Cryptorchidism: Embryological and Imaging Considerations

1. Definition and Epidemiology

• Affects ~30% of premature male infants (100% of those <900 g birthweight) and 3-5% of full-term males
• Falls to <1% by 1 year of age as most descend spontaneously
• Usually right-sided; bilateral in 10-25% of cases
• The most common defect found in neonates overall

2. Embryology of Testicular Descent

2a. Gonadal Differentiation

• Gonads are first identifiable on the medial aspect of the urogenital ridge at 32 days postovulation
• SRY (sex-determining region, Y chromosome) expression starts at ~40 days' gestation, triggering testicular development via a cascade: SRY + SF1 → prolonged SOX9 expression → Sertoli cell differentiation
• By 6 weeks, primordial germ cells (PGCs) migrate from the yolk sac; by 8 weeks they are localized within testicular cords as gonocytes
• Leydig cells (producers of testosterone and INSL3) differentiate from mesenchymal precursors

2b. Two Phases of Testicular Descent

• Gubernacular swelling causes cellular proliferation and extracellular matrix expansion, enlarging the inguinal canal
• Transinguinal passage rarely occurs before 22 weeks; the majority of testes are scrotal after 27 weeks
• The scrotum itself forms from genital swellings first visible at 7 weeks, fusing by 10-12 weeks under dihydrotestosterone influence
• INSL3 and androgens synergistically stimulate gubernacular growth

2c. Normal Positions vs. Cryptorchidism vs. Ectopia

3. Pathology and Histological Changes

1. Thickening of the basement membrane of spermatic tubules - the earliest change
2. Loss of spermatogonia - tubules retain only Sertoli cells ("Sertoli cell-only" pattern)
3. Tubular atrophy with hyaline connective tissue replacement
4. Increased interstitial stroma - Leydig cells are spared and thus appear relatively prominent
5. Germ cell neoplasia in situ (GCNIS) - may be present even at early ages

4. Imaging Considerations

4a. Ultrasound (US)

• High-resolution linear array transducer required
• Begin with the scrotum to confirm bilateral absence
• Search in transverse planes with patience - the testis may be atrophic
• Trace the inguinal canal from the internal ring to the external ring

4b. CT

4c. MRI

• Ambiguous genitalia
• Severe hypospadias with non-palpable gonads
• Intra-abdominal testes not seen on US

• Typically hypoplastic
• Low T2 signal (distinguishes from normal testicular tissue)
• Small, dysplastic testes may be indistinguishable from non-specific nodules

4d. Diagnostic Laparoscopy

5. Clinical Implications and Management

5a. Complications of Untreated Cryptorchidism

5b. Management Algorithm

• Spontaneous descent expected by 1 year of age; observation until then
• If undescended at 1 year: orchiopexy at 6-12 months of age (or as soon as identified after 1 year)
• Early surgery is critical because histological changes begin by 6 months

• Palpable inguinal testis: Standard dartos pouch orchiopexy
• Non-palpable / intra-abdominal: Diagnostic laparoscopy first
  • Blind-ending vessels = monorchia (no testis present; consider excision of remnant)
  • Vessels exiting internal ring = inguinal orchiopexy
  • Intra-abdominal testis = Fowler-Stephens orchiopexy (two-stage: ligate testicular vessels first to develop collateral circulation; orchiopexy 6 months later) or laparoscopic single-stage orchiopexy
• Post-pubertal bilateral: Consider orchiectomy given malignancy risk; a 2024 systematic review evaluated post-pubertal management from the malignancy risk standpoint

6. Associated Conditions

• Unilateral testicular agenesis: Associated with ipsilateral renal agenesis - a crucial point for imaging workup
• Prader-Willi syndrome and Noonan syndrome - systemic causes of cryptorchidism
• Prune belly syndrome, Beckwith-Wiedemann syndrome, congenital rubella, renal agenesis - associated anomalies
• Hypospadias - when combined with bilateral non-palpable testes, raises concern for DSD

Summary

Cryptorchidism: Embryological and Imaging Considerations

Definition (0.5 marks)

Embryology of Testicular Descent (4 marks)

Normal Descent - Two Phases

• Regulated by INSL3 (insulin-like factor 3) secreted by Leydig cells, acting via receptor RXFP2
• Also controlled by Müllerian-inhibiting substance (MIS/AMH) from Sertoli cells
• Gubernacular swelling draws the testis from the retroperitoneum to the inguinal brim
• Mutations in INSL3 or RXFP2 lead to cryptorchidism

• Regulated by androgens (testosterone/DHT via androgen receptor)
• Intact genitofemoral nerve (GFN) is essential; CGRP (calcitonin gene-related peptide) is the downstream mediator
• Gubernaculum is untethered distally; rapid transinguinal passage follows
• Majority of testes are scrotal by 27-32 weeks

Gonadal Differentiation Cascade

• SRY gene (Y chromosome) expressed at 40 days → activates SOX9 via SF1 → Sertoli cell differentiation
• Sertoli cells → testicular cord formation; Leydig cells → INSL3 and testosterone secretion
• Primordial germ cells migrate from yolk sac to testicular cords by 8 weeks

Sites of Arrest (in order of frequency)

1. Inguinal canal (~80% - most common)
2. Just outside the deep inguinal ring
3. Abdominal (5-10%)
4. Just above the scrotum

Pathological Consequences (1.5 marks)

• The contralateral descended testis also shows similar changes in unilateral cryptorchidism - indicating an intrinsic gonadal dysgenesis, not purely a thermal/positional effect
• Associated conditions: inguinal hernia (10-20%), torsion, trauma, 4-10x increased cancer risk (seminoma most common)

Imaging Considerations (3 marks)

