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Summary Table

Category	Details
Risk Factors	Prematurity, low birth weight (<2.5 kg), family history, maternal smoking, small for gestational age. Associated syndromes: Prader-Willi, Down syndrome, Noonan syndrome, prune-belly syndrome, Kallmann syndrome, disorders of sex development (DSD).
Etiology	Failure of testicular descent due to defects in INSL-3 and androgen (testosterone/DHT) signaling through the gubernaculum; also requires intact genitofemoral nerve innervation of the gubernaculum. Genetic or environmental disruption of any of these pathways impairs descent.
Patient Presentation	Male infant or child with an empty hemiscrotum (unilateral ~70% of cases) or bilateral empty scrotum. Often discovered at well-child examination. Parents may report "missing" testis. May present with ipsilateral inguinal hernia.
Classic Physical Exam	Non-palpable testis in the scrotum. Testis may be palpable in the inguinal canal. Ipsilateral hemiscrotum appears hypoplastic/flat. Contralateral testicular compensatory hypertrophy if the other testis is absent. Examine in a warm room, in the frog-leg/cross-legged position.
Key Diagnostic Results	Clinical diagnosis via physical examination (palpable vs. non-palpable). Imaging (ultrasound/MRI) is NOT routinely recommended and does not reliably confirm or exclude a testis. Diagnostic laparoscopy is the gold standard for localizing a non-palpable testis. In bilateral non-palpable testes + ambiguous genitalia: karyotype + hormonal workup (LH, FSH, testosterone, AMH/MIS, hCG stimulation test).
Management	Referral to surgery by 6 months of age. Orchiopexy should be completed by 12 months, maximum 18 months. Palpable testis: inguinal or scrotal orchiopexy. Non-palpable testis: diagnostic laparoscopy followed by laparoscopic orchiopexy or Fowler-Stephens procedure if the testis is too high. Post-pubertal unilateral UDT with normal contralateral testis: consider orchiectomy (malignancy risk). Hormonal therapy (hCG/GnRH) is NOT recommended for unilateral UDT; may be considered in bilateral UDT for fertility preservation.
Keywords	"Empty scrotum," "testis not palpable in scrotum," "inguinal mass in male infant," "premature male neonate," "undescended testis," "retractile testis" (distractor), "gubernaculum," "orchiopexy before 12 months."

1. Pathophysiology

Cryptorchidism results from the failure of the testis to complete its descent from the abdomen into the scrotum during fetal life. Understanding the mechanism of normal testicular descent is the key to understanding every testable concept about this condition.

Normal descent occurs in two phases. The transabdominal phase (weeks 10 to 15 of gestation) is primarily driven by insulin-like factor 3 (INSL-3), a hormone secreted by fetal Leydig cells. INSL-3 acts through its receptor, RXFP2, to induce gubernacular swelling and outgrowth. During this phase, the gubernaculum enlarges and anchors the testis near the future inguinal canal while the Mullerian ducts regress under the influence of anti-Mullerian hormone (AMH). This is why the testis in its highest arrested position tends to be found near the internal inguinal ring: the transabdominal phase completed, but the inguinoscrotal phase did not.

The inguinoscrotal phase (weeks 25 to 35 of gestation) is driven by androgens (testosterone and dihydrotestosterone) acting via androgen receptors on the gubernaculum. These hormones stimulate the gubernaculum to migrate through the inguinal canal and into the scrotum, pulling the testis along with it. This phase also requires intact innervation of the gubernaculum by the genitofemoral nerve, which releases calcitonin gene-related peptide (CGRP) to guide gubernacular migration. The processus vaginalis forms within the gubernaculum during this migration.

By approximately week 35, the testis has reached the scrotum, the gubernaculum is reabsorbed, and the processus vaginalis remains attached to the scrotal skin. This is precisely why premature infants have a much higher incidence of undescended testis: they are born before the inguinoscrotal phase is complete. The incidence is 1 to 4.6% in full-term neonates but rises to as high as 45% in preterm neonates.

The clinical implication of this two-phase model is that any disruption of INSL-3, androgen signaling, gubernacular development, or genitofemoral nerve function can result in an arrest of testicular descent. This explains why cryptorchidism is associated with conditions involving hypogonadism (Prader-Willi syndrome, Kallmann syndrome, Klinefelter syndrome), androgen insensitivity, and anatomic anomalies of the abdominal wall (prune-belly syndrome).

