CHAPTER 29Testicular Descent and Cryptorchidism

TESTICULAR DESCENT

In the normal equine male fetus, the testes descend into the scrotum between 30 days before and 10 days after birth.¹ Three anatomic structures are involved in normal descent: the vaginal process, the gubernaculums, and the inguinal canal.

The vaginal process is a flask-shaped evagination of the peritoneum in the inguinal region. This evagination penetrates the abdominal wall at the inguinal canal and passes into the scrotum. In stallions, it is a large, flask-shaped structure that is divided into 2 parts: (1) a long, narrow neck containing the spermatic cord, which consists of the blood vessels, nerves, and lymphatics of the testis, the epididymis, the vas deferens, and the internal cremaster muscle, and (2) a dilated body containing the testes, epididymis, and the proximal section of the vas deferens.^2,³ The opening of the vaginal process, through which the gubernaculum proprium passes, forms the vaginal cavity.¹^–⁴ The vaginal ring is at the level of the internal inguinal ring.^2,⁵ The testes are encased in the tunica albuginea and lie within an out pocket of abdominal peritoneum known as the tunica vaginalis. The tunica vaginalis consists of visceral and parietal tunics. The visceral tunic is tightly adhered to the testis, ducts, and vessels, and the parietal tunic is continuous with the parietal peritoneum.

The fetal gubernaculum is divided into 3 parts: (1) a portion connecting the testis to the tail of the epididymis—the proper ligament of the testis, (2) a portion between the tail of the epididymis and vaginal process—the caudal ligament of the epididymis, and (3) a remaining portion extending from the vaginal process to the scrotal fold—the scrotal ligament.^2,^3,^6,⁷ The gubernacular ligament passes from the end of the vaginal process to the scrotum but does not form a firm anchor to the process. The gubernaculum helps to keep the passageway open by expanding in size and stretching the vaginal ring.¹–⁴ ^6,⁸–¹²

The inguinal canal is a passageway between the internal inguinal ring and the external inguinal ring. The internal inguinal ring is formed from the internal abdominal oblique muscle, and the external inguinal ring is formed from both the internal and external abdominal oblique muscles.^2,³ The external cremaster muscle, which lies on the outside of the vaginal tunic, is derived from the internal abdominal oblique muscle and is continuous with the parietal layer of the tunica vaginalis, inserting at the caudal pole of the testis. It helps bring the testis closer to the external inguinal ring when it contracts.^3,¹³

During embryonic development, the equine gonad becomes differentiated into testes at 5½ weeks of gestation and lies ventral to the mesonephric kidney.²–⁴ ^8,¹⁴–¹⁶ At 4 months, the cranial pole of the testis overlies the caudal pole of the mesonephric kidney, while its caudal pole is still some distance from the vaginal ring. The equine gonad begins to hypertrophy at 6 weeks of age through an increase in the number of interstitial cells, and by 5 months of gestation this hypertrophy results in the caudal pole of the testis lying close to the vaginal ring.^1,^4,^8,^14,¹⁷–¹⁹ The high growth rate of the fetal testes corresponds to a period of high blood estrogen in the pregnant mare.^17,¹⁹ The equine fetal testis weigh about 20 grams at 5 months of gestation, 50 grams at 8 months, and 30 grams at 10 months.^1,^8,^9,¹⁴

After about 7½ months, the testes begin to decrease in size due to degeneration of the interstitial cells, and by 10 months there is a clear separation between the testes and the kidneys.^1,^4,^8,^14,^16,¹⁹ Between 5 months and 10 months the inguinal canal is too small for the passage of the testes. The developing gubernaculum and vaginal process hold the testes at the vaginal ring. Other abdominal viscera also aid in retaining the testes in the pelvic region. After 5 months of gestation, the enlarged distal end of the gubernaculum proprium becomes the main force preventing inversion of the vaginal process. Firm fibrous connections between the process and the inguinal canal also exist. After 8 months of gestation, the gubernaculum proprium begins to decrease in size and only the fibrous connections in the inguinal canal hold the vaginal process outside the vaginal ring.^1,^2,^4,⁸–¹⁰ ^14,^16,^17,¹⁹

