Managing dystocia in the mare (Proceedings)
Dystocia is a significant event in equine reproduction. In one study, dystocias were the second most common cause of equine pregnancy loss, accounting for 19% of losses (second only to infectious placentitis at 33%; Giles et al. 1993). Approximately 4% of Thoroughbred births and up to 10% of Belgian Draft births are dystocias (Vandeplassche 1993). Every equine dystocia is an emergency and requires clinical skill and quick decisions (Norton et al. 2007).
Stage One DystociaThe normal course of stage one of parturition is for the chorioallantoic membrane to rupture at the cervix (in the area of the "cervical star") and the fetus to exit through the birth canal via this hole in the chorioallantois. Detachment of the chorioallantois should happen after expulsion of the fetus. Delays in normal stage II labor, however, increase the chances of premature placental separation and consequent asphyxiation of the fetus. In some cases, the chorioallantois will detach before rupturing and appear at the vulvar lips as a red, velvety, fluid-filled bag. This is known as premature placental separation, or a "red bag" presentation, and is an absolute emergency. Every second counts, as the fetus is without a source of oxygen due to detachment of the chorionic microvilli from the endometrium. The bag must be manually ruptured immediately to give the fetus access to outside oxygen. If available, supplemental oxygen should be given. These foals, if viable, will likely be "dummy" foals.
Stage Two Dystocia: Be Clean, Be Gentle, Use Lots Of Lube!!!
Mutations must be done with clear goals and a workable plan. Do not start moving fetal limbs around haphazardly. Many decisions in the face of a dystocia will depend on fetal viability. Limb withdrawl, suckle reflex, eye reflexes, umbilical blood flow and anal tone can be assessed to determine fetal viability. Assuming a viable fetus, time is of the essence. Minutes count. There exists a strong inverse correlation between length of dystocia and fetal survival, with only 13.6 minutes separating the mean times of those foals that survived and those that did not (Byron et al. 2002). Another retrospective study reported a mean time for stage II labor of 71 minutes for those neonates delivered alive and 44 minutes for those actually discharged alive (Norton et al. 2007). Even if the fetus is dead, a prolonged dystocia can lead to significant cervical damage and/or ischemia to the caudal reproductive tract leading to scarring, adhesion formation and possibly systemic disease.
Many different kinds of effective lubrication are available for correcting dystocias. Care should be exercised to remove as much lubricant as possible from the uterine lumen following mutations (via uterine lavage). This is especially important for the mare that requires a c-section.
On the farm, place the mare in a clean area that will allow the mare to get up and down without trapping the clinician. Most mutations are performed after first repelling the fetus back into the uterus to give more room to maneuver the fetus. Performing mutations can be tiring, so it is useful to have a skilled colleague to share the task.
If referral is an option, early referral is desirable. Time should be carefully monitored while attempting to manually correct a dystocia. With a live fetus, manipulations should be limited to no more than 30 minutes. If the fetus is dead, manipulations ideally should not exceed an hour. If referral is not an option, one should strongly reevaluate the situation after an hour to determine the best outcome for the mare.
Sedation of the mare may help, but in the case of a live fetus, should be used cautiously in order to avoid detrimental effects on the fetus. Acetylpromazine (2-3 mg/100 kg bw IV) has minimal effects on the fetus. Xylazine (0.17 mg/kg bw)-lidocaine (2 or 3 ml) q.s. to 8-10 ml in normal saline can be used. Xylazine or detomadine are best used in conjunction with an agent such as butorphanol as alpha agonists used alone may cause mares to become hypersensitive in the hind quarters (Frazer et al. 1999a).
General anesthesia for dystocia management on farm should be reserved for cases where referral is absolutely not an option. An epidural may be given, but requires extra time, does not prevent myometrial contractions, and increases the risk of the mare becoming recumbent. Maintaining a mare under general anesthesia on the farm using intravenous anesthestic agents (xylazine, ketamine guaifenesin) can facilitate manipulation of the fetus and removal. This is best achieved if the mare's hindquarters can be hoisted using ropes over the beam of a barn, a branch or with a front end loader. If hoisting the mare is not possible, general anesthesia may be of little benefit in correction of a dystocia. A marked amount of lubrication is pumped into the uterus. Once the mutations are successful in placing the fetus into a normal presentation, position, and posture, the mare's hindquarters are lowered before placing traction on the fetus. Never at any time are mechanical aides used to pull the fetus from the mare. Two strong people, at most, should be sufficient to pull the fetus. The fetus should be pulled in a downward arc from the mare's vulva (not straight out, not straight down). Initially traction should be placed on one leg, then the other leg, until the shoulders clear the birth canal. If the fetus will not move under these conditions, the situation must be reevaluated to determine why the fetus is not moving (ie, flexed hind legs, hip lock). If the blockage cannot be corrected efficiently via more mutations and lubrication, fetotomy or c-section are the next options.
Feto-maternal disproportion: While very common in bovine dystocias, this phenomenon is very uncommon in equine dystocias (< 2%; Frazer et al. 1997). The size of the mare's uterus plays a much greater role in limiting the size of the fetus (Allen et al. 2002). Thus, it is possible and safe to inseminate a pony mare with semen from a full-sized stallion. The resulting fetus will be born the size of a normal pony neonate, but then quickly catch up in growth (Allen et al. 2004). When this does occur, it can only be resolved with c-section or fetotomy.
Hydrocephalus: This condition reportedly occurs with some frequency in horses (5% of referred dystocias in one hospital, Frazer et al. 1997), especially pony breeds (Vandeplassche 1993). Hydrocephalus occurs when increased intracranial pressure causes a deformity of the skull bones greatly increasing the size of the skull. This condition is to be differentiated from encephalocele, which is an accumulation of fluid in the lateral ventricles that has forced a soft sac through a split in the skull, forming an increase in head size with no bony involvement. Correction of the dystocia requires reduction of the head size with a palm knife, removal of the dorsal part of the skull via a partial fetotomy, or caesarean section.