We performed a systematic review and critique of the literature on the frequency of undescended testis (UDT) among boys from birth to adolescence. Special attention was given to whether previous testicular position was taken into account to distinguish between congenital and acquired UDT. We searched Medline, Embase, Cinahl and the Cochrane Library. Any study reporting on the frequency of UDT was included. Study population age, number of boys studied, period of examination, primary examiner, area of study, study design, ethnicity, definitions used and previous testicular position were analysed. A total of 46 studies met the inclusion criteria. Twenty-three of the 46 (50%) studies involved newborns. Definitions were described in half of the studies; however, the definitions used were heterogeneous. Previous testis position was described in 11% (5/46) of the studies. At birth, in term and/or birth weight >2.5 kg infants, the UDT rate ranged from 1.0 to 4.6%, and in premature and/or birth weight <2.5 kg infants from 1.1 to 45.3%. At the age of 1 year UDT in term and/or birth weight >2.5 kg infants was seen in 1.0-1.5%, at 6 years in 0.0-2.6%, at 11 years in 0.0-6.6% and at 15 years in 1.6-2.2% of boys. The frequency of UDT shows variable figures in the literature. The actual frequency of acquired UDT essentially remains unclear because of the shortage of studies performed at an older age, and of studies reporting on previous testicular position.

RESULTSOn referral, 340 testes were not in the scrotum (182 uni-and 79 bilateral). Of the 340 testes, 82 (24%) in 61 boys were diagnosed as retractile, whereas the remaining 258 in 221 boys were undescended. The previous testicular position was known in 208 of 221 boys (94%), with 244 UDTs. In 65 of these 244 (26.6%) the testis had never been scrotal (congenital UDT); in 179 (73.4%) a previous intrascrotal position was recorded in early childhood (acquired UDT) at least once, in 149 (61%) at least twice and in 117 (48%) at least three times. The mean age at referral for congenital UDT was 2.1 years and for acquired UDT was 8.4 years. CONCLUSIONSThese results show that acquired UDT is frequent, and occurs at about three times the rate of congenital UDT. Because these boys are referred for treatment later in childhood, the acquired UDT probably accounts for the high rate of (late) orchidopexy.

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SummaryAcquired undescended testis is now a well-recognized disorder. It is seen in 1.5% of pre-pubertal boys and accounts for the 1-2% orchidopexy rate in older boys. Its pathogenesis remains largely unclear, but it may be caused by a fibrous remnant of the processus vaginalis. There is much controversy over its management, and the proper management awaits a randomized-controlled trial. Until now, follow-up data are available only for cases of spontaneous descent or pubertal orchidopexy. It is speculated that acquired undescended testis is in fact congenital and because of a short funiculus at birth, allowing a lowscrotal position early in life. However, as the boy grows, the testis might evolve into an undescended state. When testosterone surges at puberty, spontaneous descent occurs in three of every four cases.

We assessed spontaneous descent of acquired undescended testis (UDT) at puberty. 299 Boys (aged 1.2-16.5 years, mean 9.4) with 350 acquired-UDT were examined annually during a 12.6-year period (mean 3.1). An acquired-UDT was defined as a previously intrascrotal testis which can no longer be manipulated into a stable scrotal position. Each year, position of the testis and pubertal development according to Tanner's stages were assessed. Early puberty was defined as puberty stage G2 (testicular volume 4-9 mL), mid-puberty as puberty stages G3 (testicular volume 10 mL) and G4 (testicular volume 11-15 mL), and late puberty as puberty stage G5 (testicular volume >15 mL). Follow-up was completed if spontaneous descent had occurred, if mid-pubertal orchidopexy (ORP) had to be performed, if the boy was lost for follow-up, or if pre-pubertal ORP was performed in another hospital. In 139 boys with 164 acquired-UDT follow-up was meanwhile completed. Twelve boys with 14 UDT were lost for follow-up. In an additional 16 boys with 21 UDT, ORP was performed in another hospital. In 98 of the remaining 129 (76.0%) acquired-UDT spontaneous descent at puberty occurred. Mean follow-up was 2.5 years (range 0.2-8.5). In 70 of 98 testes (71.4%) descent occurred in early puberty, in 26 of 98 testes (26.5%) in mid-puberty, and in two testes in late puberty. In 31 of 129 testes (24.0%) ORP had to be performed at mid (30 cases) or late (one case) puberty. In this series, 98 of 129 acquired-UDT (76.0%) descended spontaneously at puberty, whereas in 31 of 129 (24.0%) pubertal ORP was performed. If ORP is postponed until puberty stage G3 (testicular volume of 10 mL) three of four acquired-UDT will descend spontaneously.