NORMAL DIAMETER OF FILUM TERMINALE IN CHILDREN - IN-VIVO MEASUREMENT

The hallmarks of tethered cord syndrome are a low-lying conus medullar is and a thick filum terminale. In diagnosing the tethered cord syndro me, 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 myelog raphic measurements a few decades ago, and the true normal diameter of the filum terminale diameter in children or adults remains unknown. W e measured the diameters of the filum terminale in vivo in the operati ng room on 31 children (age range = 2-14 years; mean age = 5 years) un dergoing selective dorsal rhizotomy for spastic cerebral palsy. None o f them had clinical evidence of tethered cord syndrome. The conus medu llaris 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 extraoperative ly 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 mu m (mean +/- SD), respectively. In all children except one, the conus m edullaris ended above the L2 level. The data indicate that filum termi nales greater than 2 mm in diameter in children are abnormally thick.