Anorchia or vanishing testis syndrome is a rare condition characterized by the complete absence of one or both testes in a phenotypical male. Clinical examination can be inconclusive and specific hormonal studies are required to confirm the diagnosis.
Anorchia, also known as embryonic testicular regression is characterized by the unilateral or bilateral absence of functioning testicular tissue in an individual who has 46, XY karyotype and is phenotypically male   . The congenital form has an incidence of one in twenty thousand  while an acquired form has been identified based on the presence of testicular tissue in the postnatal period but which disappears later . Anorchia has also been reported in one out of 175 cases with cryptorchidism . It is known to have a familial tendency    . The etiology of the condition is still being debated with several proposed theories ranging from genetic to 46, XY abnormal gonads  to vascular occlusion during embryonic testicular descent .
Most patients with anorchia present with normal male external sexual characteristics but may also present with microphallus , ambiguous external genitals , and feminine appearance of external genitalia . Cases of neonatal testicular torsion as a presenting symptom has been reported by various authors  . Individuals with bilateral anorchia typically develop normally but the onset of puberty is either delayed or absent depending on the extent of functioning testicular tissue in the body. Therefore secondary sexual characteristics may develop partially or not at all. Other organ anomalies like renal ectopia can also be present in these individuals   along with low bone density.
A clinician should suspect anorchia if a patient presents with complaints of delayed puberty and has masculine external genitals but no palpable testes. A thorough family history must be elicited as other family members are also likely to have complete or partial gonadal abnormalities with 46, XY karyotype . Physical examination may reveal microphallus  or normal external male genitalia with bilateral or unilateral palpable intrascrotal   or inguinal masses  or no palpable masses .
Confirmation of the diagnosis requires hormonal evaluation such as gonadotrophin-releasing hormone (GnRH), follicle stimulating hormone (FSH), luteinizing hormone (LH), testosterone, inhibin B levels and provocation test with human chorionic gonadotrophin (hCG). In bilateral anorchia, low levels of testosterone along with elevated FSH levels and an absence of a rise in levels of testosterone after hCG     is noticed. hCG provocation test is not required in 46, XY patients with elevated FSH and undetectable levels of anti-müllerian (AMH) and inhibin B as this scenario is diagnostic for anorchia .
Although ultrasonography can be performed as part of the workup, it cannot distinguish between testis and inguinal lymphadenopathy. Magnetic resonance imaging (MRI) is more useful for detecting testicular tissue especially if cryptorchidism is suspected. But imaging studies may not be required if hCG provocation fails to cause a rise in plasma testosterone levels .
Some authors have suggested surgical exploration for the testicular tissue as the most definitive method of diagnosis  especially in unilateral anorchia . However, others  suggest that surgery is unnecessary in the workup of bilateral anorchia if endocrine tests confirm the absence of functioning testicular tissue.