Once a testis fails to descend, it remains in an environment where the temperature is 2 to 3 degrees Celsius higher than the scrotum. This supraphysiologic temperature causes progressive germ cell loss, decreased spermatogonia, and Leydig cell dysfunction that worsen with time. Histological studies demonstrate that germ cell deterioration begins as early as 6 months of age and becomes irreversible after 12 to 18 months. This time-dependent damage is the entire rationale behind the recommendation for early surgical correction.

The undescended testis is also prone to malignant transformation. The risk of testicular germ cell tumor is approximately 2 to 5 times higher than the general population. The most common precursor lesion is intratubular germ cell neoplasia (ITGCN). Orchiopexy performed before puberty reduces this risk (relative risk 2.2x with prepubertal repair vs. 5.4x with postpubertal repair), but does not eliminate it entirely, suggesting a common embryological origin for both UDT and testicular cancer (the testicular dysgenesis syndrome hypothesis).

2. Classification and Clinical Manifestation

The most clinically useful classification divides UDT by whether the testis is palpable or non-palpable on physical examination. Approximately 80% of undescended testes are palpable, and the remaining 20% are non-palpable. This distinction directly determines the surgical approach.

Category	Subtype	Location	Key Features	Can It Descend Spontaneously?
Palpable (~80%)	True undescended (inguinal)	Along the normal path of descent, arrested before scrotum (inguinal canal or at external ring)	Most common type. May or may not be palpable depending on exact position within the canal.	Possible before 6 months of age
	Ectopic testis	Outside the normal path of descent (superficial inguinal pouch, femoral, perineal, penile, contralateral scrotum)	Superficial inguinal pouch is the most common ectopic site. Must be distinguished from true UDT because it cannot descend spontaneously.	Never
	Retractile testis	In the scrotum but intermittently pulled to the suprascotal position by an overactive cremaster reflex	Can be manually brought back into dependent scrotal position and remains there without tension. Normal size and consistency. Does NOT require surgery but requires annual follow-up because >1/3 become ascending testes.	N/A (already descended)
Non-palpable (~20%)	Intraabdominal	Anywhere within the abdomen, most commonly near the internal inguinal ring; less often near kidney, anterior abdominal wall, or retrovesical	Accounts for 50 to 60% of non-palpable testes. Requires laparoscopy for localization.	No
Non-palpable (~20%)	Absent (vanishing testis)	No testis present	Caused by testicular agenesis or intrauterine torsion with atrophy. Monorchidism ~4% of UDT cases, anorchidism <1%. Laparoscopy shows blind-ending vas deferens and vessels.	N/A

Laterality and Epidemiology

Feature	Detail
Unilateral UDT	~70% of cases; right side more common than left
Bilateral UDT	~30% of cases; warrants DSD workup if combined with hypospadias or ambiguous genitalia
Incidence at birth (full-term)	1.0 to 4.6%
Incidence at birth (preterm)	1.1 to 45%
Incidence at 1 year (full-term)	~1% (reflecting spontaneous descent in the first 6 months)

3. Diagnostic Workup

Test	Role	When to Order	Key Findings
Physical examination	Best initial test and most important diagnostic tool	Every male newborn and every well-child visit	Testis palpable vs. non-palpable; position along inguinal canal; can it be brought to the scrotum? Retractile vs. true UDT. Assess contralateral testis size (compensatory hypertrophy suggests absent contralateral testis).
Ultrasonography	Limited role; NOT routinely recommended	Only if investigating DSD (to look for Mullerian structures) or certain selected cases	Low sensitivity for intraabdominal and non-palpable testes. Cannot reliably confirm or exclude a testis. Does NOT change management.
MRI	Limited role; NOT routinely recommended	Selected cases only (e.g., DSD evaluation for Mullerian structures)	Slightly better sensitivity than ultrasound for inguinal testes, but still unreliable for intraabdominal testes.
Diagnostic laparoscopy	Gold standard / most accurate test for non-palpable testis	Non-palpable testis after examination under anesthesia fails to localize it	Directly visualizes the testis or confirms absent testis (blind-ending vas deferens and spermatic vessels). Can transition to therapeutic laparoscopic orchiopexy in the same session.
Hormonal workup (LH, FSH, testosterone, AMH/MIS, hCG stimulation test)	Rule out DSD and confirm testicular tissue presence	Bilateral non-palpable testes, especially with hypospadias or ambiguous genitalia	Elevated FSH + low AMH/MIS + absent testosterone response to hCG suggests anorchidism. Detectable AMH confirms functioning testicular tissue exists.
Karyotype	Rule out DSD	Bilateral non-palpable testes with any genital ambiguity	Confirms 46,XY or identifies chromosomal DSD (e.g., 45,X/46,XY mixed gonadal dysgenesis)

The diagnosis of cryptorchidism is clinical. Physical examination is both the best initial test and the cornerstone of management decisions. The examination must be performed in a warm room with warm hands and a relaxed patient to avoid triggering the cremasteric reflex, which can pull a normally descended testis out of the scrotum and mimic an undescended testis. The frog-leg or cross-legged sitting position is the best position for reducing the cremasteric reflex.

