PATIENT SELECTION

In consideration of the client, patient, and general perception of our professional abilities as well as the merit of our interference, care should be exercised to select only those patients that are likely to respond favorably to surgery (Box 24-1). For example, it is pointless to perform a sophisticated, expensive surgery for vesico vaginal reflux if the mare has sustained chronic uterine damage and fibrosis that will render her infertile despite an excellent surgical outcome. Full reproductive evaluation is necessary before surgery in any candidate subjected to physical insults such as recto-vaginal lacerations and fistulas for longer than 1 year.

SEDATION AND ANESTHESIA

Most procedures are performed with the mare standing, using local infiltration or epidural anesthesia. Apart from minor procedures (Caslick’s vulvoplasty), we prefer to have mares tranquilized and restrained in appropriate stocks such as facilities for cross tying and/or tail elevation. Tranquilization and analgesia are provided most commonly with intravenous (IV) detomidine (mg) and butorphanol (mg). Longer surgeries may require the repeated IV use of low doses of xylazine (100 mg). Cost and length of tranquilization/analgesia are important considerations. Some have other physiological effects, such as increased urinary output with xylazine. Detomidine provides excellent tranquilization and analgesia; however, effects are often still apparent hours after administration. Epidural anesthesia may be extremely effective; however, variation in response and individual susceptibility and time before appropriate anesthesia is obtained occasionally make it less rewarding than tranquilization and local infiltration for some of the procedures discussed here. Techniques for epidural anesthesia have been discussed elsewhere.¹ One word of warning: epidural administration of more than 7 to 9 ml of 2% local anesthetic to a 500 kg mare can be associated with loss of motor control to the hind limbs and recumbency. These mares are difficult to manage and may require support for 2 to 4 hours.

Epidural injection of xylazine for perineal analgesia has been reported to be very efficacious. A dose of 0.17 mg/kg (around 75-100 mg in an average horse) was as effective as local anesthetic agents for analgesia and had nil or minimal effects on depression of motor nerves in the lumbosacral intumescence. Another advantage was demarcation of the area of analgesia with an area of sweating (dermatome) that was consistent both temporally and topographically.² In our experience the perineal dermatome is not always obvious. In addition it takes around 35 to 40 minutes for good analgesia to be recognized associated with xylazine epidural anesthesia. Currently we prefer to use a combination of xylazine (70 mg) and lidocaine (60 mg [3 ml]), q.s. saline to 10 ml for most horses. With this regime, loss of tail tone has occurred associated with a successful “block” within 5 to 10 minutes and surgery generally has begun by approximately 30 to 45 minutes post administration. Our most common method of epidural anesthesia is to begin with the infusion of 1 ml of local anesthetic through a 25 gauge needle subcutaneously above the first moveable coccygeal/sacral joint (usually C1 and C2). Next, a 38-mm, 18-gauge needle (bevel forward and introduced at 45 degrees) that has a small amount of fluid left to create a meniscus on the needle hub is introduced through the local bleb. Recognition of the fluid being aspirated into the needle infers correct placement, and the appropriate amount of solution is injected slowly into the epidural space whilst continually evaluating the ease of administration to ensure that the needle tip is still in place.

Occasionally epidural anesthesia is difficult to achieve (infusion outside the spinal cord before proper epidural placement, hemorrhage through the needle and tissues, anatomic abnormalities and fat horses). Provided local anesthesia can be obtained by infiltration without adverse affects on wound healing, we prefer to perform surgery without the epidural and provide incremental IV administration of xylazine (i.e., 50-100 mg to already tranquilized patients when or if minor surgical discomfort is recognized).

In some instances we have been able to identify correct epidural placement by injecting the anesthetic slowly while moving the needle until there is an obvious change in the amount of pressure needed to advance the solution. These situations are usually associated with a mare with abnormal anatomy (i.e., after a traumatic foaling) and lack of movement of fluid in the needle hub.

VESTIBULE AND VULVAL LIPS

Pneumovagina

There are 3 barriers to aspiration of air and contaminants into the cranial reproductive tract.

1. The vulval/vaginal lips

2. The vestibular sphincter

3. The cervix

It would appear that the vestibular barrier is more important than previously recognized; however, when any of these protective barriers are rendered incompetent, contamination and concomitant vaginitis, cervicitis, and metritis may result. Pneumovagina can result from faulty perineal conformation, previous injury to perineal tissues, or effects of poor body condition. Pneumovagina can also be iatrogenic (i.e., reproductive examination or breeding).

Recognition of potential candidates for pneumovagina are made from anatomical observations ³ (i.e., predisposition is associated with a low pelvis, tilted vaginal lips, and a sunken anus) (Figure 24-1). If possible, mares should be examined during estrus when the reproductive tract is relaxed. Parting the vulval lips results in loss of integrity of the vestibular sphincter and an audible ingress of air to the cranial vagina. Recognition of foamy exudates from the vaginal lips may often be noted during rectal examination. In addition, ultrasonography has been used to visualize air in the uterus.⁴ If in doubt after clinical evaluation regarding a mare’s potential to aspirate air into the vagina, we recommend treating by surgery all mares that have been barren one or more breeding seasons, unless other causes of reduced fertility are obvious (i.e., stallion, farm management).

Figure 24-1 A, Retraction of vulval lips reveals an intact vestibular sphincter. B, Retraction of vulval lips reveals an open vestibular sphincter that is likely associated with pneumovagina.

Caslick’s Vulvoplasty

Caslick’s vulvoplasty was first described by Dr. E. A. Caslick in 1937 and involves apposition of the vulval lips.⁵ Local anesthesia of vulval lips, removal of a thin portion of mucosa from the mucocutaneous junction, and apposition of the cut edges is simple, quick, and effective. Length of apposition is determined by height of pelvis relative to vulval lips. In general the join should continue to a point at least 3 to 4 cm below level of the pelvis, and approximately 3 cm should be left unopposed to allow for urination. If this is not possible, other surgical techniques (see below) may be necessary.

Points for consideration

• For suture material we use 0 catgut (Metric 4) on a continuous spool. The benefit of gut is that it is gone or degraded by either pregnancy testing (∼15 days) or the next breeding cycle (if the mare is empty). Other materials that are nonabsorbable or delayed-absorbable may damage the stallion if the mare is rebred.

• For suture pattern, a simple continuous pattern is quicker and just as effective as a simple pattern interrupted. However, failure to appose the deepest layers of the exposed tissues may result in fistula formation. Failure to exit/enter in the middle of the cut tissue caused by taking bites too deep will result in wound breakdown.

• Method of mucosa removal:

• Tissue should be removed from the mucosal surface of the vestibule, not the skin of the vulva lip.

• Removing too much will result in fibrosis and difficulty for the next veterinarian (Figure 24-2, A).

• If multiple procedures are performed in 1 season, freshening with a scalpel blade is sufficient, provided a careful suture pattern is placed (Figure 24-2, B).

• Incision with a scalpel blade rather than mucosal removal is recommended for maidens (i.e., race mares) (Figure 24-2, C). This results in a thinner join that is easily opened at the breeding farm.