Assessing the Depth of the Subarachnoid Space by Ultrasound

Methods: Thirty-one patients (25 males and 6 females), ASA I or II participated in this study. The transversal ultrasound image of the lumbar spine was obtained at the level of the L3-L4 space. An anesthesiologist without prior information performed the spinal anesthesia through the predicted target area. The distance between the skin and the anterior portion of the flavum ligamentum which is supposedly the bottom limit of the intrathecal depth or an approximation of the depth of the epidural space (ED-US) was measured by ultrasound and it was compared with the distance between the skin and the anterior portion of the flavum ligamentum on the needle (ED-N). Results: ED-US and ED-N were respectively 5.15 ± 0.95 cm and 5.14 ± 0.97 cm; these distances were not significantly different (p > 0.0001). A significant correlation r = 0.982 [95% CI 0.963-0.992, p > 0.0001] was observed between the ED-US and ED-N measurements. Bland-Altman analysis showed an accuracy of 0.18 cm; tolerated variations ranged from -0.14 cm to -0.58 cm. Conclusions: This study supports the idea that the US transversal plane allows the identification of axial anatomical structures and provides physicians with efficient information to perform spinal anesthesia.