The examiner's fingers should sweep along the inguinal canal from the anterior superior iliac spine toward the pubis, milking any palpable structure downward toward the scrotum. The critical clinical question is: can the testis be brought into the dependent portion of the scrotum and stay there without tension? If yes, this is a retractile testis (no surgery needed, but annual monitoring). If the testis cannot reach the scrotum, it is a true undescended testis requiring surgical correction.

If the testis is non-palpable on the affected side, the examiner should check for contralateral compensatory hypertrophy, which, when present, suggests the contralateral (non-palpable) testis is absent or atrophic. However, this finding is not reliable enough on its own to rule out surgical exploration.

Imaging is NOT recommended routinely. This is a heavily tested concept. The AUA guideline, EAU guideline, and the IAUI guideline all state that ultrasound and MRI have limited sensitivity for non-palpable testes and do not change management. A retrospective study from RSCM Jakarta confirmed that non-palpable UDT can be adequately diagnosed through history and physical examination alone, followed directly by laparoscopic orchiopexy. The exam may test this by offering ultrasound as a "next step" for a non-palpable testis, which would be incorrect. The correct next step is examination under anesthesia followed by diagnostic laparoscopy.

The one scenario where a formal workup beyond physical examination is essential is the bilateral non-palpable testis, particularly with signs of DSD (ambiguous genitalia, hypospadias, scrotal hyperpigmentation). In this setting, urgent endocrine and genetic evaluation is mandatory because the differential includes congenital adrenal hyperplasia (a virilized 46,XX female), persistent Mullerian duct syndrome, or gonadal dysgenesis.

4. Management and Treatment

Scenario	Management	Timing/Details
Spontaneous descent	Observe	Most spontaneous descent occurs within the first 6 months of life. After 6 months, spontaneous descent is rare (<7% will reach the scrotum after this point).
Palpable UDT	Inguinal or scrotal orchiopexy (orchidofuniculolysis + orchiopexy)	Begin treatment at 6 months. Surgery should be completed by 12 months, maximum 18 months. Success rate of inguinal orchiopexy: ~92%. Testis is mobilized, spermatic cord freed, processus vaginalis ligated at the internal ring, and testis placed in a subdartos pouch.
Scrotal orchiopexy	Indicated for low-lying palpable testes	Scrotal approach used for testes in a low position. In >20% of cases, an inguinal incision is additionally needed to correct a concurrent inguinal hernia.
Non-palpable UDT	Examination under anesthesia, then diagnostic laparoscopy	If the testis becomes palpable under anesthesia: standard inguinal orchiopexy. If still non-palpable: laparoscopy to localize.
Intraabdominal testis (near internal ring, <2 cm above)	Laparoscopic or open orchiopexy (standard)	Testis can reach the scrotum without dividing testicular vessels.
Intraabdominal testis (high, >2 cm above internal ring)	Fowler-Stephens orchiopexy (1-stage or 2-stage)	Testicular vessels are divided; the testis survives on collateral blood supply via the vas deferens artery and cremasteric artery. Two-stage is used when the collateral supply needs time to develop. Atrophy rate: 28.1% (1-stage) and 8.2% (2-stage).
Absent testis (blind-ending vessels at laparoscopy)	No further intervention	Diagnosis confirmed; laparoscopy shows blind-ending spermatic vessels and vas deferens.
Post-pubertal unilateral UDT with normal contralateral testis	Consider orchiectomy	Elevated malignancy risk (intratubular germ cell neoplasia ~2%). Orchiectomy is recommended per consensus.
Retractile testis	No surgery; annual follow-up until puberty	>1/3 may become ascending testes, requiring surgical intervention later.
Hormonal therapy (hCG or GnRH)	NOT recommended for unilateral UDT; may be offered for bilateral UDT	Success rate only ~20%, location-dependent. Side effects: scrotal erythema, pigmentation, premature pubic hair growth, penile enlargement, injection site pain. Does NOT improve fertility index. Consensus limits use to bilateral UDT for fertility preservation.

