Since the lateral holes of the epidural catheter (inserted as part of a CSE anaesthetic) may lie in close proximity to the dural puncture site, a bolus of local anaesthetic or narcotic injected via the epidural catheter has the potential to leak through the dural puncture hole into the subarachnoid space.
Collier (1) performed an epidurogram by injecting contrast into the epidural catheter of three patients who had had a CSE anaesthetic. In one of these patients he documented a spread of contrast from the epidural into the subarachnoid space.
The implications of an intrathecal leak arising from a bolus epidural injection give cause for serious concern with anecdotal evidence supporting this in the literature (2).
The incidence of leakage from the epidural into the subarachnoid space through the dural puncture site is still not known. Contrast epidurography studies which look at such leakage following the insertion of a catheter as part of a CSE anaesthetic using Tuohy epidural and pencil-point spinal needles (in cases where there is no clinical evidence of dural puncture by the Tuohy needle) are needed.
At the Royal Hospital for Women, a preliminary study of this potential problem is being undertaken (3). To date, fifteen CSE women have received an injection of 10-13mls of the contrast iohexol 300mg/ml via the epidural catheter within three hours of catheter insertion. The spread of contrast has been followed fluoroscopically. No evidence of intrathecal leakage has been demonstrated.
Nevertheless, following a combined CSE anaesthetic, the fact that the dura has been breached should be kept in mind. This is particularly so when bolus injections are given via the epidural catheter soon after placement of the CSE.
References:
1. Collier C. Cardiorespiratory arrest following combined spinal epidural anaesthesia (letter) Anaesthesia 49 (3):259, 1994.
2. Myint Y. Cardiorespiratory Arrest Following Combined Spinal Epidural Anaesthesia for Caesarean Section Anaesthesia 48; 684-686, 1993.