvic cavity. This makes examination difficult, and often it is not possible to palpate a fetus through the voluminous fetal fluids. Transrectal and transabdominal ultrasound can be useful in confirming diagnosis by ruling out twin pregnancy and assessing fetal viability. Values for maximal normal vertical depths of allantoic fluid (47 to 221 mm) and amniotic fluid (8 to 185 mm) in late gestation have been published; however, ultrasonography does not always allow differentiation between the two fluid compartments. The specific gravity and electrolyte composition of allantoic and amniotic fluid differ substantially, and these parameters may be used to distinguish hydrops allantois from hydrops amnion. Whether and to what degree the compositions of these fluids change in the course of dropsical condition is unknown. In general, the prognosis for a positive outcome for the foal is poor, and the possibility of an adverse outcome for the mare increases as the condition develops, with some of the complications including body wall, prepubic tendon, and uterine rupture.
When hydrops has been diagnosed, treatment should not be delayed. The treatment goal is termination of pregnancy by slow, gradual release of fetal fluids to allow the mare to adapt systemically to the sudden change in abdominal pressure. Typically this is achieved by manual dilation of the cervix and controlled siphoning of fetal fluids by use of a stomach tube. The expulsion of fluid on dilation of the cervix may be less than expected, given the volume of fluids, because of uterine atony that has resulted from stretching of the myometrium. Similarly, use of oxytocin may also yield disappointing results for the same reason. Caution should be exercised with regard to the speed at which the fetal fluids are released to avoid hypovolemic shock in the mare, thought to result from pooling of blood in the abdomen secondary to the sudden decrease in pressure that has chronically compressed the abdominal blood vessels. Collapse has been reported in mares in which a high volume of fluid was drained over 30 minutes, so operator patience is necessary. Mares should have at least one large-bore venous catheter in place before initiating fluid evacuation to facilitate prompt fluid therapy should it be necessary. During this procedure, when the fetus can be palpated, it should be removed. Generally, the fetus is small and not mature enough to survive; however, if the fetus is large, dystocia may result, and care must be taken to avoid damage to the cervix. The fetus is commonly alive, and its preterm delivery can be a disturbing experience for those present. Intravenous euthanasia of the fetus should be avoided while the umbilicus is still attached to the placenta inside the mare. The mare should be monitored for retention of fetal membranes, which is a common sequel to hydrops.
Although it is difficult to draw conclusions from the limited number of cases of hydrops reported, it appears that future breeding soundness is unaffected, with one publication reporting six of eight mares delivering normal, healthy foals subsequent to hydrops.