Diagnosis of Cryptorchidism

• Anti-Mullerian Hormone is very accurate and the test of choice
• Oestrone sulphate may be tested in horses over 3 years (not donkeys)
• hCG stimulation testing might be performed as a further test

Bilateral cryptorchids are usually infertile and unilateral cryptorchids have reduced sperm production. The Leydig cells of retained testicles remain capable of testosterone production and cryptorchids therefore exhibit stallion-like behaviour. Stallion-like behaviour may be observed in up to 20% of geldings and therefore is not always an indication of cryptorchidism.

The condition is likely to be hereditary though the mode of inheritance may be different between stallions, as some have a high incidence of cryptorchid foals whilst others do not. Percherons, Quarter Horses, Friesians and cross-bred horses or ponies may have a higher incidence of cryptorchidism than other breeds, whilst Thoroughbreds, Arabs, Standardbreds and Morgans may be less likely to be affected.

Testicles should descend by 2 weeks of age. Failure of descent of right and left testicles occurs with equal frequency overall although the right testicle is more likely to be retained in the inguinal region (60% vs 40% in the abdomen) whereas the left is more likely to be retained in the abdomen (75% vs 25% in the inguinal region). Although most cryptorchids are unilateral, about 10% of horses with retained testicles have the condition bilaterally. Where the condition is bilateral, both testicles are usually retained at the same site; two thirds being intra-abdominal and the remaining third being inguinal. Cryptorchidism may on very rare occasions (1 in 500) be a sign of intersexuality. Anorchidism and monordichism are exceedingly rare.

A diagnosis of cryptorchidism can often be made by careful examination, especially if the animal’s
history is known:

• External palpation under sedation: Careful palpation of the scrotum and inguinal region. Beware palpation in foals as the gubernaculum is relatively large and may be mistaken for a testicle.
• Internal palpation: Rectal palpation or transrectal ultrasound may identify the retained testicle.
• Transcutaneous abdominal ultrasound: with experience and a co-operative patient, ultrasonography is a very accurate means of diagnosis.

ANTI-MÜLLERIAN HORMONE (AMH)
AMH is the test of choice for identifying cryptorchids of any age.
In the horse, AMH is strongly expressed in Sertoli cells of the fetal, neonatal, and pre-pubertal testes, as well as cryptorchid testes, Sertoli cell tumours, and male intersex gonads. At puberty, maturation of Sertoli cells is accompanied by reduced production of AMH, however normally descended testes continue to produce some.
AMH is a reliable and specific test for the presence of testicular tissue in horses of any age. AMH concentrations are significantly higher in cryptorchid stallions (mean 33 ng/mL) versus intact stallions (mean 15 ng/mL), and AMH is undetectable in geldings. Following castration, serum AMH concentrations decline rapidly (biological half-life of 1.5 days) making AMH a useful test if there is doubt over the success of a recent castration procedure.

In horses over 3 years of age, measurement of serum oestrone sulphate concentration is a good indicator of the presence of testicular tissue. In younger animals and in donkeys, cryptorchids do not reliably produce sufficient oestrone sulphate so whilst a positive result is useful, a negative result does not rule out the possibility of retained testicular tissue.
Expected Serum Oestrone Sulphate Concentrations:
Gelding <0.02 ng/ml
Cryptorchid 0.1-10 ng/ml
Stallion >10 ng/ml

For results between 0.05 and 1 ng/ml, further testing with an alternative test (preferably AMH) is
recommended.

Measurement of testosterone following injection of human chorionic gonadotrophin (Chorulon) can be used where the results of other hormone assay measurements are equivocal.

• Measure serum testosterone concentration
• Administer 6000iu human chorionic gonadotrophin IV
• Re-measure serum testosterone concentration 24 hours later (Serum testosterone will be increased from 30 minutes to 48 hours after injection of hCG so timing of the follow-up sample is not critical).