Damage to the conus medullaris following spinal anaesthesia: 2

We read with interest the recent case report describing damage to the conus medullaris following spinal anaesthesia (Reynolds. Anaesthesia 2001; 56: 238–47). Atraumatic (pencil-point) needles have at least 1 mm of blind tip beyond the hole and thus a tendency to insert further into the subarachnoid space than is necessary with a bevelled needle [1]. As Dr Reynolds states, this can only partly account for the cluster of the conus medullaris injuries following combined spinal–epidural (CSE) or pencil-point needles. We would like to suggest further explanations. It is well known that during spinal anaesthesia bevelled needles are deflected more than pencil-point needles, and that the degree of deflection is correlated with gauge and depth of insertion [2–4]. Bevelled spinal needles are deflected in the opposite direction to its bevel. In contrast, pencil-point needle deflect mainly in the direction of the distal side port because of increased ‘drag’ induced by the irregularity of the side port or, if present, the tip bend made at the area of weakness (level of side port) [4]. When spinal and epidural needles are assembled during the CSE technique, the spinal needle emerges at a deflected angle of 4–5°[5] or approximately 10°[6] from the long axis of the epidural needle because of the bent tip of the Tuohy needle. Tuohy needles are also deflected to the bevel (end-hole) side by the curved tip [3]. During CSE block, the spinal needle deflects further to the bevel side of the Tuohy needle than the simply assembled state because the shaft of the Tuohy needle makes a curved route to the epidural space. The variation in conus position follows a normal distribution with the peak incidence at the lower third of the body of L1 with a range from the middle third of T12 to the upper third of L3 bodies [7]. During attempts at spinal anaesthesia at L2/L3, the more cephalad the needle tip is directed, the higher the probability of needle trauma. Even during the midline approach, a slight cephalad angulation is generally recommended. In Dr Reynolds' report, all the cases were women, six obstetric patients and one obese patient. It is clear that their interspinous spaces could not be widened as much as the general population, because such patients cannot flex their hips fully even in the lateral decubitus position [8]. The paramedian approach, which needs a longer and more cephalad path to the subarachnoid space, may be favoured when the interspinous space is not sufficiently wide or to minimise the risk of postdural puncture headache in high-risk groups such as parturients. While performing a spinal or CSE block with the bevel orientated cephalad, bevelled spinal needles direct more caudad than the axis of insertion into the skin. In contrast, CSE or pencil-point spinal needles course more cephalad. Because the clinical use of Tuffier's line requires palpation through a variable amount of subcutaneous fat, high placement is especially likely in obese patients [9]. Term parturients with pre-eclampsia and generalised oedema might have the same tendency. When performing spinal anaesthesia with isobaric bupivacaine, injection at a high lumbar space produces a higher spread of analgesia [10]. The temptation to use a lesser amount of ‘potentially neurotoxic’ spinal anaesthetic drug may force the anaesthetist to choose the L2/L3 interspace. These arguments may help to explain some incidences of spinal cord damage incurred during CSE or spinal anaesthesia attempted at the L2/L3 interspace. As Dr Reynolds states, however, the main reasons for the conus medullaris injuries following spinal or CSE anaesthesia are an error of identification of the correct intervertebral space from surface markings, and a failure to remember that the conus may extend as low as L3.