Safety and technique with pencil‐point subarachnoid needles

Abstract

Recent correspondence related to the competing merits of Quincke-point versus pencil-point spinal needles is gradually evolving toward a coherent solution, first in choice of instrument, and second in deciding how that instrument should be used [1, 2]. Another recent in vitro study confirms that the pencil-point 27 G Whitacre needle is a good choice, with only 25% of the volume of cerebrospinal fluid lost via dural holes created by Quincke needles of the same calibre [3]. Drs Ali & Samsoon are absolutely correct [2]; subarachnoid and epidural needles should always be advanced with continuous, machine-like precision and never intermittently [2, 4]. The need for continuous advance is intuitively and anatomically obvious, because of the vulnerability of low-lying spinal cords that may extend as low as L5 in the occasional case of spina bifida occulta with a tethered cord [5]. A third piece must be added to this debate in terms of the much neglected technical imperatives of the subdural space − aptly referred to as ‘the third place to go astray’ by Dr Felicity Reynolds [6]. In the days of myelography, most radiologists and anaesthetists were aware of the technical pitfalls of this space [7] and pains were taken to avoid them by using the loss-of-resistance-to-negative-pressure test, or what may be called the ‘suck-and-see’ technique; an approach that is de rigueur for cisternal punctures to avoid the danger of pithing the patient. The basic physics of the subdural space, the hydraulics of fine needles and the technical advantages of the ‘suck-and-see’ test to collapse the subdural space have been described elsewhere [8]. I believe all who are strangers to this technique would be gratified by instant conversion both to pencil-point needles and to the method, after a little preliminary corroboration on a training manikin. Unfortunately, the ‘suck-and-see’ technique cannot be used with the needle-through-needle combined spinal-epidural technique, and for that reason Morris et al. [1] are also correct that a two-needle approach, through separate vertebral interspaces is a sound choice, at least for those who are committed to a belt-and-braces philosophy in their practice of neuraxial blockade.