[Subdural intra-arachnoid spread of local anesthetics. A complication of spinal anesthesia]

Accidental subdural injections and catheterisations are a complication of epidural and spinal anaesthesia. The incidence of subdural spread in myelographies is estimated to be over 10% by the spinal technique. With spinaloscopy in an anatomic human model, we analysed the puncture process and the influence of different needle types on the incidence of subdural injection. We compared 22-gauge Sprotte, Quincke, and 18-gauge Tuohy needles in median and paramedian approaches with various bevel orientations. METHOD. The studies were performed in a preserved and recently expired cadaver donated to the Institut für Anatomie, Westfälische Wilhelms-Universität, Münster. The spinal column from T12 to S1, together with the back musculature (in order to preserve the normal curvature of the spine), were removed from the cadaver. Spinaloscopy was performed with a 4-mm endoscope with a 0 degree optic (Storz, Tuttlingen, Germany). All observations were made in the lumbosacral region of the dissected preparation. The endoscope was inserted from the caudal end of the spinal canal and, depending on the observations being made, the spinal canal was filled with air or artificial cerebrospinal fluid (CSF). To obtain information on the distribution of local anaesthetics injected into the subarachnoid space, 0.5% bupivacaine was coloured with a small amount of 1% methylene blue. The distribution of the coloured anaesthetic was clearly visible during and after injection. RESULTS. Needle insertion: Multiple observations were made using median or paramedian advancement of the needle into the spinal canal. With all needles, including the pencil-point, we saw an unexpected inward movement of the dura to the epidural space before penetration. This dural movement was independent of the direction of the dural fibres in the lumbar area. Distribution of local anaesthetics: Our observations indicate that difficulty with injecting drugs occurred when needle insertion was stopped too close to the dura, especially with the Sprotte needle. After manually registered penetration of the dura, the lateral opening of the needle only partially penetrates the dura. This allows CSF to appear in the needle hub, and injection into the vertical subdural space is possible. In all cases with the Sprotte needle, we could reproduce deposition of methylene-blue-coloured local anaesthetics into the subdural space. With the Quincke and Thuohy needles, it was not possible to deposit local anaesthetics into the subdural space in this model. CONCLUSION. Spinaloscopy was done in a non-fixated anatomic preparation of a spinal column with a 4-mm, 0 degree endoscope. From these observations we conclude that both manually registered penetration of the dural and the appearance of CSF in the needle hub can mimic correct needle position. Especially with the lateral opening of the Sprotte needle, deposition of local anaesthetics in the subdural space is possible.