The vaginal process and gubernaculum continue to lengthen at about the same rate up to 8½ months of gestation. During this time, the caudal gonadal ligament, between the testis and caudal epididymis, increases in length faster than the gubernaculum, resulting in the caudal epididymis becoming progressively displaced from the caudal pole of the testis. The caudal epididymis does in fact descend into the inguinal canal at about 5 months of gestation in advance of the testes.¹–⁴ ^11,^14,^15,^17,¹⁸ After 8½ months there is a gradual decrease in the length of the gubernaculum while the vaginal process remains static, so that the caudal pole of the regressing testis is drawn into the opening of the vaginal ring, which has been dilated by the increasing diameter of the caudal gonadal ligament and the body of the epididymis.^2,^4,¹⁴ Owing to the length of the caudal gonadal ligament, hypertrophy of the fetal testis, the small diameter of the vaginal ring, and the lack of tension on the gubernaculum, descent of the testes through the inguinal canal occurs late in gestation.¹⁸

The actual passage of the testis through the vaginal ring is a rapid process, which occurs during the last month of pregnancy. The gubernacular length continues to decrease while there is a rapid increase in the length of the vaginal process, which increases tension of the gubernaculum on the small flaccid testis. The testis does not pass immediately into the scrotum because the gubernaculum mass remains below it. At birth, most testes lie in the inguinal canal while the gubernaculum lies in the scrotal region and can easily be mistaken for a testis. Further passage of testis into the scrotum after birth is affected by progressive reduction in length and mass of the gubernaculum, which finally becomes a small, dense, fibrous band.^1,^2,^4,^6,^17,¹⁹ During the first 2 weeks of neonatal life, the internal vaginal ring constricts to about 1 centimeter in diameter and is fibroses; thus the testis cannot be forced through in either direction after this time.^4,^6,^8,^9,^16,¹⁷

At birth the weight of each testis ranges from 5 to 20 grams and testicular size does not change much throughout the first 10 months of life. There is slight growth between 11 and 16 months of age, and rapid development of the testes starting around 18 months of age.¹ Puberty in pony stallions was reported to occur between 11 and 15 months of age based on the appearance of sperm in the ejaculate.²⁰^–²² Puberty in Quarter horse stallions was found to occur between 13 months and 23 months of age.²³ Although stallions produce spermatozoa throughout the year, their testes show seasonality where maximum production of sperm occurs from May to July. In one study, 17 stallions were looked at and their testes were found to weigh an average of 167 grams, with an average length of 8.5 cm and an average width of 5.0 cm.²⁴ In another study, 48 stallions were looked at and their average weight was 164 grams, with an average length of 10.4 cm and an average width of 5.6 cm.²⁵

Normal testicular descent involves a process in which the essential physical components are a testis with sufficient freedom of attachment to allow it to be moved from its original dorsal abdominal location to its final scrotal position; patent inguinal rings leading to a properly developed tunica vaginalis; proper size relationship between the testis and the inguinal ring; and sufficient force to move the testis to the inguinal ring.

Surgical Approaches to Castration of Normal Stallions

Castration is probably the most common surgical procedure performed in male horses. It is performed to prevent or eliminate aggressive male behavior in animals not intended to become breeding stallions. Other indications for castration include prevention of testicular neoplasia, testicular trauma, spermatic cord torsion, orchitis, hydrocele, and most cases of inguinal or scrotal hernia. Castration may be safely performed at any age, without complications, even before weaning. Some owners prefer to delay castration until a later age than weaning to allow for development of a more masculine appearance.^13,²⁶

Castration can be performed safely and acceptably either with the horse standing, using chemical restraint and local anesthesia, or with the horse in recumbency, under general anesthesia. Factors for choosing one method or the other include the surgeon’s preference, practice tradition, owner’s desire, behavior of horse, descent of the testes, and location where the surgery will be performed.^13,^18,²⁶ One must always make sure that the horse has been immunized for tetanus within the last 6 months before any surgical procedure. If not, then the horse should be vaccinated right before or right after the surgical procedure. Postoperatively the horse should be confined for 12 to 24 hours so that he may be monitored for bleeding or herniation. After that, moderate exercise is important so that drainage occurs and healing proceeds normally.^13,^18,²⁶

