Perineal lacerations occur during unassisted foaling, most commonly in primiparous mares. Lacerations are caused by a combination of foal limb malpositioning and the violent, unpredictable expulsive efforts that accompany equine parturition. The foal’s hooves can engage the roof of the vestibule during forceful contractions and may lacerate the dorsal wall of the vestibule. The resulting injury is classified as either a first, second, or third-degree perineal laceration, or a rectovestibular fistula. First-degree lacerations involve only the vestibular mucosa and vulvar skin at the dorsal commissure. Second-degree lacerations involve the vulvar mucosa, submucosa, and perineal body musculature. Third-degree lacerations result from the foal’s foot perforating the rectum and tearing all the structures between the rectum and vestibule caudally to include the dorsal vestibular wall, rectum, perineal body, and anal sphincter. Third-degree lacerations result in a common opening between the rectum and vestibule. Rectovestibular fistulas result from the foal’s foot perforating the rectum but then withdrawing the foot before subsequent normal delivery. The result is a fistula of variable size between the rectum and vestibule, usually cranial to the perineal body. The external genitalia in these cases will appear normal.
Diagnosis of these conditions is made during the post-partum examination, although third-degree perineal lacerations will be immediately obvious to the owner and may cause serious alarm (). Injury to the perineal body, anal sphincter, and dorsal vulvar commissure are readily apparent on visual inspection. Even with less severe injuries, external examination of the perineum will usually reveal edema, bruising, stretching, and splitting of the vulvar skin and mucosa. Speculum and manual examination are necessary to determine the degree of injury to the vestibule and vagina. Rectal palpation will confirm the presence of a rectovestibular fistula. Although other peri-parturient injuries may simultaneously occur, no reported associations exist with the occurrence of perineal lacerations. Thus no reason exists to be unduly concerned about the potential for other foaling-related injuries that may not be detected on a routine postpartum examination.
Classification of the type of laceration on the basis of the involved structures is useful because it will accurately predict the required treatment. First-degree lacerations will heal uneventfully with no surgical intervention other than possibly a Caslick’s procedure. Second-degree lacerations frequently will heal completely without intervention because the tissues are maintained in apposition. Damage to the perineal body or vestibular musculature that does not completely heal can be repaired at a later date. Third-degree lacerations and rectovestibular fistulas will always require surgical repair. However, immediate surgery is ill advised regardless of the type of injury. In the acute phase the wounds are extremely edematous, contaminated with feces, and some tissues may not be viable. Repair must be delayed at least 4 to 6 weeks until complete healing has occurred before reconstruction of the damaged perineum is attempted. During this interval, any devitalized tissue will slough and second intention healing will occur. Fibroplasia will take place and remodeling of the deposited fibrous tissue provides greater holding power for sutures. Complete epithelial resurfacing occurs so that visual examination of the laceration will reveal a line of junction where the rectal mucosa meets the vestibular mucosa (). Reconstructive surgery may be performed at any time after complete healing has occurred. Surgery performed 6 to 8 weeks postfoaling is optimal; however, other management concerns such as weaning of the foal or getting the mare in foal for next year often factor into the timing of the surgical repair. It is advisable to forewarn owners contemplating surgical repair that more than one attempt is often required to achieve reconstruction.
Treatment of Perineal Lacerations
Immediate treatment should be directed at medical management to minimize discomfort, prevent infection, and promote wound healing. Tetanus prophylaxis is indicated according to the recommended guidelines. Pain and inflammation secondary to trauma should be treated with phenylbutazone (4.4 mg/kg PO q24h). The wounds are invariably severely contaminated and thus benefit from broad-spectrum antimicrobial treatment. Antimicrobial therapy may be discontinued in 7 to 10 days once a healthy bed of granulation tissue is present. Affected mares may experience difficulty in defection as a result of perineal discomfort, and impaction can be a secondary complication. The feces can be kept soft by initial prophylactic administration of mineral oil through a nasogastric tube, and then with subsequent doses administered in the feed. Mares on lush green pasture may not require assistance to achieve fecal softening. Local wound care involves providing as clean an environment as possible. Manual evacuation of feces from the vestibule is beneficial in the early stages of healing for third-degree lacerations and rectovestibular fistulas. Gross fecal contamination of the vagina and uterus is seldom a concern because the injuries invariably occur caudal to the vestibular sphincter. Daily, or twice daily, lavage of the wound with antibacterial solutions such as dilute povidone iodine (10 ml of 10% stock solution/L of water) is beneficial for the first few days. The practitioner should resist the urge to aggressively surgically debride the lacerated tissues. Surgical repair depends on reconstruction of the shelf between the rectum and the vestibule. Retention of as much viable tissue as possible will improve the chances for successful surgical reconstruction. Tissue that is clearly non-viable should be promptly removed; however, the trauma and associated inflammation may make it difficult to differentiate between viable and nonviable tissues. This difficulty can easily result in removal of viable tissues. It is preferable to take a more conservative approach and to de-bride the laceration over several days, each time removing only the definitively nonviable tissue.
Mares with first- or second-degree lacerations seldom require surgery. Third-degree lacerations and fistulas result in chronic, low-grade bacterial contamination of the vagina and uterus; therefore surgical repair will be required for breeding soundness. Uterine contamination and subsequent endometrial degeneration secondary to these injuries has been documented by culture and biopsy. Any inflammatory uterine changes are reversible after surgical repair. Studies have shown that 75% of mares are able to successfully carry a foal after surgical repair of third-degree perineal lacerations or rectovestibular fistulas. Mares that have undergone surgical repair are predisposed to reinjury on subsequent foalings; however, clinical experience indicates that the incidence of recurrence is low.