1 Glasgow and 2 Dundee, UK
EditorThe editorial by Fettes and Wildsmith1 discusses the important issue of neurological damage after spinal anaesthesia with pencil point needles. They mention several factors about these needles and how these may predispose to neurological damage. I would like to comment on three of the four points they make in the section on equipment.
Their first point is that pencil point needles require to be inserted for a greater length because the needle orifice is situated approximately 1 mm proximal from the tip. Whilst the latter is true, there is no evidence of the former. We performed a pilot trial, which did not support this point (see below).
Their third point was that pencil point needles may require a slightly greater force to advance and that this may predispose to overshoot and risk neurological damage. An in vitro study comparing 25 gauge Becton Dickinson Whitacre and Quincke spinal needles, showed that a threefold increase in force (0.12 kg) was required to puncture bovine dura mater with the former compared to the latter (0.04 kg).2 It would be logical to presume that greater force is required to advance pencil point needles through the other tissues in the back. The risk of overshoot depends on how the force is applied; if it is applied in a controlled manner, there should be no overshoot.
Their fourth point concerned tenting of the dura. Tenting of the dura does occur before puncture. Spinaloscopy in a system open to atmospheric pressure (cadaver dissection) has shown that the dura tents and then retracts to its original position after needle puncture. This occurs with both types of needle tip but is more pronounced with a pencil point tip.3 The normal practice of spinal anaesthesia, however, occurs in a system which is closed to atmospheric pressure.
We studied epidural pressures and distance to dural puncture in patients undergoing subarachnoid anaesthesia performed either with a Whitacre (pencil point) 25G or Quincke 25G spinal needle.4 The subarachnoid depth and dural tenting was similar in both groups, implying that a Whitacre needle is not inserted closer to the spinal cord. However, the median (95% CI) pressure recorded just prior to dural puncture was significantly more negative in the Whitacre group (32 (52 to 10 mm Hg) vs 2.5 (10 to 5.5 mm Hg, P<0.02 in the Quincke group)). The higher pressure gradient would favour retraction of the dura back to its original position even more than in the open system. This would obviate the need to advance the needle further to ensure the orifice is entirely within the subarachnoid space.
The production of a greater negative epidural pressure with the Whitacre needle generates the higher force required to puncture the dura and also limits the distance of tenting, which therefore limits the distance of the needle to the neural tissue. This would not be the situation in an open system, such as the needle through the needle combined spinal epidural technique, where the negative pressure would not be generated.
Our study involved only a small number of patients and we were only able to measure distances as small as 0.5 mm.4 However, we concluded that in a closed spinal anaesthetic technique, there is no evidence that pencil point needles require to be inserted any further than Quincke needles. Therefore they should not predispose to any increased contact with the neural tissues.
M. G. Serpell
Glasgow, UK
EditorWe thank Dr Serpell for his interest in our editorial.1 Much of the editorial was based on first principle analysis; data are always to be preferred if they are available, but those data must be accurate.
Dr Serpell disagrees with our view that it is necessary to insert more of a pencil point needle into the subarachnoid space than a standard Quincke point, the basis for this being a study with a small number of patients, published only in abstract form 5 yr ago.4 Reviewing that abstract, we found that changes in the pressure recorded from the needle as it was advanced through the ligaments were used to identify key anatomical landmarks (e.g. the point of moving from the ligamentum flavum into the epidural space). The length of needle remaining outside the patient was measured at each stage by some unidentified method. This strikes us as a less than reliable technique because it makes assumptions about the meaning of the pressure changes recorded as the needle moves forward, and appears to take no account of skin tethering as it does so. Dr Serpell admits to being able to measure distances no smaller than 0.5 mm, and we wonder about even that accuracy given the methodology. In addition, it is important to recognize that the side port of the Whitacre needle will mean that approximately 1 mm of needle will be inserted into each space before a pressure change is observed (whereas it will be much less than this with the Quincke needle). Perhaps confirming our concerns about the method, the data presented in the abstract are for only 10 out of 16 Whitacre, and 9 out of 12 Quincke patients because of inability to identify two out of the four key landmarks.
The actual measurement of epidural pressure is more straightforward, and we note that Serpell4 found a statistically significantly greater decrease in pressure as the Whitacre needle was advanced. It is now well recognized that much of the negative pressure identified in the epidural space is due to dural tenting as the needle is advanced,5 so a greater decrease in pressure would, to us, suggest that the needle has been inserted further, even before the dura has been breached. Serpells analysis of the differences in the dynamics of pressure change with the two different needles is not sustainable given the problems with the measurement of distance.
Finally, we are grateful to Dr Serpell for providing additional evidence that the force needed to insert a pencil point needle is greater.2 4 He argues that the risk of overshoot depends on how well controlled is the force used to insert the needle, but the real issue is that the greater the force needed, the greater the risk of overshoot, especially in inexperienced hands. That is one of our main concerns.
P. D. W. Fettes
J. A. W. Wildsmith
Dundee, UK
References
1 Fettes PDW, Wildsmith JAW. Somebody elses nervous system. Br J Anaesth 2002; 88: 7603
2 Westbrook JL, Uncles DR, Todd Sitzman B, Carrie LES. Comparison of the force required for dural puncture with different spinal needles and subsequent leakage of cerebrospinal fluid. Anesth Analg 1994; 79: 76972[Abstract]
3 Mollmann M. Dural trauma; audiovisual presentation. Presented on September 9, 1993 at the 12th Annual European Society for Regional Anaesthesia, Dublin, Ireland
4 Anderson L, Marshall S, Brydon C, Serpell MG. Comparison of dural indenting between Whitacre and Quincke tipped spinal needles. Br J Anaesth 1997; 78: 4689P
5 Aitkenhead AR, Hothersall AP, Gilmour DG, Ledingham IM. Dural dimpling in the dog. Anaesthesia 1979; 34: 1419[ISI][Medline]