Standing Castration

Standing castrations have been done for years. However, to be able to use this approach, both testes must be descended into the scrotum. Some of the types of chemical restraint used are the sedative-hypnotics, tranquilizers, and opioids, and include acepromazine, xylazine hydrochloride, romifidine, detomidine hydrochloride, and butorphanol tartrate. Detomidine (0.011-0.022 mg/kg intravenously [IV]) in combination with butorphanol (0.011-0.022 mg/kg IV) has been shown to provide excellent sedation and analgesia for standing castrations. Xylazine (0.3-0.5 mg/kg IV) has been added to this combination for a longer lasting sedative effect.¹³ Once sedation takes place, anesthesia of the testis and spermatic cord is accomplished by injecting 10 to 25 ml of local anesthetic into the parenchyma of each testis and also in a subcutaneous line block along each side of the scrotal raphae.^13,^18,²⁶

Standing castration should be performed with both the surgeon and handler positioned on the same side of the horse, with a right-handed surgeon standing on the left side of the horse, facing toward its rear. Both spermatic cords are grasped with the left hand, and the testes are displaced ventrally to tense the skin of the scrotum. Two parallel incisions are made through the scrotal skin on either side of the median raphae. The left incision is deepened so that the cut goes through the tunica dartos, scrotal fascia, and vaginal tunic. This allows the left testis to prolapse through the incision. The mesorchium is then separated so that separate emasculation can be done of the vascular spermatic cord and the vaginal tunic. This procedure is then repeated on the right testis.^13,²⁶

Care must always be taken to ensure that the emasculator has been applied so that the crushing component is proximal to the cutting blade (nut to nut), which means that the crushing will occur on the part of the tissue that is to be left in the horse. This should prevent major bleeding from occurring. Once both testes have been removed, any tissue that is protruding from the incisions should be excised and the wounds left open to heal by secondary intention.^13,²⁶

Recumbent Castration

Many veterinarians prefer general anesthesia for castration because surgical exposure is improved and it carries less risk for both the surgeon and patient.¹³ A requirement for recumbent castration is a clean, safe area for induction and recovery. Again, before induction, a physical exam is necessary to make sure both testes are palpable in the inguinal/scrotal area and that the horse is healthy and a good candidate for general anesthesia. Ideally the animal should be taken off feed for 12 to 24 hours before anesthesia is carried out.^13,²⁶

In most cases only a short-term, general anesthetic is necessary and there are many safe and effective choices. The most commonly used anesthetic regimen is still xylazine (1.1mg/kg IV) administered first, then, once the horse is fully sedated, Ketamine (2.2-3.0 mg/kg IV). This protocol provides recumbency in 1 to 2 minutes and 10 to 15 minutes of surgical anesthesia. The administration of butorphanol (0.022-0.044 mg/kg IV) or diazepam (0.05-0.1 mg/kg IV) simultaneously with or just after xylazine may provide additional anesthetic time. If more time is necessary, more xylazine or Ketamine can be given intravenously at ⅓ to ½ the dose. If additional time is necessary, it is wise to use a longer lasting anesthetic from the start, for example, continuous infusion of 5% guaifenesin containing 2 mg/ml Ketamine and 0.5 mg/ml xylazine.^13,²⁶

Once the animal is anesthetized, he is placed in either lateral or dorsal recumbency, depending on surgeon’s preference and the status of the animal. Lateral recumbency is easier on the cardiovascular status of the animal; however, it is easier to reach both testes in dorsal recumbency.^13,^18,²⁶

The surgeon should stand behind the horse’s croup and reach over the operative field. The lower testis is removed first in case there is dripping of blood. Parallel skin incisions are made on either side of the median raphae, as in the standing castration. The incision on the down testis is made deeper so that the scrotal fascia is cut exposing the vaginal tunic. The scrotal fascia is then stripped from the vaginal tunic as far proximally as the surgeon can go. Once this is done, the surgeon must make a decision as to whether to carry out a closed castration or an open castration. Young horses or horses with small testicular cords may be safely emasculated across the entire spermatic cord within the vaginal tunic (closed castration). In mature horses, it is safer to open the tunic, separate the vascular spermatic cord and ductus deferens from the vaginal tunic, and emasculate and/or ligate separately (open castration). One will have less chance of heavy bleeding postoperatively if an open castration is done.²⁶

