Ultrasonographic evaluation of the scrotal contents in the stallion includes the spermatic cord (the spermatic artery, ductus deferens, and spermatic venous network (pampiniform plexus), cremaster muscle, nerves, lymphatics), the epididymis (head, body and tail), the testis, vaginal cavity, and scrotum (skin, dermis). The clinician should be able to identify position, location, and the normal echoic pattern of these structures. The examination of the scrotal contents, in addition to determining normalcy, should also include measurement of the length, width, and height of each testis. These measures are then used to calculate testis volume and determine the efficiency of sperm production.

Stallion Position and Location

The clinician is required to perform the examination in the vicinity of the flank area of the stallion. For obvious reasons this is a risky position for the clinician. If semen collection is performed in conjunction with the evaluation, stallions tend to be more tractable following semen collection. Regardless, it is recommended to sedate the stallion to facilitate a thorough and complete evaluation. The ultrasound evaluation of the scrotal contents should be considered a stand-alone primary procedure and therefore sufficient time and patience should be allotted to allow for a thorough examination of the scrotal contents as well as an accurate measurement of the testes dimensions. An inadequate exam­ination can result in erroneous testis measurements leading to an incorrect clinical interpretation.

The stallion can be evaluated from the left flank, either confined in a stock or free-standing in the stall following sedation.

Probe Type

The location of the scrotum is in a relatively restricted area between the hind legs of the stallion, which can limit manipulation and probe placement. The linear array probe commonly used for per rectum examination is satisfactory, but its size (i.e. length) can limit examination of discrete structures such as the epididymis. Sector probes can also be used, particularly the “finger” type probe, that is small (surface area ∼2.5 cm) and allows easier access to specific areas of interest. The T-type linear probe allows ease of handling and placement in the scrotal area. See Figure 13.1.

Examination

Ultrasound evaluation should be preceded by a thorough manual evaluation of the scrotum and its contents to detect any specific areas that require scrutiny. Ultrasound gel or a similar lubricant can be applied to the probe. Since the scrotum in the stallion has very little hair there is no need to clip the scrotum. However, since sweat glands are present, artifactual changes due to lather and bubble formation require gel removal and gel reapplication to maximize image resolution.

Testis Measures

Measurement of the testis dimensions (width, height, and length) can be performed first.

Height

The height is measured by placing the probe ventrally and directing the beam dorsally so that the central vein is approximately two thirds of the distance from the surface and the spermatic artery can be visualized dorsally. There is no need grasp or manipulate the testis for this measure (Figures 13.2, 13.3).

Length

The length is usually measured from the caudal aspect of the scrotum in the vicinity of the tail of the epididymis, directing the beam cranially. The ultrasound probe should be rotated slightly in a horizontal direction so that the maximum length is visualized. The maximum distance can be determined by visualizing the hyperechoic cranial edge of the tunica albuginea of the testis. Similar to the height measurement, the length should be measured in situ (Figure 13.4).

Width

The left testis is measured by placing the probe on the left lateral surface of the scrotum and directing the beam horizontally and medially (Figure 13.5). At the same time the right testis should be pushed dorsally so that the shape of measured testis is not distorted. For the measurement of the right testis, the left testis is pushed dorsally to allow measurement. Grabbing of the scrotal neck, similar to the technique used to measure ruminant testes, should be avoided for several reasons. First, it is easier to perform the examination using only one hand to manipulate the probe on the testis, while the testis hangs freely. Even if the testis is in an inguinal position, the testis can be accurately measured without the need to manually draw the testis into the scrotum. Second, it tends to cause strong contraction of the cremaster muscle, which may elicit a similarly strong response from the stallion (i.e. kick).

Qualitative Evaluation of the Scrotal Contents

The echotexture of the scrotal contents can be evaluated following testes measurement. A routine should be followed to ensure a thorough evaluation is completed. The routine followed by the author starts at the neck of the scrotum and visualizes the spermatic cord above the testis.

Spermatic Cord

The primary structure visualized is the tortuous spermatic artery as it winds to the testis (Figure 13.6). Pathology of the spermatic cord includes spermatic cord torsion, which commonly occurs 1–3 cm dorsal to the testis (Figures 13.7, 13.8, 13.9). Varicocele (Figure 13.10), dilation of the pampiniform plexus, while uncommon, may also be detected here. In addition, generalized edema resulting from inflammation, such as orchitis or epididymitis, may present as hyperechoic inconsistencies in the spermatic cord (Figure 13.11).

Testis

Following the examination of the spermatic cord the ultrasound probe is passed laterally on the testis to examine the parenchyma, which should be homogeneous in echotexture, devoid of hyper- or hypoechoic foci, commonly associated with neoplasia (Figures 13.12, 13.13) or benign structures (Figure 13.14). In cross-section the testis shape may be round to oblong.

Epididymis

The epididymis includes head, body, and tail regions that are located craniolaterally, dorsolaterally and caudally respectively, on the horizontally oriented testis. The epididymal tail (Figure 13.15) should be evaluated for location and size. Spermatic cord rotation of 180° occurs where the epididymal tail is located dorsocranially. In addition, the size and ultrasonographic appearance of the tail can vary from small, in the case of hypoplasia, to prominent, when the tail is distended with sperm or due to inflammation. A tail distended by sperm only, may be common in stallions at sexual rest or in cases of sperm accumulation when sperm accumulates from the ampulla all the way back to the ductus deferens and epididymis (Figures 13.16,13.17).

Vaginal Cavity

The vaginal cavity is a potential space that communicates with the peritoneal cavity through the inguinal canal and therefore, peritoneal contents such as fluid and intestines have the potential to pass through the canal and occupy the vaginal cavity. In addition, circulatory (artery, vein, lymphatic) compromise to the spermatic cord can result in free fluid accumulation (hydrocele). This fluid tends to accumulate around the epididymal tail (Figure 13.18).

Measurement and Interpretation of Testes Volume

Measurement of testes size is an important part of the stallion breeding soundness evaluation since testes size is associated with sperm production. The number of sperm produced by the stallion may impact fertility and, thus, the number of mares that a stallion can breed. Historically, testes size was determined using the linear measure, total scrotal width, in which calipers were used to measure the combined width of the testes, the mediastinum testes, and the scrotum. This technique, however, does not determine the three-dimensional shape (i.e. volume) of each testis. While the relationship of length, width, and height tends to be proportional, there are instances where they are not, and the determination of each dimension will more accurately determine volume.

Subsequently, ultrasonography has been introduced as a technique to measure and evaluate the testes. This technique has the advantage of being specific and accurate since the clinician can visualize the testis parenchyma and identify landmarks that assure measurement of the length, width, and height.

Measurements (length, width, and height) from each testis are performed as described in the preceding section. The measures from each testis are then inserted in a formula that approximates the volume of an ellipsoid [1]:

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