The reproductive tract of the mare is protected against ascending contamination by three anatomical barriers. The most caudal and external of these barriers or seals is the vulva, followed by the vestibulovaginal fold and finally, the cervix. Together, these barriers significantly reduce the risk of air and debris entering the uterus (Inoue and Sekiguchi, 2017) and help minimise contamination of the reproductive tract by potential pathogens (McCue, 2008).
For an adequate vulval seal, a normal perineal conformation is necessary. A natural and good perineal conformation is achieved when the vulval labia are full and firm, and meet evenly at the midline (McCue, 2008; Papa et al, 2014; Inoue and Sekiguchi, 2017). Around 80% of the vulval opening should be located below the pelvic brim (Pascoe, 1979, 2007), and the vulval lips should be in a nearly vertical orientation (a deviation of <10°) (McCue, 2008) (Figure 1).
A compromised vulval seal is often associated with a condition referred to as pneumovagina, or wind sucking, which can predispose ascending contamination with microorganisms. Contamination of the caudal reproductive tract can initially cause vaginitis, which may ascend further to cause acute endometritis and, in mares in late gestation, will predispose placentitis (Papa et al, 2014; Inoue and Sekiguchi, 2017). Continued aspiration of air can elicit an inflammatory response, even in the absence of bacterial infection (Slusher et al, 1984; Dascanio, 2011). Pneumovagina and its sequalae often result in subfertility and reproductive failure (Papa et al, 2014).
A study confirmed the positive correlation between vulval conformation and cytological status of the endometrium, and validated an adequate vulval seal as a requirement for optimal fertility in Thoroughbred mares (Hemberg et al, 2005). The latter study also demonstrated that a mare's age and reproductive status (maiden vs pluriparous mares) significantly influenced vulval conformation (Hemberg et al, 2005). Moreover, Hemberg et al (2005) demonstrated a significant effect of vulval conformation on conception and live foal rates, confirming the importance of perineal conformation for reproductive health and fertility, a topic first addressed by Dr Caslick in 1937. Caslick further described Caslick's vulvoplasty as a procedure to appose the dorsal parts of the malconformed vulval labia in mares (Caslick, 1937; Dascanio, 2011).
Abnormalities in perineal conformation are most often corrected by performing a Caslick's vulvoplasty (Dascanio, 2021), which is a procedure that can be succinctly described as the surgical closure of the dorsal part of the vulval labia (Hemberg et al, 2005). The goal is to ensure apposition to below the pelvic brim, in order to restore an effective vulval seal as a barrier to spontaneous ingression of air.
Indications for Caslick's operation
In general, a Caslick's operation is indicated when malconformation or inadequate closure of the vulva is present, for example as a result of trauma, weight loss, or congenital deviation from the ideal conformation. Trauma to the vulva during foaling (McCue, 2008), or other causes of damage leading to tissue loss, can also compromise adequate apposition of the two labia. A decrease in muscular tone and tissue function as a result of ageing may also predispose a horse to pneumovagina (McCue, 2008). In addition, pneumovagina may occur in mares with a low body mass index or poor body condition score, since loss of perineal fat often results in a displacement or sinking of the anus and related cranial sloping of the dorsal commissure of the vulva (Dascanio, 2011) (Figure 2). Mares with a flattopped croup and a tail setting level with the sacroiliac joint are also predisposed to poor vulval conformation (Pascoe, 2007; Dascanio, 2011). Although a congenital predisposition is less prevalent than the causes listed above, it is described for certain breeds, including the Arabian horse (Rais et al, 2013).
Pascoe (1979) described a method to categorise vulval conformation, the so-called ‘Caslick's index’. This index is calculated by multiplying the effective length of the vulva in centimetres by the angle of declination of the vulva in degrees (Pascoe, 2007; Crabtree and Pycock, 2020). The Caslick's index is the only way to objectively quantify the degree of abnormality and correlates positively with the likelihood of developing pneumovagina (Pascoe, 1979, 2007; Dascanio, 2011; Crabtree and Pycock, 2020). A value of >150 is considered indicative of the need for a Caslick's vulvoplasty (Pascoe, 2007; Dascanio, 2011).
