A study on the diagnostic value and classification of sirenomelia by prenatal ultrasonography.

objectiveTo investigate the prenatal ultrasonographic features of sirenomelia and perform classification, aiming to improve diagnostic accuracy during the early and mid-trimester.

methodsClinical data of pregnant women with a prenatal diagnosis of sirenomelia were retrospectively analyzed. The ultrasonographic characteristics were described, and cases were classified into three groups according to Stocker's classification: bipedal sympodia group (Group A, including types I, II, III), monopedal sympodia group (Group B, including types IV, V), and apodal sympodia group (Group C, including types VI, VII). Based on the origin of the umbilical artery and the position of the umbilical vein, single umbilical artery (SUA) was categorized as Type I or Type II. The correlation between relevant vascular parameters and the severity of lower limb malformations was analyzed across different SUA types and groups. Pregnancy outcomes and related imaging findings were followed up, and the consistency between post-termination fetal appearance and prenatal diagnosis was compared.

resultsThe incidence of sirenomelia in our institution was approximately 0.37 per 1,000 (20/54,196). The primary prenatal ultrasonographic features in the 20 cases included sympodia (fusion of the lower limbs), single umbilical artery, and foot deformities. Major associated anomalies were oligohydramnios (12 cases), bilateral renal agenesis (20 cases), non-visualization of the bladder (14 cases), and a slender abdominal aorta with its branches (15 cases). The classification of sympodia was as follows: Type I (3 cases), Type II (2 cases), Type III (4 cases), Type IV (2 cases), Type V (2 cases), Type VI (5 cases), and Type VII (2 cases). Type I SUA included 3 cases from Group A and 2 from Group B. Type II SUA included 6 cases from Group A, 2 from Group B, and 7 from Group C. In Type I SUA, there were no statistically significant differences in the mean internal diameter at the origin of the umbilical artery (t = 4.420, P = 0.126) or the mean internal diameter of the distal abdominal aorta (t = -1.777, P = 0.076) between Groups A and B. In Type II SUA, no statistically significant difference was found among the three groups regarding the mean internal diameter at the origin of the umbilical artery (F = 0.679, P = 0.525). However, a statistically significant difference was observed in the mean internal diameter of the distal abdominal aorta among the three groups (F = 9.865, P = 0.007). Bivariate Pearson correlation analysis revealed that the mean internal diameter of the distal abdominal aorta was negatively correlated with the severity of lower limb malformations in both Type I (r = -0.889, P = 0.044) and Type II SUA (r = -0.818, P < 0.001). Post-termination examination of all 20 fetuses showed fusion from the hips downward, with absence of the gluteal cleft, external genitalia, anus, and urethra. The postnatal external findings were consistent with the prenatal ultrasonographic diagnoses.

conclusionPrenatal ultrasonography enables accurate diagnosis and classification of sirenomelia. In Type II single umbilical artery (SUA), differences were observed in the mean internal diameter of the distal abdominal aorta among the three groups. Furthermore, in both Type I and Type II SUA, the mean internal diameter of the distal abdominal aorta was negatively correlated with the severity of lower limb malformations. As a rare and lethal malformation involving multiple systems, the pregnancy outcome after diagnosis is typically termination.