Monorchidism: One testis is absent. Usually left testis is absent and remaining one undergoes compensatory hypertrophy. Hormonal patterns remain normal. Histologically, finding blindly ending spermatic vessels makes the diagnosis. If hypoplastic epididymis is seen, the diagnosis cannot be made.
Testicular regression syndrome: both testes absent; several etiologies:
Rudimentary testes syndrome: normal male phenotype with small cryptorchid tests; regression occurs at 14-20 weeks gestation.
Congenital bilateral anorchidism: normal external male genitalia; regression after 20 weeks gestation.
Vanishing testes syndrome: patients develop hypergonadotropic hypogonadism after puberty with infantile phallus, hypoplastic scrotum, and impalpable prostate; regression between birth and puberty.
Leydig cell only syndrome: patients are anorchid but not eunuchoid, due to clusters of Leydig cells producing testosterone.
Both monorchidism and testicular regression syndrome show similar histology:
Absence of testicular parenchyma.
Presence of vas deferens, epididymis, or small groups of seminiferous tubules (69-83% of cases) (image A).
Spermatic vessels are small in 83% of cases
Areas of dystrophic calcifications, hemosiderin deposition, or giant cell reaction may be found within the mass that is noted instead of a testis (image B).
Etiology: 2 hypotheses: either it represents a primary anomaly of the gonad or it represents atrophy of the testis secondary to a vascular lesion, such as thrombosis or intrauterine torsion – a theory may be supported by common finding of hemosiderin in area adjacent to regressed testis.