Comparison between cold and sharp modalities for testing sensory block in neuraxial labour analgesia: a prospective observational trial

Editor—Neuraxial (spinal or epidural) labour analgesia blocks transmission of afferent nerves from the uterus, vagina, and perineum; breakthrough pain can occur if the appropriate nerves are not blocked. Study of spinal anaesthesia demonstrates differential neural block: the extent of sensory block to cold differs from that to sharp, but this concept has not been studied for neuraxial labour analgesia.1 Neuraxial labour analgesia results in a block that is significantly less 'dense' than that required for surgical anaesthesia. Although an anatomical study showed that sensory nerves from the uterus enter the spinal cord at T10 to L1,2 it is not clear whether differential sensory block exists for neuraxial labour analgesia or how the clinician should assess the adequacy of block of sensory impulses that transmit labour pain in the clinical environment. We performed a prospective observational study to test the hypothesis that after initiation and during maintenance of neuraxial labour analgesia, the extent of cephalad sensory block assessed using a cold stimulus differs from that of a sharp stimulus, and that parturients with breakthrough pain will not have a bilateral sensory block to a cold stimulus between S2 and T10.