Ultrasound (US)

• High-resolution linear array transducer; search in transverse planes
• Begin by confirming bilateral absence from the scrotum

• AUA recommendation: Routine US should NOT be used for standard cryptorchidism - poor sensitivity, does not change management
• Exception: obese child where inguinal US may simplify surgical planning
• US role in DSD: detects Müllerian structures (uterus/fallopian tubes) to guide workup

MRI

• Far superior for localizing non-palpable, near-normal testes
• Preferred in: ambiguous genitalia, severe hypospadias, intra-abdominal testes not seen on US
• Cryptorchid testis appears hypoplastic with low T2 signal
• Small dysplastic testes may be indistinguishable from non-specific nodules

CT

• Advantage: retroperitoneal imaging
• Limited use in children due to radiation; 5 CT patterns described based on spermatic cord + testis morphology

Diagnostic Laparoscopy

• The definitive investigation for non-palpable testes
• Allows concurrent biopsy and surgical correction
• Makes preoperative imaging of limited additional value to patient management

Management Outline (1 mark)

• Observe until 1 year (spontaneous descent possible)
• Orchiopexy at 6-12 months if still undescended (before histological damage progresses)
• Non-palpable testes → laparoscopy:
  • Blind-ending vessels = monorchia
  • Vessels exiting internal ring = inguinal orchiopexy
  • Intra-abdominal testis = Fowler-Stephens orchiopexy (two-stage) or laparoscopic orchiopexy
• Bilateral non-palpable → hCG stimulation test first to confirm viable testicular tissue

Key Points to Remember

• Two-phase descent: INSL3 (transabdominal) → Androgens/GFN (inguinoscrotal)
• Inguinal canal is the most common site of arrest
• Histological damage from 6 months → orchiopexy should not be delayed beyond 1 year
• Unilateral testicular agenesis is associated with ipsilateral renal agenesis (important for imaging workup)
• Cancer risk persists after orchiopexy and involves the contralateral testis too
• US is unreliable for non-palpable testes; laparoscopy is gold standard

Testicular Torsion vs. Epididymitis: Radiological Differentiation

1. Clinical Context and Why Imaging Matters

Key principle: Surgery must NOT be delayed for imaging if torsion is clinically certain.

2. Ultrasound: The First-Line Modality

• B-mode (grey-scale): testicular echotexture, size, epididymal appearance, hydrocele, scrotal wall
• Colour/power Doppler: intratesticular and epididymal blood flow
• High-resolution cord imaging (10-20 MHz): spermatic cord morphology

3. Comparative Radiological Features

A. Blood Flow on Colour Doppler (Most Critical Finding)

• Absent intratesticular flow on CDUS: 86% sensitive, 100% specific, 97% accurate for torsion and ischaemia
• Hyperaemia of epididymis +/- testis is the hallmark of epididymo-orchitis
• Caution: False negatives occur with CDUS in torsion - newer equipment may detect residual/collateral flow even in early torsion, and partial/intermittent torsion can show preserved flow

B. Spermatic Cord Signs (Pathognomonic of Torsion)

• In epididymitis, the cord appears normal with no twisting
• The whirlpool sign increases sensitivity from 76% (CDUS alone) to 96%

C. B-Mode Grey-Scale Appearances

• Heterogeneous parenchymal echogenicity = 100% necrosis at surgical exploration
• Homogeneous echogenicity predicts lower orchiectomy risk even with absent flow
• A hypoechoic, swollen testis with absent flow = likely non-salvageable

D. Epididymal Findings

4. Scintigraphy (Radionuclide Scan)

• Torsion: Central photopenic ("cold") area with peripheral rim of increased activity
• Epididymitis: Diffuse increased ("hot") uptake throughout the scrotum

5. MRI

• Dynamic contrast-enhanced MRI: Shows absent perfusion in torsion
• Diffusion-weighted imaging (DWI): Torsed testis shows lower apparent diffusion coefficient (ADC) vs. normal (does not require gadolinium)
• Epididymitis on MRI: Enlarged, T2-hyperintense epididymis with enhancement on contrast sequences

6. Contrast-Enhanced Ultrasound (CEUS)

• CDUS sensitivity/specificity: 76%/45%
• CEUS sensitivity/specificity: 96%/100% for testicular torsion
• Detects microvascular perfusion not visible on standard Doppler
• Useful when CDUS is inconclusive in the emergency setting

7. Torsion of Testicular Appendages (Important Differential)

• US: hypoechoic/avascular nodule at the upper pole or epididymis
• Surrounding reactive hyperaemia of normal epididymis/testis - can mimic epididymitis
• Testis and epididymis themselves show normal flow
• Kremasteric reflex is usually intact

8. Decision Rule Summary

Acute Scrotum
     │
     ├─ Clinical suspicion of TORSION (high-riding testis, absent cremasteric reflex, 
     │   sudden onset, nausea/vomiting, horizontal lie)
     │        │
     │        ├─ HIGH suspicion → Immediate surgical exploration (do not delay for imaging)
     │        │
     │        └─ EQUIVOCAL → CDUS +/- HRUS cord imaging
     │                          │
     │                          ├─ Absent flow + whirlpool/snail sign → TORSION → OR
     │                          ├─ Normal flow + linear cord → NOT torsion
     │                          └─ Heterogeneous parenchyma + absent flow → Non-viable
     │
     └─ Gradual onset, fever, dysuria, intact cremasteric reflex → EPIDIDYMITIS
              │
              └─ CDUS: Increased epididymal flow → Epididymo-orchitis
                       Consider renal US in prepubertal boys (urinary anomaly screen)

9. Key Numbers to Remember

10. Summary Comparison Table