Managing dystocia in the mare (Proceedings)

Introduction

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 Dystocia

The 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.

Uterine torsion: Uterine torsion is relatively common in late-term mares. Although most occur prior to 10 months gestation, uterine torsion can occur at parturition. It is common for maldisposition of the fetus to accompany the torsion. Torsions may be diagnosed via transvaginal/cervical palpation, but may be more easily diagnosed by transrectal palpation of the broad ligament. The most effective treatment is surgical. If surgery is not an option, rolling the mare may be attempted using the "flank on the plank" technique. This may have complications including torsion of the intestines or further torsion of the uterus.

Twins: The frequency of twin dystocias has greatly decreased since the advent and adoption of ultrasound in equine veterinary reproduction. Diagnosis during dystocia requires that the clinician identify two separate fetuses via transcervical palpation (e.g., two heads, two tails, three forelimbs or three hind limbs, etc), to differentiate this condition from a transverse singleton presentation. Delivery via mutations followed by manual assistance is possible in some cases. The parts belonging to each separate twin must be carefully differentiated and one twin must be repulsed back into the uterus while the other twin is mutated into the correct disposition for delivery. Once one twin is delivered, the process is repeated with the second fetus. Depending on fetal viability and financial concerns, c-section or fetotomy may also be considered.

Maldisposition: Transverse presentations: In general, transverse presentations are very rare (0.1% of pregnancies; Vanderplassche 1987), but when they occur they are very difficult to correct (8% of light breed and 18% of draft breed dystocias referred to one referral clinic were transverse presentations; Frazer et al. 1997). Ventro-transverse presention is much more common than dorso-transverse (Vanderplassche 1987, Frazer et al. 1997). In the case of a fetal transverse presentation, there is no "pregnant" horn versus a "non-pregnant" horn. The fetus is in a bicornual position, taking up roughly equal space in each horn, with the majority of the pregnancy held in the body of the uterus. Since these cases may present as multiple feet and or muzzle in the birth canal, it is very important to differentiate this condition from twins. Ventro-transverse presentations can only be corrected by mutations with great difficulty. Dorso-transverse presentations are virtually impossible to correct manually. A transverse presentation warrants a c-section (preferred) or fetotomy.

Caudal presentations: Caudal, or posterior, presentations are also quite rare (making up 1% of equine pregnancies; Vanderplassche 1987), but also quite difficult to correct (comprising ~15% of dystocias referred to a referral hospital; Vanderplassche 1987, Frazer et al. 1997). Nearly all cases involve either bilateral hip flexion (true breech) or bilateral hock flexion. Roughly half of fetal caudal presentations are also malpositioned (dorso-ilial or dorso-pubic), making correction that much more difficult. Delivering a live foal in caudal presentation is unlikely due to pressure on or premature rupture of the umbilicus leading to fetal asphyxiation. In some cases it is possible to deliver these fetuses with mutations alone or with the assistance of a partial fetotomy. In the case of a hip flexion, the fetus must first be mutated to a hock flexion, and then to full extension. Correction of the hock flexion may be particularly dangerous as the risk of perforating the uterus during this mutation is high. A partial fetotomy cut just below the hock joint through the distal row of tarsal bones may facilitate the removal of the fetus. A c-section is also a good option, even in the case of a dead foal.

Cranial presentations: Cranial, or anterior, presentations account for 98.9% of dystocias due to maldispositions. The cranial presentation, itself, is the correct presentation. Dystocias occur in the cranial presentation when position and/or posture are incorrect. Many of these are amenable to correction on the farm and do not require referrals to tertiary clinics. Some are still very difficult, however, so remember that increased time and vaginal/cervical manipulation are undesirable for both fetus and mare. If it is possible, sometimes a quick referral is the best decision. Different combinations of malpositioning and posture deviations are discussed below:

Head and neck flexion: This is the most common maldisposition in referred cases (Vanderplassche 1987, Frazer et al. 1997). Due to the length of the fetal head and neck, this condition can be very difficult to correct. Correction involves repulsing the entire fetus back into the uterus before attempting to bring the head and neck into the extended position. Care should be taken to palpate for facial abnormalities and mobility of the neck to try to diagnose wry neck. A fetus with wry neck will not be able to be corrected with mutation; the practitioner must perform a partial fetotomy or c-section.

Two fetotomy options are available. The neck may be severed directly, or the opposite forelimb may be removed. The latter course may allow the practitioner more room to reach the flexed head. In the case of neck amputation, the cut should be made as close to the fetal thorax as possible. In the case of forelimb removal, all efforts should be made to cut proximal to the scapula, removing the entire scapula. If unsuccessful in these attempts, scapular remnants must be removed with a fetotomy (palm) knife. Leaving these bony remnants leaves the mare at very high risk for reproductive tract trauma.