Timing is the single most testable concept in UDT management. The treatment window is narrow and defined by histological evidence. After 6 months of age, spontaneous descent is unlikely (one study found only 6.9% of infants with UDT before 12 months achieved scrotal position by age 1 year or later). By 12 to 18 months, histological studies show progressive and irreversible loss of germ cells and Leydig cells.

For a palpable testis, the standard procedure is inguinal orchiopexy. The surgeon makes an inguinal incision, performs orchidofuniculolysis (freeing the spermatic cord), ligates the processus vaginalis at the level of the internal inguinal ring, and places the mobilized testis into a subdartos pouch in the scrotum. This pouch is critical because it creates a tissue layer between the testis and the dartos fascia that prevents retraction. The success rate is approximately 92%.

For a low-lying testis that is easily palpable near the scrotum, a scrotal orchiopexy can be performed through a scrotal incision alone. However, the processus vaginalis must still be assessed for patency, and in more than 20% of cases, an additional inguinal incision is required to repair a coexistent inguinal hernia.

For a non-palpable testis, the management begins with re-examination under anesthesia, because a testis that was not palpable in an awake, anxious child may become palpable under general anesthesia, allowing standard inguinal orchiopexy. If the testis remains non-palpable, diagnostic laparoscopy is the most accurate method to determine testis location. The laparoscopic findings dictate the next step:

If the testis is near the internal ring (within 2 cm), a standard laparoscopic or open orchiopexy can bring it to the scrotum.
If the testis is high (more than 2 cm above the internal ring), it cannot reach the scrotum without dividing the testicular vessels. In this case, the Fowler-Stephens technique is used. The testicular artery and vein are intentionally ligated, and the testis survives on collateral blood supply from the artery of the vas deferens and the cremasteric artery. This can be done in one stage or two stages (with the second stage 6 months later, allowing collateral vessels to mature). The two-stage approach has a lower testicular atrophy rate (8.2% vs. 28.1%).
If blind-ending spermatic vessels are found (vanishing testis), no further surgery is needed.

Regarding hormonal therapy, hCG and GnRH have been used historically but have a success rate of only around 20%, and the rate depends on the initial testis position (the closer to the scrotum, the better the response). Current AUA and EAU guidelines do not recommend hormonal therapy for achieving testicular descent. Side effects include scrotal erythema, scrotal hyperpigmentation, premature pubic hair growth, penile enlargement, and injection-site pain. The IAUI guideline recommends considering hormonal therapy only in bilateral UDT for the purpose of preserving fertility potential, not for achieving descent.

Complications of Untreated and Treated UDT

Long-term Risk	Details
Infertility	Unilateral UDT: decreased fertility but preserved paternity rate. Bilateral UDT: both fertility and paternity rates are decreased. Germ cell loss is proportional to duration of maldescent. Early orchiopexy (<12 months) preserves spermatogenic potential.
Testicular malignancy	2 to 5-fold increased risk of germ cell tumors. Prepubertal orchiopexy reduces risk (RR 2.2x) vs. postpubertal repair (RR 5.4x). Risk persists even after orchiopexy, so self-examination and surveillance during and after puberty are recommended.
Testicular torsion	Increased risk due to abnormal fixation of the testis.
Inguinal hernia	Patent processus vaginalis is found in the majority of UDT patients.
Surgical complications	Testicular atrophy (1.83% for primary orchiopexy), reascending testis, vas deferens injury, wound dehiscence, infection, hematoma.

