Canine Cryptorchidism Study Guide

Overview and Clinical Importance

Cryptorchidism is one of the most common congenital defects of the reproductive system in dogs, occurring when one (unilateral) or both (bilateral) testicles fail to descend into the scrotum. The condition is reported in 1-15% of dogs, with higher incidence in certain breeds. Understanding cryptorchidism is essential for the NAVLE because it encompasses embryology, endocrinology, oncology, and surgical principles.

The clinical significance of cryptorchidism extends beyond reproductive concerns. Retained testicles carry a 9.2 to 13.6 times higher risk of developing testicular neoplasia compared to normally descended testicles. Additionally, cryptorchid dogs face risks of testicular torsion, a painful surgical emergency. Because cryptorchidism is a heritable condition, affected dogs should never be used for breeding.

Embryology and Pathophysiology of Testicular Descent

Normal testicular descent in dogs occurs in two distinct phases, each regulated by different hormonal mechanisms. Understanding these phases is critical for comprehending why cryptorchidism occurs and how it is classified.

Phase 1: Transabdominal Phase

During embryonic development, the testes form adjacent to the caudal pole of the kidneys. The gubernaculum, a gelatinous mesenchymal structure, connects the lower pole of the testis and epididymis to the future inguinal canal. This phase is primarily regulated by Insulin-like peptide 3 (INSL3), which is produced by fetal Leydig cells. INSL3 acts through its receptor (RXFP2) to cause gubernacular growth and differentiation, resulting in the "swelling reaction" that anchors the testis near the inguinal ring as the fetus grows.

Additionally, Anti-Mullerian Hormone (AMH) may play a supportive role in this phase. The regression of the cranial suspensory ligament (CSL), mediated by testosterone, allows the testis to move caudally. In males, testosterone causes CSL regression, whereas in females, the CSL persists and keeps the ovaries in their abdominal position.

Phase 2: Inguinoscrotal Phase

The second phase involves migration of the testis through the inguinal canal into the scrotum. This phase is primarily controlled by androgens (testosterone), which act both directly on the gubernaculum and indirectly via the genitofemoral nerve (GFN). The GFN releases calcitonin gene-related peptide (CGRP), which is thought to guide gubernacular migration through a chemotactic gradient. The processus vaginalis forms inside the gubernaculum, enabling the testis to reach the scrotum while remaining within the peritoneal covering.

Timeline of Normal Testicular Descent in Dogs

Classification of Cryptorchidism

Breed Predisposition and Genetics

Cryptorchidism has a heritable basis with a complex inheritance pattern. Studies suggest it is linked to the X chromosome (sex-linked) or follows an autosomal recessive pattern with variable penetrance. Recent genome-wide association studies have identified variants in the HMGA2 gene associated with inguinal cryptorchidism risk in dogs, and KAT6A gene polymorphisms associated with global H3K9 acetylation changes.

High-Risk Breeds

Clinical Signs and Physical Examination

Most dogs with uncomplicated cryptorchidism appear completely normal aside from the absence of one or both testicles in the scrotum. The retained abdominal testis does not produce viable sperm due to elevated core body temperature, but it still produces testosterone. Therefore, cryptorchid dogs exhibit normal male behaviors including urine marking, roaming, and mounting.

Physical Examination Findings

• Scrotal palpation: Empty scrotum (bilateral) or single testis present (unilateral)
• Inguinal palpation: May detect subcutaneous testis between external inguinal ring and scrotum
• Abdominal palpation: Abdominal testes rarely palpable; may feel mass if neoplastic
• Penile spines: Present in intact males; disappear within 6 weeks of castration

Localization Technique

If only one scrotal testis is present, push it dorsally and cranially toward the prescrotal region. The testis will move to one side of the midline. The ectopic testis will be on the contralateral side.

Complications of Cryptorchidism

Testicular Neoplasia

Cryptorchid testes have a 9.2 to 13.6 times higher risk of developing testicular tumors compared to scrotal testes. The relative risk for Sertoli cell tumors specifically is 23 times higher in cryptorchid dogs. Interestingly, even the normally descended contralateral testis has an increased risk of neoplasia in unilaterally cryptorchid dogs.

