The true human tail is a vestigial organ that contains adipose tissue and other connective tissues without vertebra, whereas pseudo-tails are caused by various lesions that have superficial similarities to the true tail. Perineal lipoma is included in the pseudo-tail category [1]. Congenital isolated perineal lipoma is very rare [2], because most perineal lipomas are accompanied by other abnormalities of neighboring structures including accessory scrotum [3] or anorectal malformations [4]. Here we report a case of congenital isolated perineal lipoma presenting as a pseudo-tail with a literature review.

A 5-day-old male neonate was referred to our outpatient clinic, presenting a 5 × 1-cm-sized tail-like perianal mass found at birth (Fig. 1). The pregnancy was uncomplicated and the baby was born at 38 gestational weeks weighing 3.48 kg. He had no problems with feeding and defecation. Ultrasonography was conducted to evaluate accompanying anomalies as well as connection to the spine and other structures. There were no other anomalies or abnormal connections and the mass was confined to the subcutaneous and skin layers. Because the parents wanted to delay the operation, mass excision was performed 7 months later. Complete excision was performed under general anesthesia. The operative findings were the same as the ultrasonographic findings: confined to the subcutaneous and skin layers without abnormalities. Postoperative prognosis was uneventful. In the pathologic exam, the mass consisted of mature adipose cells surrounded by connective tissue and was covered by normal skin. There were no cellular atypia or abnormal mitotic figures. The mass was confirmed as a benign mature lipoma (Fig. 2).

Humans can have tails like other animals, but the incidence is very low. Dao and Netsky [1] proposed the following classifications for the human tail: true tail and pseudo-tail. The true tail is the most distal remnant of the embryonic tail. It consists of adipose tissue, connective tissue, striated muscle, vessels, and nerves and is covered by skin. It does not contain any bone or cartilage unlike the tails of animals that have additional vertebra. Pseudo-tails are caused by various diseases that present with lumbosacrococcygeal protrusion and only bear superficial similarity to true tails. Prominently prolonged coccygeal vertebrae are the most common cause of pseudo-tails, but perineal lipoma also produces a pseudo-tail similar to the current case [456].

Lipoma is one of the most common mesenchymal neoplasms and its average age of onset is the fourth decade of life. However, lipomas are very rare in neonates [7]. Perineal lipoma in itself is very rare. More than 80% of perineal lipomas occur with other anomalies, such as an accessory scrotum [38], anorectal malformations [4] and abnormal labial folds [9]. Congenital isolated perineal lipoma, which is not accompanied by other anomalies, is a very rare disease with less than thirty cases reported to our knowledge [2].

When encountering perineal lipoma, evaluation of the accompanying urogenital and anorectal anomalies is necessary. Evaluation of the depth and relationship with neighboring structures is also recommended and is usually done with ultrasonography and MRI [479]. The number of prenatally diagnosed cases has increased with more use of antenatal ultrasonography [210].

Though perineal lipoma has a benign nature, it should be excised not just for aesthetic purposes but also for a pathological exam. This is because perineal lipoma cannot be easily distinguished from perineal lipoblastoma, a borderline tumor with a high rate of recurrence and local invasion [10]. For perineal lipoma, complete excision and careful pathologic evaluation are recommended.