ANATOMIC REVIEW

The anatomic structures of interest during endoscopic examination in the stallion are the urethra, the colliculus seminalis, the seminal vesicles, and the urinary bladder.

The urethra generally is subdivided anatomically into the penile urethra and the pelvic urethra. The penile urethra is enclosed ventrally within the corpus spongiosum penis. It ends in a urethral process that extends freely 1.5 to 3 cm within the glans penis. This area is a common site of traumatic injuries, infectious inflammation (particularly that associated with habronemiasis), and neoplasia (squamous cell carcinoma)

The pelvic urethra extends from the neck of the bladder to the ischiatic arch. The urethra is wider in the region of the seminal colliculus than near the bladder and ischiatic arch. The dilated pelvic portion of the urethra collects seminal fluid during emission. The stratum cavernosum, the primary contractile tissue surrounding the pelvic urethra, may be visible through the distended urethral mucosa during endoscopy. During ejaculation, the contents of the pelvic urethra collected during emission is evacuated by the rhythmic contractions of the urethralis muscle, which surrounds this area.

The accessory sex glands of the stallion include the vesicular glands (seminal vesicles), ampullae of the ductus deferens, prostate, and bulbourethral glands. All of these glands contribute secretions (seminal plasma) to the ejaculate volume. The anatomic feature most noticeable on videoendoscopy is the gland ducts or openings. Only the vesicular glands have a large enough opening for direct examination by endoscopy.

The openings (ejaculatory orifices) of the bulbourethral glands and urethral glands are located just cranial to the ischial arch. The ejaculatory orifices appear as almost vertical slits on either side of the colliculus seminalis. The prostatic duct openings are located on each side of the colliculus. They sometimes are very difficult to visualize endoscopically.

The bulbourethral and urethral gland openings are found on the dorsal and lateral walls of the pelvic urethra, respectively, approximately 3 to 4 cm caudal to the colliculus seminalis.

In many stallions, a remnant of the müllerian duct system, the uterus masculinus, is found between the ampullae as they converge under the isthmus of the prostate. This structure may be visualized ultrasonographically and appears as an elongated echolucent cyst. Occasionally, it is observed as a cyst at the colliculus seminalis, where it is referred to as the prostatic utricle.⁹ Large cysts may interfere with emission, evacuation, or the passive loss of unejaculated sperm through the urethra.

PATIENT PREPARATION

Endoscopic examination is performed in the standing stallion after sedation to provide penile relaxation. Some authors suggest the use of intravenous xylazine (0.4 mg/kg); acetylpromazine (0.02 mg/kg given intravenously) has been used by others.⁴ I prefer sedation with detomidine or a combination of xylazine and butorphanol.

The tail is wrapped, and transrectal palpation and ultrasonography may be performed before endoscopic examination. Sexual stimulation for 2 to 3 minutes (teasing) may increase secretion and dilation of the accessory sex gland.

Endoscopic examination requires at least three people: one person for direct manipulation of the endoscope into the urethra, one person at the controls of the endoscope “driver,” and one person to help with stallion restraint. A fourth person may be needed for other manipulations such as rectal massage or insertion of other instruments (catheter or biopsy forceps) through the instrument channel of the endoscope.

The penis is grasped gently just above the glans and extended manually by gentle traction. The penis is then cleansed with warm water and soap. Some practitioners prefer the use of 1% povidone-iodine solution if the area is very dirty and oily. If an antiseptic solution is used, it should be rinsed off completely with sterile water before introduction of the endoscope. During the cleaning procedure, special attention is given to removal of smegma (“bean”) from the urethral diverticulum, with careful examination of the urethral process for any lesion.

EQUIPMENT AND PROCEDURE

Equipment used for endoscopy of the stallion urogenital tract consists of flexible endoscopes or videoendoscopes. In general, the most commonly used types of endoscopes are either human gastroscopes or colonoscopes.^10–12 A flexible endoscope with a minimum length of 100 cm and maximum outer diameter of 10 mm is adequate for most light breeds of horses,^4,13,14 A pediatric gastroscope is necessary for the examination of small ponies and miniature horses. Use of a pediatric gastroscope with an outer diameter of 8 to 9 mm carries an added advantage in that it can be inserted into the lumen of the vesicular glands. Longer endoscopes (120 to 140 cm) may be necessary in large breeds of horses or if the penis becomes partially erect.

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