Cryptorchidism is the absence of at least one testicle from the scrotum. It is the most common birth defect involving male genitalia. About 3% of full-term and 30% of premature male infants are born with one or both testicles undescended. Approximately 80% of cryptorchid testes descend by the third month of life. This makes the true incidence around 1%.  If the testis has not descended by 6 months of age, it is unlikely to do so spontaneously, and surgical correction should be considered.
Cryptorchidism may occur on one or both sides, but more commonly affects the right testicle.
The testicle may be anywhere along the "path of descent," such as: 
The undescended testicle can usually be palpated in the inguinal canal. In a minority of patients, the missing testicle may be located in the abdomen or be nonexistent.
Undescended testicles are associated with decreased fertility (especially bilateral cases), increased testicular germ cell tumors (overall risk under 1%), testicular torsion, inguinal hernias, and psychological problems.
Up to 10% of patients with a unilateral undescended testis will develop infertility.
Without surgical correction, an undescended testicle is likely to descend during the first three months of life. If it remains undescended, to reduce risks and minimize infertility, the testis should be brought into the scrotum with an orchiopexy starting at age six months.
Cryptorchidism, hypospadias, testicular cancer, and poor semen quality make up testicular dysgenesis syndrome (TDS). This syndrome is thought to be due to harmful environmental factors that disrupt embryonal programming and gonadal development during fetal life.
A normal hypothalamic-pituitary-gonadal axis is a prerequisite for normal testicular descent.
Birth weight appears to be the main risk factor for undescended testes, followed by family history.
Absence of an appendix testis has been linked to abdominal and cryptorchid testes especially if located proximal to the external ring. The exact role of the appendix testis in testicular descent is unclear.
In full-term infants, the cause of cryptorchidism often cannot be determined, making this a common but sporadic, idiopathic birth defect. It is thought that genetics, combined with maternal and environmental factors, may disrupt hormones and physical changes that influence testicular development and descent.
One contributing mechanism for the reduced function of cryptorchid testes is temperature. It is also likely that transient hormone deficiencies may lead to a lack of testicular descent and impair the development of spermatogenic tissue. 
Signs and symptoms of undescended testicles include the following:
Men with undescended testes may have reduced fertility, even after orchiopexy. 
Boys with undescended testicles do not tend to be effeminate, gender-disordered, or pre-homosexual. A disturbed self-image may occur when the family dynamics are destructive toward male self-esteem. When cryptorchism is surgically corrected, a healthy masculinity generally occurs.
Overall, the risk of testicular cancer if orchiopexy is done before puberty is about 3 times that of the general population. It is 5 to 6 times higher when orchiopexy is done after puberty. The risk of cancer does not seem to be different when orchiopexy is done early in infancy compared to later in childhood. 
From the AUA Guidelines: “In the hands of an experienced provider, more than 70% of cryptorchid testes are palpable by physical examination and need no imaging. In the remaining 30% of cases with a nonpalpable testis, the challenge is to confirm absence or presence of the testis and to identify the location of the viable nonpalpable testis."
Ultrasound is non-contributory in routine use, with sensitivity and specificity to localize nonpalpable testes reported at 45% and 78%, respectively. The cost and ionizing radiation exposure associated with CT scanning preclude its use. MRI with or without angiography has been more widely used with greater sensitivity and specificity but is discouraged due to its cost, low availability, and the need for anesthesia. At this time, there is no radiological test that can conclude, with 100% accuracy and reliability, that a testis is absent; "...regardless of preoperative radiological findings, these studies rarely assist in the decision making and may at times yield misleading information (such as absence when actually present or vice versa).” 
A karyotype can confirm or exclude dysgenetic primary hypogonadism. Hormone levels such as gonadotropins and Anti-Mullerian Hormone (AMH) may confirm hormonally functional testicles worth salvation, as can stimulation with human chorionic gonadotropin to elicit a rise in the testosterone level. In some cases, further testing is crucial and has a high likelihood of detecting intersex conditions. 
The AUA Guidelines state: "Providers should not use hormonal therapy to induce testicular descent as evidence shows low response rates and lack of evidence for long-term efficacy."
The American Pediatric Association Guidelines do recommend the use of hormones for cases of undescended testis associated with Prader-Willi Syndrome. Their reasoning is that a therapeutic trial of human chorionic gonadotropin (HCG) is indicated for treatment of undescended testes before surgery, because avoidance of general anesthesia is desirable for infants with low muscle tone and at high risk for underlying respiratory compromise.
