The Influence of Spinal Flexion in the Lateral Decubitus Position on the Unilaterality of Spinal Anesthesia

Abstract

For unilateral spinal block, local anesthetics should affect the spinal nerves of 1 side. With full flexion of the spine, the sunken cauda equina becomes tightened and is suspended in the middle of the subarachnoid space. We performed this study to assess whether spinal flexion facilitates unilateral spinal anesthesia. Hyperbaric bupivacaine (8 mg) was administered at the L3-4 interspace through a 25-gauge Quincke needle at a rate of 0.02 mL/s. Patients were randomly allocated to group F (with full spinal flexion) or group N (the hips and back straightened). After maintaining the lateral position for 15 minutes with or without spinal flexion, patients were gently returned to the supine position. Spinal blockade was assessed by loss of pinprick sensation and the modified Bromage motor scale. While the lateral position was maintained, sensory block was noted on the nondependent side in 14 of 16 patients in group N (87.5%) but only in 1 of 16 patients in group F (6.3%) (P < 0.001). The median level of sensory block in group N was L5 on the nondependent side just before turning to the supine position. When patients were returned to the supine position, sensory blockade on the nondependent side was noted in all group N patients (100%) and 15 group F patients (93.7%). The sensory level on the nondependent side between group N and group F were similar after turning supine. Strict unilateral sensory block was not achieved even after lateral decubitus positioning with spinal flexion, when 8 mg hyperbaric bupivacaine was administered manually at a conventionally slow rate through a beveled spinal needle. However, maintaining flexion of the spinal column during lateral decubitus positioning altered the initial onset of sensory block with respect to laterality.