Alternatively, the need for a Caslick's operation is suggested by a positive wind sucker test (parting the vulval labia and listening to the sound of air rushing into the vagina), which indicates an incompetent vestibulovaginal fold, or the detection of air in the vagina and uterus during per rectum ultrasound examination (Dascanio, 2021). Occasionally, a vulvoplasty is performed as a preventive measure in mares with no obvious deviation of normal vulval conformation. This is justified in cases where the mare is sub-fertile, has repeated episodes of endometritis, or is expected to lose significant body mass, risking an acquired angulation of the vulva. This explains why Caslick's operation is frequently performed on young race horses with little intrapelvic fat (Dascanio, 2021).
Furthermore, the performance of a racehorse with poor vulval conformation can suffer as a result of discomfort and behavioural problems provoked by vaginitis, since an increase in negative pressure in the pelvic cavity during forward movement can trigger wind sucking. A Danish study by Christoffersen et al (2007) suggested that hypersensitivity in dermal areas correlated with irritating processes in the caudal reproductive tract may result in poor performance. In the latter study, they treated 14 mares exhibiting performance problems with a vulvoplasty, resulting in either elimination of the problems or improvement of performance in 86% of the mares (Christoffersen et al, 2007). These results support the common practice of performing a Caslick's operation in racehorses, and the reported better trainability and performance of Thoroughbred and Standardbred horses following vulvoplasty (Burger et al, 2008).
How to perform a Caslick's operation
Permanent technique
A Caslick's operation should be performed on the standing, restrained mare under local anaesthesia (Bradecamp, 2011). Adequate restraint can be achieved by placing the mare in examination stocks, using a twitch and/or by positioning the back-end of the mare just inside a stable door (Bradecamp, 2011). Sedation is not always necessary, especially if the mare is adequately restrained in examination stocks, but an alpha-2-agonist such as detomidine hydrochloride, in combination with an opioid such as butorphanol, is recommended if sedation is required.
In preparation for surgery, the mare's rectum should be emptied, with the tail bandaged and held or tied to one side. The perineal region should then be gently cleaned with disinfectant soap such as povidone iodine soap, dried with paper towels or cotton, and prepared with a minimally-irritant antiseptic solution (0.5% chlorhexidine or 30% alcohol). It is advisable to wear medical gloves and to work as cleanly as possible. However, it is neither necessary to wear sterile gloves nor realistic to prepare a completely aseptic surgical field. A total of approximately 15–20ml of local anaesthetic (2% lidocaine) should be injected subcutaneously using a 20-gauge or smaller needle along the mucocutaneous junction of both vulval labia, starting at the most ventral aspect of the proposed vulvoplasty and progressing to the dorsal commissure of the vulva (Dascanio, 2021) (Figure 3). The ventral extent of the Caslick's operation should continue to about 2–3cm below the level of the ischial tuberosities, and in the case of a cranially sloping vulva, to below the level where the vulva orifice assumes a vertical orientation. At least 3cm of the vulva must be left open to allow unconstrained urination (Bradecamp, 2011). If further treatments are to be expected, such as vaginal palpation, vaginal inspection with a speculum, artificial insemination, or natural breeding, the remaining opening of the vulva should be 4–5cm. If natural breeding is planned, it must be ensured that the sutures will not cause trauma to the stallion's penis (Bradecamp, 2011).
After local anaesthesia of the surgical area, a thin strip of mucosa is removed from the mucocutaneous junction of the labial rim using tissue scissors, to expose a strip of submucosa around 8–10mm wide. To allow removal of the tissue, thumb forceps can be used to grasp a ribbon of mucosa (Hendrickson, 2013). Only mucosal tissue should be removed, avoiding the skin (Dascanio, 2021) (Figure 4). It is worth noting that the surgical wounds are frequently much wider than anticipated by the surgeon, because of tissue swelling caused by infiltration of the local anaesthetic (Hendrickson, 2013). In this respect, the aim is to remove a minimal strip of mucosal tissue, while still creating an approximately 8mm strip of exposed submucosa. If too much tissue is removed, excessive fibrosis might develop, which can compromise good apposition after repetitions of the Caslick's operation in subsequent breeding seasons after foaling (Bradecamp, 2011). If not enough tissue is removed and the fresh wounds are too narrow, healing may be incomplete, leaving fistulas, or the apposition may break down after minimal manipulation (Bradecamp, 2011) (Figure 5). Care should be taken that the dorsal commissure is not omitted, by creating a wound with an inverted U-shape (Figure 6).
