To date, approximately 50 case reports of accessory scrota have been published; however, the exact prevalence remains uncertain. An accessory scrotum is a scrotal anomaly where ectopic scrotal tissue develops apart from the normal scrotum [4]. In contrast with ectopic testes, the accessory scrotum does not contain testes [5]. Although accessory scrota may develop in diverse locations around the normal scrotum, these frequently occur between the normal scrotum and anus. Indeed, Ikegami et al. reviewed the localization of 41 cases of accessory scrotum published between 1980 and 2020, and only one case presented posterior to the anus [3]. In cases with concomitant lipoma, the accessory scrotum typically presents continuously with the lipoma in a bead-like shape. In our case, the proximal lipoma and distal accessory scrotum were found in an uncharacteristic dumbbell-like shape; this shape, including the presence of a stalk between an accessory scrotum and lipoma, have not previously been reported.

Macroscopic appearance is not sufficient for the diagnosis of an accessory scrotum. Definitive diagnosis is based on conspicuous epidermal irregularities resembling scrotal skinfolds and histological identification of tunica dartos. Anatomically, the scrotal tunica dartos comprises prominent smooth muscle bundles in the dermis as well as epidermal skinfolds with irregularities. In the present case, the presence of prominent smooth muscle bundles positive for α-SMA in the dermis, as well as the epidermal irregularities, supporting the diagnosis as accessory scrotum. Thus, may be helpful for the diagnosis of accessory scrotum. Furthermore, the pathological findings of the accessory scrotum and lipoma were similar to those reported previously [6]; however, only blood vessels and nerve bundles were observed in the stalk between the accessory scrotum and lipoma. This is an interesting finding with potential developmental implications. Recently, cases of prenatal diagnosis of accessory scrotum have also been reported [6, 7].

A human tail is a congenital abnormality that develops in the lumbosacral region and must be distinguished from the accessory scrotum, particularly in cases where the accessory scrotum is posterior to the anus. It is thought to arise from an irregularity in the developmental process of the vertebral portion and caudal filament that differentiate from the embryonic tail [8]. The human tail has been characterized in reports from Harrison, Dao et al., and Lin et al., although no clear definition has been established [8,9,10]. Apart from our case, the human tail is typically covered with normal skin and associated with tunica dartos [11]. We ultimately diagnosed an accessory scrotum in our case.

In addition to concomitant lipoma, various congenital anomalies have been reported to be associated with the accessory scrotum, including anorectal malformation, urogenital anomalies, and vertebral or bone anomalies. Reviews have shown that lipoma and anorectal malformation are most common, with a prevalence of approximately 60–80% and 18.6%, respectively [3, 6]. MRI may be useful for screening for congenital malformations associated with the accessory scrotum.

However, simple excision is generally sufficient for treating accessory scrotums, and no recurrence has been reported.

During normal scrotal development, a pair of genital swellings forms outside the genital tubercle or cloacal fold around the fourth week of gestation, and these differentiate into scrotal swellings. Subsequently, around the 12th week of gestation, the scrotal swellings move caudally from the inguinal position, each forming half of the scrotum and fusing at the scrotal raphe. The mechanism underlying the development of an accessory scrotum remains unclear, although several theories have been postulated [1, 2, 8]. For example, Lamm and Kaplan hypothesized that the labioscrotal swelling divides into two parts early in the embryonic period, becoming an accessory scrotum [2]. In our case, the accessory scrotum was located posterior to the anus, which cannot be explained by this hypothesis. While two primordia of the labioscrotal swelling arise during normal development, Takayasu et al. postulated that three develop in case of accessory scrotum [12]. According to the explanation, in our case, it should be considered that one of them developed on posterior to the anus ectopically. In addition, Sule et al. speculated that some mesenchymal tissue may transform into the lipoma, thereby causing the labioscrotal swelling to migrate away from the scrotum [1]. However, none of these hypotheses effectively explain our case in a unified way. The presence of a stalk in our case might suggest a different underlying mechanism that warrants further investigation.