Traditionally castration wounds are left open to heal by second intention. However, primary closure of these wounds at the time of surgery is becoming more popular. It allows the wound to heal more quickly with less chance of a secondary bacterial infection. Once castration has occurred, scrotal ablation and closure of deeper fascial layers may be performed to reduce dead space. Cox described a closed castration, but instead of emasculating the spermatic cord and tunic, a transfixation suture was first placed and then the cord and tunic were emasculated distal to the ligature. The scrotal skin incision was then closed with suture. No complications were seen with this procedure, postoperative appearance of the animals was more acceptable, and convalescence time was dramatically reduced.²⁷ Castration with primary closure is probably ideal, however, it ideally should be done in a hospital using Halothane or Isoflurane, not in the field, so as to limit postoperative infection.²⁶

There can be many complications after castration. Most common are hemorrhage and infection, with intestinal evisceration, omental prolapse, penile damage, and hydrocele being less common. Also there can be accidents during physical restraint, general anesthesia, and recovery from general anesthesia.^18,^26,²⁸^–³⁰

After castration, the concentration of serum testosterone and estradiol rapidly decreases and stabilizes within 6 to 12 hours, while the concentrations of follicle stimulating hormone (FSH) and luteinizing hormone (LH) increase dramatically. It appears that testicular hormones exert negative control over FSH and LH secretion.^22,^31,³² The desire to mount (libido) and the ability to ejaculate are gradually lost; however, libido is retained for a longer period than is the ability to ejaculate. Attainment of an erection is generally the last aspect of normal sexual behavior to disappear after castration.³¹ Postcastration geldings could have enough motile sperm in their ejaculate to impregnate a mare for up to 1 week. This is due to the presence of extragonadal sperm. Frequency of ejaculation does not seem to hasten the disappearance of spermatozoa from the ejaculates. It is therefore important to keep a newly castrated horse separate from mares for at least a week to prevent unwanted pregnancy. Longer separation may help to diminish stallion-like behavior.^26,³³

PATHOGENESIS OF CRYPTORCHIDISM

Cryptorchidism, or the failure of normal testicular descent, is most frequently found in humans, swine, and horses, though it does occur in many other mammalian species.^8,³⁴ In horses, cryptorchidism, with one or both testes retained, is a common problem. The percentage of foals having cryptorchidism is about 5% to 8%, with most being unilateral cryptorchids. It is not known how many diagnosed retained testes would descend at a later date, nor how many would remain undescended.^19,³⁵ The highest incidence of cryptorchidism has been reported in Quarter horses, followed by Percheron, American Saddle horses, and ponies.^6,^35,³⁶ When both testes are retained in the abdomen, the animal is sterile due to the detrimental effect of higher temperatures on spermatogenesis, and the testes have been shown to be more prone to development of secondary tumors.^5,^19,³⁷ Unilateral cryptorchids may be fertile if spermatogenesis occurs in the descended testis.¹⁹

Cryptorchid testes are usually smaller than normal, are soft and flaccid, and weigh between 25 and 131 grams, which is considerably less than normal.^11,³⁸^–⁴⁰ Histologically, cryptorchid testes have arrested spermatogenesis, with changes seen mainly in the germ cell layers. Their Leydig cells appear normal.^11,^19,^38,^41,⁴² Spermatogenesis does not seem to proceed beyond A- or B-spermatogonia in abdominal testes, or primary spermatocytes in the case of inguinal testes.^3,^17,^18,³⁹ Mean tubular diameters and spermatogenic cell layers were significantly less in inguinal and abdominal testes when compared to normally descended scrotal testes.³⁹ In horses, development of seminiferous tubules in abdominal testes appeared to proceed to the extent normally found in foals of 3 to 4 months of age, whereas development in the inguinal testes roughly corresponded to that of a 9- to 12-month-old normal foal.^3,^6,^11,^17,¹⁸