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Congenital perianal lipoma: a case report and review of the literature

Background The surgical strategy for congenital perineal lipoma varies depending on the size, location, and accompanying congenital anomalies, with the optimum approach remaining to be determined. We herein report a case of congenital perianal lipoma that was first detected by prenatal ultrasound and review the literature. Case presentation A female neonate was referred to us for the evaluation of a perianal mass. She had been considered to be male prenatally because fetal ultrasound showed a perineal mass similar to a scrotum and penis. A postnatal examination revealed an appropriate-for-age neonate with a soft round mass 1.5 cm in diameter just to the left of the anal verge. She passed urine and stool smoothly, and contrast enema confirmed no anorectal malformation. Magnetic resonance imaging showed that the lesion had a signal intensity consistent with fat located close to the anal sphincter, and no spinal anomaly (e.g., spina bifida) was identified. We excised the lesion (pathologically confirmed to be lipoma) simply at 2 months old, taking care to avoid damaging the anal sphincter by using a muscle stimulator. She has been doing well with good bowel movement and satisfactory cosmetic results for a follow-up period of one and a half years. Our literature search revealed 49 cases of perineal lipoma reported in English in the last 25 years, and 74% of them—including ours—had other congenital anomalies, the breakdown of which was anorectal malformation in 40% of cases, labioscrotal fold or accessory scrotum in 28%, and urogenital malformation, congenital pulmonary airway malformation, and disorder of sex differentiation. The prenatal detection of the lesion, as in our case, was quite rare. Conclusion A thorough physical examination after birth, magnetic resonance imaging and contrast enema to identify the nature of the perineal lipoma and accompanying anomalies are crucial for planning the surgical strategy. The lesion may be deeply interspersed between the sphincter muscle, especially when it accompanies anorectal anomaly. A muscle stimulator is useful for preserving and repairing the sphincter muscles during resection in order to ensure satisfactory bowel movement.


Background
Congenital perineal lipoma, including perianal lesions, has been reported in about 50 cases in English literatures . Its size and location vary among patients, and it sometimes accompanies other anorectal and/or urogenital anomalies. The surgical strategy for congenital perineal lipoma may therefore differ depending on such factors, with the optimum approach remaining to be determined.
We herein report a case of congenital perianal lipoma that was first detected by prenatal ultrasound. The prenatal detection or diagnosis of such lesions is quite rare [8,14,23,25]. We will discuss the appropriate surgical strategy for such lesions and review the literature.

Case presentation
A female neonate was referred for the evaluation of a perianal mass. She was delivered at 39 weeks and 1 day of gestation weighing 3097 g. The Apgar score was 9 at both 1 and 5 min after birth. She had been considered male prenatally because fetal ultrasound showed a perineal mass similar to male genitalia at a gestational age of 20 weeks and 4 days (Fig. 1a). A postnatal examination revealed an appropriate-for-age neonate with a soft round mass 1.5 cm in diameter just left of the anal verge (Fig. 1b). She passed urine and stool smoothly. Complete blood counts, electrolytes, liver and renal function tests, and urinalysis findings were all within normal ranges. Contrast enema confirmed no anorectal malformation (ARM). Magnetic resonance imaging (MRI) showed that the lesion had a signal intensity consistent with fat located close to the anal sphincter, and no spinal anomaly (e.g., spina bifida) was identified (Fig. 1c). The operative procedures at 2 months of age were as follows: the skin incision was made around the bottom of the pedicle of the lesion. The margin between the tumor and the surrounding subcutaneous fat tissue was not clear, but the tumor did not invade into the sphincter muscle. The lesion was resected with some of the subcutaneous tissue attached, and with small area of the surface of external sphincter muscle exposed. An electrical muscle stimulator was used to confirm that the muscle was left intact. The skin was closed with interrupted sutures.
A histologic examination of the specimen showed mature adipose tissue interspersed with collagenous bands, leading to a diagnosis of lipoma. She was discharged from the hospital without any complication and has been doing well with good bowel movement and satisfactory cosmetic results for a follow-up period of one and a half years.

