Normal diameter of filum terminale in children: in vivo measurement

The hallmarks of tethered cord syndrome are a low-lying conus medullaris and a thick filum terminale. In diagnosing the tethered cord syndrome, the thick filum terminale is often defined as that greater than 2 mm in diameter. The cutoff of 2 mm in diameter was derived from myelographic measurements a few decades ago, and the true normal diameter of the filum terminale diameter in children or adults remains unknown. We measured the diameters of the filum terminale in vivo in the operating room on 31 children (age range = 2-14 years; mean age = 5 years) undergoing selective dorsal rhizotomy for spastic cerebral palsy. None of them had clinical evidence of tethered cord syndrome. The conus medullaris and filum terminale were videotaped intraoperatively and images were transferred to an image analyzer; the filum diameters at 10 and 15 mm caudal to the conus medullaris were then measured extraoperatively using the computer graphics system. The diameter of the filum at 10 and 15 mm caudal to the conus was 1,211 +/- 209 and 1,163 +/- 245 microm (mean +/- SD), respectively. In all children except one, the conus medullaris ended above the L2 level. The data indicate that filum terminales greater than 2 mm in diameter in children are abnormally thick.