Foot-nape posture: One or both forelimbs are postured over the head of the fetus. There exists a strong risk of perforation of the dorsal aspect of the cervix and vagina, creating a rectovaginal fistula. If this occurs, is not corrected, and the fetus is expelled in this posture, it may rip through the rectovaginal shelf and anal sphincter, creating a third degree perineal laceration (a cloaca, in effect). (Treatment of this condition in the postpartum mare will be discussed in the next lecture). To correct this malposture the fetus must be repelled into the uterus and the forelimbs placed under the head.

Incomplete elbow extension: This malposture is likely to be the cause of dystocia when the fetal hooves are observed at the same level as the fetal muzzle. The fetus must be repelled into the uterus and the forelimbs extended to raise the elbows over the pelvic rim. Traction should then be placed on the fetus one limb at a time, alternating, until the fetal shoulders clear the birth canal.

Carpal flexion: This condition may be unilateral or bilateral. The offending carpus is usually at the pelvic inlet. Correction via mutation is achieved by first repulsing the fetus into the uterus. The carpus is grasped with one hand and rotated laterally. The fetlock and hoof are grasped with the other hand and rotated medially and caudally (caudal in relation to the mare); the practitioner's hand should be cupped over the fetal hoof to protect the endometrium from rupture.

Flexural deformities are the most common equine congenital abnormalities (Giles et al, 1993). Care should be taken to diagnose rigidity and immobility in flexed limbs and move these cases on to partial fetotomy or c-section rather than futilely wasting time on mutations and potentially causing damage to the mare. A fetotomy cut through the distal row of carpal bones may be necessary. It is important to cut through the intercarpal joint to avoid leaving a jagged bone fragment that may potentially damage the mare, and to give a wider diameter area on the fetal leg for attachment of obstetric chains to apply traction after the fetotomy cut.

Shoulder flexion: This malposture may be unilateral ("swimming" posture) or bilateral ("diving" posture). Mutations may be able difficult to perform with the fetal head in the way. In the case of a live fetus, c-section may be indicated. To correct this malposture, as always, the first step is to repel the fetus back into the uterus. The flexed shoulder must first be mutated to a flexed carpus by grasping the distal radius and rotating it medially and caudally; the flexed carpus is then corrected as above.

Fetotomy is as described above for removal of the forelimb in the retained head/neck posture. A cut may be made with the palm knife as a guide to the wire, making sure that the cut proceeds proximal to the scapula, cutting through the muscular attachments of the scapula and removing the entire forelimb, leaving no bony fragments.

Hip flexion: This malposture may be unilateral ("hurdling" posture) or bilateral ("dog sitting" posture). In either case, repulsion of the offending limb(s) can be very difficult and carries a high risk of uterine rupture. Success may be realized with a live fetus, but should not be attempted with a dead fetus in the standing mare. Mutations may be attempted under general anesthesia with elevated hindquarters. C-section or partial fetotomy should also be considered.

References

Allen WR, Wilsher S, Turnbull C, et al. Influence of maternal size on placental, fetal and postnatal growth in the horse. I. Development in utero. Reproduction 2002 123:445-453.

Allen WR, Wilsher S, Tiplady C, et al. Influence of maternal size on placental, fetal and postnatal growth in the horse: III Postnatal growth. Reproduction 2004 127:67-77.

Byron CR, Embertson RM, Bernard WV, et al. Dystocia in a referral hospital setting: approach and results. Equine vet. J. 2002 35(1):82-85.

Frazer GS, Perkins NR, Blanchard TL et al. Prevalence of fetal maldispositions in equine referral hospital dystocias. Equine vet. J. 1997 29:111-116.

Frazer GS. Fetotomy technique in the mare. Equine vet. Educ. 2001 13(3):151-159.

Frazer GS, Perkins NR, Embertson RM. Normal parturition and evaluation of the mare in dystocia. Equine vet. Educ. 1999a 11(1):41-46.

Frazer GS, Perkins NR, Embertson RM. Correction of equine dystocia. Equine vet. Educ. 1999b 11(1):48-53.

Frazer GS, Beard WL, Abrahamsen E, et al. Systemic effects of a polyethylene polymer-based obstetrical lubricant in the peritoneal cavity of the horse. In; Proceedings American Association of Equine Practitioners 2004 (50):484-487.

Giles RC, Donahue JM, Hong CB, et al. Causes of abortion, stillbirth, and perinatal death in horses: 3,527 cases (1986-1991). JAVMA 203(8):1170-1175.

Norton JL, Dallap BL, Johnston JK, Palmer JE, Sertich PL, Boston R, Wilkins PA. Retrospective study of dystocia in mares at a referral hospital. Equine Vet J 2007 39(1):37-41.

Threlfall WR. Parturition and Dystocia. In: Current Therapy in Large Animal Reproduction, ed: RS Youngquist. WB Saunders, Philadelphia, pp141-153.

Vanderplassche M. The pathogenesis of dystocia and fetal malformation in the horse. J. Reprod. Fert. Suppl. 1987 35:547-552.

Vanderplassche M. Dystocia. In: Equine Reproduction, eds: A. McKinnon and J. Voss. Lea and Febiger, Philadelphia, pp 578-587.