Discussion
We conducted a literature search using PubMed with keywords of lipoma, perianal, perineal, and neonate, identifying 49 cases of this lesion reported in English since 1994 . The clinical features of the 50 total cases, including ours, are described in Table 1. There were 25 males and 25 females, and 37 cases (74%) accompanied other anomalies (breakdown shown in Fig. 2), including ARM most frequently (40%), followed by labioscrotal fold or accessory scrotum (28%), and urogenital malformation, congenital pulmonary airway malformation, and disorder of sex differentiation. The perianal lesion in our case was detected by a referring obstetrician using fetal ultrasound but was initially considered to be male genitalia; however, a correct diagnosis could have been achieved with more thorough assessments. Our literature search revealed only four cases in which perineal lesions were detected prenatally: case no. 34 and no. 45 in Table 1 were diagnosed as lipoma by fetal ultrasound and MRI, while case no. 35 was diagnosed with ultrasound only, and case no. 27 was detected as an uncharacterized mass using fetal ultrasound and MRI at gestational ages of 31, 32, 31, and 25 weeks, respectively. Two of these cases were found to have hypospadias and/or accessory scrotum after birth.
The differential diagnoses of perineal lesions other than lipoma include lipoblastoma, sacrococcygeal teratoma, infantile hemangioma, hamartoma, choristoma, liposarcoma, enterogenous cyst, and ambiguous genitalia    crisis. Regarding the delivery mode, one of the newborns whose perineal lesions were detected prenatally was delivered by cesarean section (one of twins, case no. 27), but the reason of the mode was not described in the paper. The maximum diameter of the reported cases was 5 cm (case no. 35, no. 43, no. 46), whose mode of delivery was vaginally or not described. If the prenatal imaging studies reveal that the diameter of the lesion is beyond the fetal head, or if the shape and vascularity of the lesion are highly indicative of injury of the lesion at the time of vaginal delivery, cesarean section would be more appropriate than vaginal delivery. Postnatally, a careful physical examination for its precise location, size, and accompanying anomalies is crucial to achieving a correct diagnosis. These lesions are typically lobulated, round, or pedunculated subcutaneous masses that are smooth, soft, mobile, and nontender [8]. Postnatal urinary and meconium passage should be carefully observed in case the lesion obstructs those areas and requires prompt surgical intervention. Ultrasonography and MRI can help identify the internal fatty content and its anatomical relationship with the surrounding structures, along with potential complications [8,23]. Contrast enema is also useful for ruling out anorectal anomalies and/or bowel obstruction, especially when the lesion is close to the anus, as in our case. Contrast-enhanced computed tomography may be needed when the above-mentioned imaging studies are insufficient to diagnose the lesion and/or other anomalies. If a neonate has no other anomalies other than the perineal lipoma as in our case, the timing of the surgery would be decided depending on two factors: the masseffect causing urinary and/or intestinal obstruction, and technical difficulty in early surgery. The former would require surgery soon after birth. Regarding the latter, if the lesion is located close to vagina, for example, it would be appropriate for the operation to be delayed for about 3 months or later, which would make the precise dissection between the lesion and the vagina easier than doing in early neonatal period.
On the other hand, most reported lesions accompanying anorectal anomalies have been resected at the time of anorectal reconstruction [5,14]. One female with anovestibular fistula had a lipoma (3 cm in diameter) close to the fistula; it was partially resected during the neonatal period to facilitate the passage of stool, followed by complete resection at the time of corrective surgery for the ARM [26]. Dissection of the perianal lesion from subcutaneous tissue, especially sphincter muscles, was successfully performed with the aid of a muscle stimulator in our case. Some perineal lipomas complicated by ARM are reportedly interspersed between the sphincter muscles [5] and may have a negative impact on the bowel functional outcome [7]. Special care should be taken to preserve the external anal sphincter using an electrical muscle stimulator during resection [5], and incised muscles or structures should be repaired accordingly.

Conclusion
We herein report a rare case with prenatally detected perianal lipoma that neonatologists and pediatricians should note. Surgical consultation should be sought, and thorough perinatal investigations of the lesion and associated anomalies are crucial for planning surgery. An electrical muscle stimulator is useful for preserving the anal sphincter during resection to achieve satisfactory bowel movement.