The Cryptorchid Horse: Work Up and Surgical Approaches

The term cryptorchid refers to a non-descending testis. It is mostly a unilateral condition, but sometimes both testicles have failed to descend.

If the testis and epididymis are both intra-abdominal, the horse is referred to as a complete abdominal cryptorchid.
If the epididymis but not the testis has descended into the scrotum, the horse is referred as a partial or incomplete abdominal cryptorchid.
If the testis and epididymis are retained within the inguinal canal the horse is referred to as an inguinal cryptorchid.

Work up – Physical examination:

Firstly it is vital to sedate the animal in order to ensure good relaxation of the cremaster muscle. Some horses may appear to be cryptorchid, but once sedated the testis will be nicely visible within the scrotum.
In both inguinal and abdominal cryptorchids there will not be a testicle visible on the affected side. However, one should look very closely for castration scars, because it may be possible that the horse has had a unilateral castration, or an attempt to castrate the horse was made, but the testicle was left in place. A good case history is very important. If the owners have bred the horse themselves, and they are sure no previous castration attempts were made, then there has to be a retained a testicle present (either inguinal or abdominal).

The second part involves palpation of the scrotum and the inguinal canal. A deep palpation of the inguinal canal should be performed. With inguinal cryptorchids, the testis can often be palpated within the inguinal canal. Abduction of the hind limb can sometimes be of assistance in palpating the inguinal canal. It should be noted that the penis and subcutaneous fat in the inguinal region can sometimes be mistaken for an inguinal testis.

The third part involves a rectal examination. The vaginal ring or internal inguinal ring should be palpated per rectum. In stallions the vaginal ring is large, and 1 or 2 fingers can be inserted into it. A rectal examination is not necessary if the diagnosis of an inguinal cryptorchid can be made by external palpation. Some young stallions are difficult to palpate, with adequate sedation and administration of a smooth muscle relaxant (Buscopan IV or Propantheline Bromide IV) prior to examination, greatly facilitating easier palpation of the vaginal ring.
If there is no vaginal ring present, it is 100% certain that the horse is an abdominal cryptorchid. If the vaginal ring is present and the ductus deferens and blood vessels are palpable as a string through the vaginal ring, the horse can either be an inguinal cryptorchid or a castrated gelding. A partial or incomplete abdominal cryptorchid is difficult to differentiate from an inguinal cryptorchid.  [...]