Types of Testicular Tumors in Cryptorchid Dogs

Feminizing Syndrome (Hyperestrogenism)

• Bilateral symmetrical non-pruritic alopecia (starts in perineal/genital region)
• Hyperpigmentation of skin
• Gynecomastia (mammary gland enlargement)
• Pendulous, edematous prepuce
• Attraction to other male dogs
• Squatting to urinate (female posture)
• Prostatomegaly
• Bone marrow suppression (15%): Non-regenerative anemia, thrombocytopenia, pancytopenia

Testicular Torsion

Retained testes are more susceptible to spermatic cord torsion than scrotal testes. Risk increases with progressive enlargement due to neoplasia. Clinical signs include:

• Acute severe abdominal pain
• "Prayer position" (forelimbs flat, rump elevated)
• Vomiting and anorexia
• Depression and lethargy
• Abdominal guarding on palpation

This is a surgical emergency requiring immediate stabilization and orchiectomy.

Diagnostic Approach

Physical Examination

Cryptorchidism is suspected when one or both testicles are not palpable in the scrotum by 8 weeks of age and confirmed if testicles have not descended by 6 months of age (when the inguinal canal closes). Systematic palpation should include:

• Scrotal sac (both sides)
• Inguinal region and subcutaneous tissue
• Caudal abdomen
• Penis examination for penile spines (present in intact males)

Diagnostic Imaging

Abdominal ultrasound is the primary imaging modality for locating intra-abdominal testes. Normal cryptorchid testes are typically smaller than scrotal testes, oval-shaped, and have homogeneous echogenicity. Neoplastic testes may show:

• Increased size
• Heterogeneous echogenicity
• Mixed anechoic to echogenic patterns
• Hyperechoic capsule

Hormonal Testing

Hormonal testing is used when castration status is unknown or to confirm presence of testicular tissue when testes are not palpable.

Treatment

The only recommended treatment for cryptorchidism is bilateral orchiectomy (castration). Both testicles must be removed because: (1) the retained testis is at high risk for neoplasia and torsion, (2) the scrotal testis must be removed to prevent breeding and passing the heritable trait, and (3) even the descended testis has increased neoplasia risk.

Surgical Approaches

Critical Surgical Considerations

• Locate the ductus deferens: Both ductus deferens enter the urethra at the prostate. Trace dorsally from the prostate to find the testis.
• Gubernaculum identification: Appears as a thin fibrous band (2-4 mm in dogs) extending from the vaginal tunic toward the scrotum.
• Verify the structure: Cryptorchid testes may be small and atrophied. Confirm you are removing the testis, not the prostate (inadvertent prostatectomy is a reported complication).
• Double ligate: Use standard double ligation technique for spermatic cord.

Prognosis

Prognosis after bilateral orchiectomy is excellent. Surgery eliminates the risk of testicular cancer and torsion. Only about 15% of testicular tumors metastasize, so surgical removal usually cures the condition. If feminization is present, signs typically regress within 4-8 weeks post-surgery. Failure to improve by 4-5 months suggests possible metastasis. Bone marrow suppression from estrogen toxicity may require supportive care and takes longer to resolve.

Differential Diagnoses

When evaluating a dog with apparent absence of one or both testicles, consider:

• Previously castrated: Confirm with hormonal testing (AMH or stimulation test)
• Monorchidism (testicular agenesis): Rare; complete absence of one testis
• Anorchidism: Complete bilateral testicular agenesis; very rare
• Retractile testis: Testis that retracts to inguinal canal due to cremasteric reflex; can be manipulated into scrotum
• Disorders of sexual development (DSD): Consider in dogs with ambiguous genitalia

"RIGHT is WRONG" Memory Aid: Right-sided cryptorchidism is more common because the right testis develops more cranially and has farther to descend.

"23x SERTOLI" Memory Aid: Cryptorchid dogs have 23 times higher risk of Sertoli cell tumors.

"INSL3 = Phase 1" Memory Aid: INSL3 (Insulin-Like 3) controls Phase 1 (transabdominal). Testosterone controls Phase 2 (inguinoscrotal).

"AMH = Always My Helper" Memory Aid: AMH is the preferred single-sample test for detecting testicular tissue at any age.

"FEMINIZING SERTOLI" Memory Aid: Sertoli cell tumors cause Feminization: alopecia, hyperpigmentation, gynecomastia, attraction to males, squatting to urinate.