The labia are sutured together with the aim of opposing the freshly created wounds using either a simple continuous, interrupted or continuous interlocking suture pattern (Bradecamp, 2011). It is advisable to use a suture material of approximate size 0 (4 metric) with a curved, round bodied, sharp needle. Suture material can be either absorbable (Vicryl), or non-absorbable (Ethilon). The suture should pass through the middle to deep part of the incised tissue to ensure adequate healing and minimise the risk of fistula formation (Bradecamp, 2011). In addition, the sutures should exit the skin approximately 1cm away from the labial rim to prevent them tearing through. The individual stitches should be placed 0.5–1cm apart to avoid gaps and fistula formation (Figure 7). Good apposition after suturing can be checked by inserting a finger under the suture line and applying gentle pressure from the vaginal aspect (Figure 8).
For a regular Caslick's procedure, postoperative topical or systemic antibiotics are not indicated (Hendrickson, 2013), and the surgical field does not need to be covered. Aftercare includes avoiding or minimising rectal palpation and manipulation of the surgical field for 10–14 days. The sutures can be removed after 7–14 days (if non-absorbable suture material was used). To prevent tearing of the vulva at foaling, a Caslick's operation should be reopened shortly before parturition (less than 14 days before the estimated date of birth) (Hendrickson, 2013), or before further breeding procedures, which are likely to exert significant force on the sutured tissue.
Opening requires sufficient local anaesthesia and involves a straight cut in the direction of the scar between the opposed labia, using either tissue scissors or a scalpel. After reopening a Caslick's closure or episiotomy for any other reason, the vulvoplasty should be redone as soon as possible. Postpartum, a Caslick's operation can be repeated 1–2 days after parturition (Hendrickson, 2013), or after the foal heat, with assurance that the mare has no fluid in the uterus and is no longer at risk of postpartum metritis.
Temporary alternatives
As a temporary alternative to the vulvoplasty described above, a non-permanent or provisional apposition of the labia can be achieved using either surgical staples, or a breeding suture that apposes the labia without first incising the mucosa. For both temporary techniques, the mare is prepared as described above. For stapling, the dorsal vulval commissure is grasped and pulled upwards to align the labia in close apposition. Individual staples are placed 1cm apart, starting at the dorsal commissure and continuing downwards to below the level of the pelvic floor (Dascanio, 2021). The suture technique is performed similarly to the permanent technique, but without incision of the mucosa. Temporary labial apposition should be replaced by a permanent technique once the mare is confirmed pregnant (Dascanio, 2021).
Alternatives and complications
Possible complications of a Caslick's operation include wound infection, suture dehiscence and recurrence of pneumovagina. However, the advantages of a Caslick's operation in mares with poor vulval conformation outweigh any potential complications associated with performing the procedure.
If the Caslick's operation is not sufficient to resolve the pneumovagina, or a urovagina that was thought to be secondary to the pneumovagina, further surgical techniques should be considered. These inlcude vestibuloplasty (also referred to as perineal body reconstruction, deep Caslick or Gadd technique); perineoplasty (also referred to as perineal body transection or Pouret's procedure); or urethral extension (Bradecamp, 2011; Hendrickson, 2013). The first two of these alternatives are most commonly indicated in mares showing severe malconformation, in which the dorsal commissure of the vulva comes to lie horizontally as a result of cranial displacement of the anus (Hendrickson, 2013).
Conclusions
A competent vulval seal is an important first physical barrier to air and microorganisms entering the reproductive tract of a mare. Abnormalities of perineal conformation and an incompetent vulval seal can ultimately lead to sub-fertility. Further investigation is required into whether performing a Caslick's operation on younger mares, with a relatively minor increase in the Caslick's index, has a positive impact on fertility.
KEY POINTS
- The vulva is the most caudal of three physical barriers between the uterine lumen and the outside world.
- An inadequate vulval seal in the mare is most often corrected using Caslick's operation.
- Caslick's operation involves the surgical closure of the dorsal part of the vulval labia, under local anaesthesia.
- Indications for Caslick's operation vary but include trauma caused by foaling, advanced age, weight loss and, in some breeds, there is a congenital predisposition.
- If a Caslick's operation is insufficient to resolve the problem of pneumovagina more invasive surgical procedures should be considered.