Spinal anaesthesia for caesarean section: an ultrasound comparison of two different landmark techniques.

Background: Spinal anaesthesia performed at levels higher than the L3-4 intervertebral space may result in spinal cord injury. Our aim was to establish a protocol to reduce the chance of spinal anaesthesia performed at or above L2-3.

Methods: One hundred and ten consenting patients at 32weeks of gestation or greater scheduled for non-emergency caesarean section under spinal anaesthesia were randomly allocated to have needle insertion performed at an intervertebral space determined by one of two landmark techniques. In Group A, if the intercristal line intersected an intervertebral space, this space was selected or if the intercristal line intersected a spinous process the space immediately above was selected. In Group B, if the intercristal line intersected an intervertebral space or a spinous process, the intervertebral space immediately below was chosen. The actual intervertebral space chosen was identified using ultrasound by a blinded investigator.

Results: In Group A, an intervertebral space at or above L2-3 was marked in 25 (45.5%) patients compared with 4 (7.3%) in Group B (P <0.001). In 5/55 (9.1%) patients in Group A, the intervertebral space initially chosen was L1-2 whereas this occurred in no patient in Group B. There was no difference between groups in number of needle passes or attempts, onset of block at 5, 10 and 15min or need for rescue analgesia.

Conclusion: Our data suggest that when performing spinal anaesthesia in pregnant patients, if the intercristal line intersects an intervertebral space then the space below should be chosen and if the intercristal line intersects a spinous process then the interspace below should be chosen. This will reduce the incidence of spinal anaesthesia performed at or above L2-3.