CHAPTER 23Vulvar Conformation

In 1937 Caslick¹ made the first reference to the importance of the conformation of the mare’s vulva in relation to infections of the genital tract and subsequent infertility of the mare. He discussed the principal factors related to this infertility as being changes in conformation and persistent relaxation of the vulva. He considered a vulval angle of 80 degrees to the horizontal to be desirable, whereas angles below 50 degrees lead to wind-sucking (pneumovagina) becoming a clinical problem. He described a simple surgical procedure to close the upper vulvar lips, now universally known as Caslick’s operation, which was found to markedly improve the fertility in the mares so treated.

This operation has also been successfully used to prevent pneumovagina in yearlings, 2-year-olds,² and racing maiden fillies.³ Other surgeons have described various techniques that must be carefully followed to ensure maximum benefits from this simple operation.^2,4

Scientific evidence was presented by Pascoe⁵ in 1979 that described a simple measuring device to determine the slope (angle of declination) of the vulva and the length of the vulvar lips (Figure 23-1).

Figure 23-1 A stainless steel measuring device (vulvometer) for determination of effective length and angle of declination (Caslick Index).

In a study on 9020 mares made on Thoroughbred, Standardbred, and Arabian mares, measurements of these two parameters were recorded; a simple mathematical formula produced a Caslick Index, which allowed a scientific determination of the necessity for Caslick’s operation to be performed in mares not exhibiting classical signs of pneumovagina.

ANATOMIC CONSIDERATIONS

The perineum of the horse is the area that includes the anus, vulva, and the adjacent skin (usually hairless) under the tail. The normal conformation of the perineum prevents the entrance of air and bacteria into the genital tract. This is a very important area because of its protective role for the internal genital tract and its implications in creating some forms of infertility in particular. It is commonly a site for injury at parturition. The anatomic arrangement of the perineum, including its length relative to the pelvic bones and its angle relative to the vertical plane, is an important feature for the fertility of the mare; for example, mares that experience bouts of infertility frequently have a flat-topped croup with a tail setting that is level with the sacral iliac joint and a sunken anus (Figure 23-2). Alterations of angle and length have been used to determine a Caslick Index, which is used to provide an index of the need to perform Caslick’s operation.

Figure 23-2 Clinical case showing sunken anus and high angle of declination—indication for Caslick’s repair.

CASLICK INDEX

Calculation and measurements (Figure 23-3):

Figure 23-3 Anatomic relationship of the vulva of the mare showing measurement site. a, angle of declination; l, effective length of vulva; tl, total length of vulva.

Interpretation

• Values of less than 100 are associated with a normal anatomic arrangement and better fertility than a score approaching 150 or higher.

• The Caslick Index increases with age and can vary with nutritional status.

• Continuous evaluation is essential, especially if loss of body condition occurs.

As a rule, a Caslick’s operation must be performed when the dorsal vulvar commissure is located more than 4 cm (1½ inches) dorsal to the pelvic floor.

• Conformational characteristics that correlate with high fertility include a long sloping hip, a sacral iliac joint located dorsal to the tail setting, and a vulva that is no more than 10 degrees from the vertical. Various degrees of vulvar length and angulation and their relationship to fertility are illustrated in Figure 23-4.

• The normal/ideal anatomic arrangement is that the vulva should be vertical (or at least less than 10 degrees from the vertical) and that more than 80% of its length (from dorsal commissure to ventral commissure) should lie below the level of the ischial tuberosities (Figure 23-5). Variations in either the relative length below the ischium or an increase in the angle of declination (or both) result in a tendency for the vulva to be drawn forwards into the perineum below the anus (Figure 23-6). This combination results in an increased chance of aspiration of air into the vagina and a higher Caslick Index.

• Endometritis secondary to pneumovagina may resolve spontaneously after the operation.

• Natural defense mechanisms can become effective again after eliminating the continued irritation from aspirated contaminants.

• Once pneumovagina is corrected, it is essential that the mare remains Caslicked for the remainder of her reproductive life.

Figure 23-4 Vulvar changes in three conformation groups as effected by increasing angle of declination (α) and length of vulva (l).

Figure 23-5 Application of vulvometer with angle of 5 degrees and l of 8 cm; Caslick Index = 40. Caslick’s operation would not be required.

Figure 23-6 Application of vulvometer with angle of 30 degrees and l of 8 cm; Caslick Index = 240. Caslick’s operation is necessary.

Breeding soundness examinations are performed to determine a mare’s suitability as a broodmare and for identifying causes of infertility. Portions of the examination are performed routinely on normal mares during the breeding season to determine when they should be bred. Common uses of new technologies, such as artificial insemination (AI) of mares with shipped, cooled or frozen semen and embryo transfer, require that the veterinarian excel in reproductive examination skills. A complete breeding soundness examination is most commonly performed to identify a cause of infertility.

EXTERNAL EXAMINATION (RELATED TO VULVAR CONFORMATION)

• Examination of the external genitalia should include the entire perineal area, as well as the tail and buttocks, to detect changes in conformation, presence of signs of equine herpesvirus type 3 (EHV-3), signs of vulvar discharge, and the presence of tumors (e.g., melanoma or squamous cell carcinoma).

• Perineal and pelvic conformation needs to be examined thoroughly because the anatomic alignment of the mare’s perineum is important for reproductive soundness. The perineal body may be defective in older, pluriparous mares. The defect most likely occurs from repeated foalings and poor reproductive conformation and results in a sunken anus and lack of tissue between the rectum and the vagina.

• The pelvic conformation should be viewed laterally and caudally.

• Mares that experience bouts of infertility frequently have a flat-topped croup with a tail setting that is level with the sacral iliac joint and a sunken anus. These conformational defects lead to pneumovagina, urovagina, and accumulation of intrauterine fluid.

• The vulva is best evaluated during estrus, when relaxation and elongation of the vulvar lips are greatest. The integrity of the vulvar lips, the angulation of the vulva, and the location of the dorsal commissure of the vulva in relation to the pelvis need to be evaluated. The vulvar lips should meet evenly and appear full and firm. They function as a seal against external contamination of the uterus. The lips should lie vertically with a cranial to caudal slope of no more than 10 degrees from the vertical. The dorsal commissure of the vulva should be no more than 4 cm dorsal to the pelvic floor. If it is more than 4 cm, the vulva is predisposed to cranioventral rotation, leading to pneumovagina and contamination.

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