Reliability of the 'pop' sign as an indicator of dural puncture during obstetric spinal anaesthesia: a prospective observational clinical study.

Anaesthetists rely upon a loss of resistance and flow of cerebrospinal fluid to indicate when a spinal needle has breached the dura. The loss of resistance is not always felt, with the danger that the needle may be advanced into neurological tissue. One hundred women undergoing elective caesarean section were recruited and spinal anaesthesia, using a 27-G Whitacre needle, was performed using an incremental advancement technique. After each advancement of the needle, and before removing the stylet, it was recorded whether the anaesthetist had perceived any sign that the dura had been breached, and then whether cerebrospinal fluid had been obtained. Six patients were not included in the study due to technical difficulties. In 27 of 94 patients (29%) there was no clear tactile sign that the dura had been breached when cerebrospinal fluid was obtained. This study demonstrates that loss of resistance is not always felt when a 27-G spinal needle breaches the dura in the pregnant woman.