Discussion

Cryptorchidism is the most common nonlethal developmental disorder in the horse, occurring in one-sixth of 2- and 3-year-old colts presented for castration at a teaching hospital (Haynes 1986). It occurs when one or both testicles fail to descend from the abdomen into the scrotum. Retained testicles retain the ability to produce testosterone, resulting in the development of secondary sexual characteristics and behaviour. Castration is necessary to abolish the adverse behavioural characteristics, a procedure more technically challenging compared to routine castration.
Cryptorchids have been classified into 3 groups within the literature. If both the testicle and the epididymis are found within the abdomen the horse is a 'complete' abdominal cryptorchid. In these cases, the testicle is usually very mobile, being found near the deep inguinal ring, mixed up with small intestine, dorsal to the rectum, or occasionally high in the abdomen adjacent to the ipsilateral kidney. If the tail of the epididymis enters the inguinal canal, leaving the testicle and remnants of epididymis within the abdomen, the horse is classified an 'incomplete' abdominal rig. In both cases, testicles are removed by suprapubic paramedian or parainguinal laparotomy under general anaesthesia, or by one of several laparoscopic methods carried out standing or under general anaesthesia. If the testicle and epididymis are retained within the inguinal canal the horse is an inguinal cryptorchid, which can be temporary or permanent. Temporary retention is mostly seen in ponies and such testes usually complete their descent into the scrotum by 3 years of age. In these cases, 3 out of 4 cases are right sided. Permanent inguinal retention occurs in all breeds of horse with equal frequency left and right.
Inguinal testicles can be removed using either a direct inguinal or indirect scrotal approach (Turner and McIlwraith 1989), or via retraction into the abdominal cavity for laparoscopic removal.
In the author's practice, laparoscopic removal in the standing sedated animal is the modus operandi for complete and incomplete abdominal cryptorchids. In unilateral cases, the descended testicle is then removed using an open approach under standing sedation. Inguinally retained testicles are removed using a scrotal approach in dorsal recumbency. It was necessary to develop a reliable method of locating cryptorchid testicles before surgery in order to make the correct approach.

Technique:
All animals undergoing cryptorchid castration are starved for 36 h before examination. On presentation, individuals are heavily sedated using a routine intravenous alpha-two opiate combination. The technique is derived from that first described by Schambourg et al. (2006). The groin area is palpated and liberally soaked with surgical spirit; no clipping is carried out. The scrotal and connective tissue abaxial to the penile shaft and external to the superficial inguinal ring are then examined methodically using a 4 MHz curvilinear ultrasound probe set on 8 cm depth. Testicles have a characteristic appearancewith a uniform grey stroma and bright acoustic signal of the tunica albuginea. If evidence of a testicle was present the case was treated as inguinal retention; if no testicle was seen, the individual was treated as an abdominally retained case. It was not possible to distinguish complete and incomplete abdominal retention because the location of this testicle was not visualised.

Results:
All rig castrations carried out by the author from June 2009 to April 2012 were identified retrospectively. The results of inguinal ultrasound determination of location were then compared with the results of surgery to calculate sensitivity, specificity, positive and negative predictive values. Fisher's exact test was used to compare the effect of side on location.
In total, 36 cryptorchid testicles were removed from 33 individual cases, 17 left- and 19 right-sided. Median age at presentation was 2 years, range 1 - 4. Breeds included 12 Welsh Cob (Section C and D), 9 Welsh Pony (Sections A and B), 8 Standardbred, 4 Irish Draught, 2 Thoroughbred and one Warmblood. No cases underwent rectal examination and only 2 had undergone blood test before examination; none reported previous attempted surgery. Overall 21 (58%) of cases were located inguinally and 15 (42%) abdominally; the proportion of left and right testicles in each location was identical (P = 1.000). Sensitivity and specificity of inguinal ultrasound examination was 0.95 and 0.87, respectively; positive and negative predictive values were 0.61 and 0.39, respectively.

Conclusion:
Inguinal ultrasound carried out in this way provided an accurate method of determining the location of cryptorchid testicles before surgery. Time spent attempting visualisation of abdominal testicles by percutaneous ultrasound of the flanks (Schambourg et al. 2006) was not necessary. One false negative case occurred; this case underwent an abortive laparoscopic examination and was then anaesthetised for inguinal removal. Two false positives occurred, both 2-year-olds, in which a small incomplete abdominally retained testicle was retrieved via a scrotal approach.

It seems most likely that these had fallen back into the abdomen in dorsal recumbency.