Anthony Claes, James A. Brown
Diagnosing and Managing the Cryptorchid
Failure of one or both testes to descend into the scrotum is known as cryptorchidism. Cryptorchidism is more frequently observed in American Quarter Horses, Percherons, American Saddlebreds, ponies, and cross-bred horses than in Thoroughbreds, Standardbreds, Morgan Horses, Tennessee Walking Horses, and Arabians. Even though it is commonly accepted that equine cryptorchidism is heritable, no scientific evidence is available to date to support this statement. The different stages of testicular descent and associated regulatory factors are beyond the scope of this chapter, but a review can be found in the Suggested Readings that follow. An accurate diagnosis and treatment of cryptorchidism is essential not only to avoid unwanted stallion behavior but also to prevent torsion or neoplastic transformation of the cryptorchid testis. Cryptorchid stallions should be differentiated from monorchid stallions and false rigs because the management differs considerably. Monorchidism is a rare condition characterized by the complete absence of one testis, whereas false rigs are horses that continue to have stallion-like behavior after being fully castrated.
Diagnosis
Cryptorchidism should be suspected if only one testis is present in the scrotum (Figure 153-1) or if stallion-like behavior is observed in a horse without scrotal testes. Taking a thorough history is the first diagnostic step, and questioning must be focused primarily on behavioral characteristics and whether castration took place. Unfortunately, unless the horse was born and raised by the current owner, the obtained information is often scarce and vague. Before proceeding with the physical examination, the examiner should note whether the horse has secondary sex characteristics. An important part of the physical examination consists of inspection and palpation of the scrotal and inguinal area. If palpation fails to reveal testicular or epididymal tissue, some effort should be made to detect scar tissue from previous castration. The availability of more specific tests, such as hormone analysis, palpation per rectum, and ultrasonography (either transrectally or transcutaneously), is indispensable in the diagnosis of cryptorchidism.
Endocrine Testing
The most convenient and least expensive method to detect retained testicular tissue in supposedly castrated horses is hormone analysis that includes measurement of serum testosterone, estrone sulfate, and anti-Müllerian hormone concentrations, or some combination thereof. To interpret test results accurately, values must be compared with the reference ranges provided by the diagnostic laboratory because they can vary considerably among laboratories.
Measurement of baseline testosterone concentration is often used by equine practitioners as a quick screening test for cryptorchidism. Additional testing is required in 11% to 14% of cases because of inconclusive results. When a human chorionic gonadotropin (hCG) stimulation test is used, the percentage of inconclusive results can be decreased to 6.7%. An hCG stimulation test is based on the principle that hCG interacts with the luteinizing hormone receptor, which stimulates synthesis of testosterone by the Leydig cells. Although variations exist, the protocol involves collection of a blood sample immediately before, as well as 30 to 120 minutes after, intravenous administration of 6000 to 12,000 IU hCG. The testosterone response, which varies considerably among individual stallions, is evaluated by comparing baseline with post-hCG testosterone concentrations. The diagnostic accuracy of an hCG stimulation test is 94.6%. Notwithstanding its usefulness, an hCG stimulation test is time consuming and has limited reliability in horses younger than 18 months of age. In addition, the testosterone response is influenced by location of the cryptorchid testis and season, with a reduced response seen in stallions with an abdominal testis and during the winter. In more complicated cases, such as persistent stallion-like behavior despite negative test results, it can be valuable to extend the blood collection period up to 24 to 72 hours after the administration of hCG. The hCG-induced testosterone response is biphasic, with a first peak observed within 2 hours after the administration of hCG, followed by a second, more prominent peak approximately 24 to 72 hours later.
Another diagnostic test that has been used to distinguish geldings from cryptorchid stallions is measurement of estrone sulfate. Because this hormone is produced in the testis of stallions, its concentration is higher in cryptorchid stallions than in geldings. In general, measurement of estrone sulfate is not recommended for horses younger than 3 years of age or for cryptorchid donkeys.
The most recent endocrine marker for the presence of testicular tissue is anti-Müllerian hormone (AMH). Serum AMH concentrations are significantly higher in cryptorchid stallions, compared with intact stallions or geldings. Moreover, concentrations in geldings are at the lower limit of detection. Therefore determination of AMH concentrations can be a valuable diagnostic tool for cryptorchidism.
Testing to Localize a Cryptorchid Testis
Diagnostic tests that reveal the location of a cryptorchid testis are extremely valuable in the diagnostic workup of cryptorchidism. They are able to differentiate not only between inguinal and abdominal cryptorchid stallions but also between unilateral cryptorchid, bilateral cryptorchid, and monorchid stallions, whereas endocrine tests are of no particular value for these anatomic determinations. Moreover, planning the surgical approach often depends on accurate localization of the cryptorchid testis or testes.
Transrectal Palpation and Ultrasound
Less commonly used in first-opinion practice but still useful in differentiating inguinal from abdominal cryptorchid stallions is transrectal palpation. The location of the undescended testis can be detected with 87.9% accuracy by palpating the inguinal ring per rectum to determine whether the ductus deferens enters the vaginal ring. If it does, the testis is likely to be located inguinally. However, an abdominal testis can be misdiagnosed as an inguinal testis if the ductus deferens both enters and exits the inguinal ring. In addition, the feasibility of accurately identifying an abdominal testis based solely on transrectal palpation is limited. A more reliable technique is transrectal ultrasonography because the cryptorchid testis is sonographically easy to distinguish from other intraabdominal structures. An abdominal testis is typically identified by its ovoid shape, smaller size, and reduced echogenicity, compared with the normally descended testis. Another important landmark of testicular tissue is the testicular vein. The main disadvantage of transrectal palpation and ultrasound is that the procedure is not very well tolerated by young horses. Therefore adequate sedation and restraint are required to avoid rectal tears or injury to the veterinarian. Furthermore, the technique is not applicable in immature horses or small breeds such as Miniature Horses.
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