The cost of hormone treatment is less than surgery, and the chance of complications are minimal. However, a recent meta-analysis of seven randomized clinical trials concluded that hormonal therapy was no more effective than placebo. 
Surgery is recommended for congenital undescended testes between the ages of 6 and 18 months (AUA Guidelines). Many experts are recommending surgery early, at around 6 months, to optimize testicular growth and fertility. For premature babies, corrected age is used to determine surgery timing. Fertility is improved if the orchidopexy is performed early. Patients with bilateral undescended testes who receive orchidopexies as adults are almost always infertile and azoospermic; but there are now a few anecdotal reports of pregnancies achieved through assisted reproduction in this group. 
For acquired (testis documented normal before diagnosis) and entrapped (after hernia repair) undescended testes, surgery is recommended shortly after diagnosis.
For retractile testes, a yearly physical examination is recommended because of the 2% to 50% reported risk of a retractile testis becoming an acquired undescended testis.
Technique of Orchiopexy
For palpable undescended testes, an inguinal or scrotal orchiopexy is recommended. 
For nonpalpable testes under anesthesia, exploratory laparoscopy is recommended. If a testis is found during exploratory laparoscopy, the options are: 
If no testis is found during exploratory laparoscopy, one has to determine the presence of either blind ending vessels or a testicular nubbin to completely rule out a missing testis. The vas can be dissociated from the testis and thus is not always a good guide to find the gonad. 
If the internal ring is closed but vessels are going into it, a scrotal exploration usually will find a testicular nubbin. Look for a small structure with a brown spot.
If vessels are going into an open inguinal ring, one can usually push the testis into the abdomen but if not, an inguinal or scrotal exploration would be warranted.
A common diagnostic challenge is distinguishing a retractile testicle from a testicle that is not spontaneously descending into the scrotum. Retractile testes are more common than undescended testes and do not require surgical correction. In normal males, as the cremaster muscle contracts, the testicles retract into the upper scrotum and inguinal canal. This reflex is more active in infants.
A retractile testicle high in the scrotum is difficult to distinguish from one positioned in the lower inguinal canal. Maneuvers used to assist identification include the cross-legged position, soaping the clinician's fingers, and examining the patient in a warm bath.
The American Urological Association (AUA) Guidelines define a retractile testis as "...one that is initially extrascrotal on examination or moves easily out of the scrotal position, (often associated with a vigorous cremasteric reflex), but that can be manually replaced in a stable, dependent scrotal position and remain there without tension at least temporarily."
When properly diagnosed and treated, the prognosis is excellent. Testicular cancer risk and infertility remains somewhat increased compared to the general population. However, a recent Danish study of over 6,000 men suggested testicular hypofunction, smaller testis volume (by 3.5 cc), 28% reduction in sperm concentration and reduced Leydig cell function in men with a history of cryptorchidism compared to normal men.
Orchiopexy is associated with two major testicular complications: atrophy and testicular ascent. For palpable testes, these occur less than 5% of the time. For laparoscopic orchiopexies, the rate is also around 5%. For Fowler-Stephens orchiopexies (dividing the vessels) in one or two stages, the testicular atrophy rate is around 20% to 30% (worse for the one stage procedure).
There may be a slight decrease in penis size in cryptorchid patients.
It has been suggested that adjuvant LHRH treatment be offered to cryptorchid patients with a high risk for infertility due to their insufficient gonadotropin stimulation at the time of surgery. Sperm cryopreservation is another prophylactic option in case of resultant infertility in spite of the adjuvant LHRH.
Non-union, a rare anomaly involving the epididymis and testicle, can sometimes be mistaken for a testicular nubbin (atropic testis) when it's actually the vas and epididymis. The actual testicle in this condition is located proximally and can be identified via laparoscopy.
An undescended testicle is the most prevalent male congenital defect affecting the male genitalia. Most (about 80%) will descend by the third month of life. It is more frequently found in premature infants. Risks for male infertility and cancer are increased compared to the general population but still relatively small. Optimal timing of surgical orchidopexy (<18 months of age) is not always followed. Close collaboration and communication between nurse practitioners, physician assistants, pediatricians, and urologic surgeons will result in better outcomes and avoid unnecessary procedures while increasing the incidence of timely surgery when indicated.
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