Sir,
Read with interest the article ‘No rent is small for migration of epidural catheter into sub-arachnoid space’.[1] I have few observations regarding this article and need further clarifications before believing that catheter could migrate through such a small opening in the sub-arachnoid space.
First, 26-gauge 1½ inch (3.81 cm) long needle was used for infiltration of local anaesthesia and understandably, the needle must have been introduced up to the junction of the shaft with the hub as cerebrospinal fluid (CSF) was aspirated due to puncture of the dura. However, when epidural space was located, it was found at the depth of 4 cm, marginally more than the sub-arachnoid depth. How it can be explained? Considering some thickness for dura (0.5 mm) and a few mm of epidural space, the distance to be covered by the needle to meet the sub-arachnoid space should have been a little more than 4 cm.
Second, if authors could confirm the returning fluid to be CSF (temperature and sugar analysis) from the epidural catheter, they could as well have done the same for the returning fluid from the needle used for infiltrating local anaesthetic. The injected saline returning into the catheter or needle might easily mislead us to believe it to be CSF. Often, the local anaesthetic that is injected into soft tissues flows back into the needle under pressure. I believe, this was the case in this episode also. Authors have not mentioned what medium was used to ascertain loss of resistance technique, whether air or saline. Further, since the epidural needle was inserted up to 4.0 cm, it seems more probable that the epidural needle created a large rent in the dura, leaving the arachnoid membrane intact and catheter must have penetrated arachnoid membrane with ease before entering into sub-arachnoid space.[2,3,4,5]
Dura mater is thin posteriorly and relatively thicker on its anterior aspect; this fact predisposes to dural puncture by epidural catheters with a pointed tip. In this report, CSF flow was seen when the catheter was inserted up to 14 cm and, therefore, another possibility is that catheter tip might have perforated the dura and catheter entered in the sub-arachnoid space.[6]
I believe entry of epidural catheter through opening made by the 26-gauge needle is solely based on a presumption as it is not confirmed by any investigation. I hope authors may be able to answer the raised queries to justify their claim.
REFERENCES
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