Filum terminale lipoma revealed by screening spinal ultrasonography in infants with simple sacral dimple

Spine ultrasound (US) examinations are widely performed in neonates and infants with sacral dimple to exclude associated congenital malformations. Increased utilization of, and improvements in, sonographic technology has resulted in the frequent identification of echogenic filum terminale (FT) containing fat below the conus medullaris, termed FT lipoma or fatty FT. We examined the value of screening US, especially for diagnosis of FT lipoma, in infants with "simple sacral dimple."

This study was a retrospective review of spinal US and magnetic resonance imaging (MRI) findings performed in our hospital on full-term and preterm infants with simple sacral dimple. We reviewed the clinical records and spinal sonographic scans of 337 patients, and the MRI scans of 40 patients with simple sacral dimple. We evaluated the thickness and echogenicity of FT on US, and the presence of fat in the FT was determined by MRI.

In total, 50 of the 337 patients showed strong echogenic FT on US. Correlation with MRI data was possible in 40 of the 50 patients with strong echogenic FT. MRI scans of all 40 of these patients revealed the presence of fat in the FT. The maximum thickness in the strong echogenic area of the FT lipomas of these 40 patients was 1.7 ± 0.4 mm (range 1.2-2.5 mm), and the minimum thickness was 1.1 ± 0.2 mm (range 0.8-1.4 mm). Among the 40 infants, 13 (32%) had maximum FT thickness > 2 mm (mean thickness 2.2 ± 0.2 mm; range 2-2.5 mm) and 27 (68%) had echogenic FT < 2 mm thick (mean thickness 1.5 ± 0.2 mm; range 1-1.9 mm). The length of the echogenic segment on US was about 1.9-5.1 cm (mean length 3.4 ± 1.1 cm). All 40 patients showed FT lipoma without any associated abnormalities, such as cord tethering or low-lying conus. Among the total population of 337 patients, 30 (8.9%) showed borderline low-lying conus medullaris located at the L2-3 disc space or midpoint of the L3 vertebra. All 30 of these patients had borderline low-lying conus without other associated abnormalities. Filar cyst was noted in 36 of 337 patients (10.6%), without other associated abnormalities. No patient underwent surgery associated with a risk of neurological problems within a follow-up period of 10-37 months.

Although screening US in infants with simple sacral dimple yielded benign imaging findings, the presence of FT lipoma was common in these infants. Therefore, it is necessary to carefully observe changes in echogenicity during measurement of FT thickness on US. All FT lipomas were low risk and had no associated abnormalities, such as cord tethering or low-lying conus.