5. Differential Diagnosis and Distractors

Differential	Why It Is Similar	Key Discriminator
Retractile testis	The most common mimic. The scrotum may appear empty at the time of examination, simulating an undescended testis.	The retractile testis can be manually brought into the dependent scrotal position and stays there without tension. A true undescended testis cannot. The key exam maneuver is the "milking" technique with the patient in frog-leg position. Retractile testes have normal size and consistency.
Ectopic testis	The testis is not in the scrotum, just like a true UDT. The examiner may palpate a mass outside the inguinal canal.	An ectopic testis is located outside the normal path of descent (e.g., femoral triangle, perineum, penile shaft, contralateral scrotum). A true UDT is arrested along the normal path. Both require surgery, but the vignette location outside the inguinal canal is the giveaway.
Testicular torsion (neonatal)	May present with an absent or non-palpable testis in a newborn. The affected hemiscrotum may be discolored.	Testicular torsion presents with acute pain, swelling, erythema, and tenderness. The scrotum is usually edematous and discolored (blue/dark). UDT is painless and the hemiscrotum is flat and hypoplastic, not swollen. In neonatal torsion, there may be a firm, non-tender scrotal mass (infarcted testis).
Inguinal hernia	A palpable inguinal mass in a male infant. May coexist with UDT.	An inguinal hernia presents as a reducible bulge that increases with crying or straining. It transilluminates if it contains bowel/fluid. In UDT, the inguinal mass (the testis) is firm, ovoid, and does not reduce into the abdomen or into the scrotum.
Disorder of sex development (DSD)	Bilateral non-palpable testes in a neonate, especially with ambiguous genitalia or hypospadias.	The vignette will include clues such as ambiguous genitalia, hypospadias, or scrotal hyperpigmentation. Bilateral non-palpable testes with any genital anomaly demands a karyotype and hormonal workup to exclude a virilized 46,XX female (e.g., congenital adrenal hyperplasia).
Prader-Willi syndrome	Cryptorchidism is present in nearly all affected males. The vignette may show an infant with bilateral UDT.	The vignette will also describe neonatal hypotonia, poor feeding/sucking, failure to thrive in infancy, followed by hyperphagia and obesity in childhood. The combination of bilateral cryptorchidism + severe hypotonia + feeding difficulties in a neonate should trigger Prader-Willi as a differential.
Ascending (acquired) testis	A testis that was previously documented in the scrotum is now found in a suprascotal position.	History is the discriminator. In congenital UDT, the testis was never documented in the scrotum. In ascending testis, there is prior documentation or parental confirmation that the testis was previously scrotal. This is a distinct entity from a retractile testis because the ascending testis does not return to the scrotum.

6. Traps and High-Yield Pearls

The single most common way students get questions on cryptorchidism wrong is by ordering imaging as the next step for a non-palpable testis. A vignette will describe a male infant with an empty hemiscrotum and offer answer choices including "order scrotal ultrasound" or "order abdominal MRI." The correct answer is nearly always referral for surgical evaluation (examination under anesthesia, then diagnostic laparoscopy). Ultrasound has low sensitivity for non-palpable testes and does not change management. This is explicitly stated in the AUA guideline, EAU guideline, and the IAUI Pediatric Urology guideline (which cites RSCM data confirming that non-palpable UDT can proceed to laparoscopic orchiopexy without imaging). Imaging is only justified in the context of evaluating for Mullerian structures in a patient with suspected DSD.

The second major trap involves timing of surgery. The exam may present a premature infant with UDT and ask the correct timing for intervention. The answer is to correct for gestational age when determining the 6-month referral window. The key nuance is: treatment should begin at 6 months (corrected age), and surgery should be completed by 12 months, with a maximum deadline of 18 months. Students who select "observe until 2 years" or "reassess at 3 years" are falling for outdated recommendations.

A third trap involves retractile testis management. Students may select orchiopexy for a testis that can be manually brought to the scrotal position and stays there. The correct answer is observation with annual follow-up until puberty, not surgery. However, students must also recognize that more than one-third of retractile testes will eventually become ascending testes, requiring later surgical intervention.

A fourth trap is the bilateral non-palpable testis with hypospadias. This combination is a red flag for a disorder of sex development. The vignette is testing whether the student recognizes that the "next step" is not orchiopexy but rather a karyotype and endocrine evaluation. Failure to evaluate for DSD in this scenario is the intended wrong answer.

Finally, students must understand that orchiopexy reduces but does not eliminate the risk of testicular malignancy. A question may ask whether a patient who had orchiopexy at age 1 still needs surveillance for testicular cancer. The answer is yes. The relative risk is reduced (2.2x vs. 5.4x for postpubertal repair), but the risk never returns to baseline. Self-examination and screening during and after puberty remain indicated.

Core competencies being tested: (1) Ability to distinguish true UDT from retractile testis based on physical examination maneuvers. (2) Knowledge that imaging is NOT indicated for isolated non-palpable UDT. (3) Understanding the correct timing of surgical intervention (6 to 12 months, maximum 18 months). (4) Recognition that bilateral non-palpable testes with genital anomalies requires DSD evaluation before surgical planning. (5) Awareness that malignancy risk persists after orchiopexy and long-term surveillance is needed.

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