In Reply:--We appreciate Joshi's support of our data. Success with the combined spinal-epidural (CSE) needle-through-needle technique relates to some degree to the protrusion length of the spinal needle beyond the tip of the epidural needle. In our study and in clinical practice, we have found that some patients require up to a 15-mm clearance. In this same study, when a 3-inch Weiss epidural needle was used, there was adequate needle length in all patients. However, the technical price that one pays with an exceedingly long protrusion length is diminished stability of the needle-through-needle setup. We prefer a spinal needle that protrudes at least 12 mm but no longer than 15 mm.
However, we disagree with Joshi regarding the use of an epidural needle with a separate "back hole." No data support the use of a back hole in terms of CSE success. To the contrary, in the study by Joshi and McCarroll, the needle combination using an epidural needle with a separate back hole had a 15% failure rate, whereas the normal Tuohy needle was associated with a 100% success rate in 13 patients. Further, there are no data from a prospective, blinded study supporting the better "feel" of dural penetration with an epidural needle that has a separate back hole. I have been able to perceive dural penetration using a 27-G Whitacre spinal through a Weiss epidural needle in the vast majority of patients. However, this also is anecdotal, unblinded, and operator-dependent. This claim must be evaluated with the investigators properly blinded and dural "feel" graded and confirmed (e.g., by the presence of cerebrospinal fluid) if meaningful conclusions are to be drawn.
In a study of fresh cadavers using epiduroscopy, Holmstrom et al. concluded that "it was impossible to force 16 or 18 gauge epidural catheters through the dural hole made by a single dural puncture with a 25 gauge spinal needle." No data show a reduction in dural penetration with the catheter when a needle with a separate back hole is used. We agree that it is important to select a matched spinal and epidural needle combination of proper length to achieve acceptable success. Similar to Joshi, we recommend a protrusion length of the spinal needle of at least 12 mm. However, beyond this, there are inadequate data to conclude what constitutes the "optimal" needle combination.
William F. Urmey, M.D., Jennifer Stanton, B.S., Nigel E. Sharrock, M.B., Ch.B., Department of Anesthesiology, The Hospital for Special Surgery, Affiliated with The New York Hospital and The Cornell University Medical College, 535 East 70th Street, New York, New York 10021.