New Avenues of Epidural Research

To the Editor: The case report by Kasai et al. (1) and the accompanying editorial by Rose (2) remind us that the potential for serious risk is inherent with every neuraxial procedure performed. Rose (2) has provided us with some commonsense guidelines to help us prevent such complications. We personally concur with his conclusions but wish to bring attention to two new avenues of research. Techniques are being developed to help make neuraxial procedures safer, particularly in heavily sedated or anesthetized patients. Entry into the epidural or intrathecal space can be reliably demonstrated electrically (3,4). Previous studies (5–7) have demonstrated that a motor response evoked by 1 mA or less indicates the catheter is either in the subarachnoid space (SA) or close to a nerve root (subdural, 0.3 mA; SA, 0.4 mA; immediate proximity to a nerve root, 0.5 mA). These observations support the potential application of electrical epidural stimulation or Tsui test (3–8) as an adjunct method to identify the precise location of a needle or catheter in the epidural space. A modification of the Tsui test can be used to help guide an insulated needle into the caudal or epidural space (9–10). In a porcine model, Tsui et al. (10) have demonstrated that the test can be used to reliably detect entry of an insulated Tuohy needle into the epidural space. By using supramaximal delivered currents and ensuring the subject has not been paralyzed, proximity to any motor neuron (nerve, nerve root, or spinal cord) can be reliably detected. This is done by sequentially reducing the current to a level where the motor response just disappears, while advancing the Tuohy needle using a continuous loss-of-resistance (LOR) technique. Entry into the epidural space will be signaled by a LOR and the simultaneous recurrence of the motor response (at an appropriate myotomal level) with a delivered current well above 1 mA. If, at any time, a motor response occurs at a current below 1 mA, proximity to a nerve structure is assured and further advancement of the needle is not advised as it may risk injury. The use of a nerve stimulator to perform peripheral nerve blocks in anesthetized patients has not been demonstrated to enhance safety (11). However, epidural stimulation, unlike peripheral nerve localization, uses a supramaximal current sufficient to stimulate any motor nerve structure within several centimeters. The principle goal when performing a peripheral nerve block is to seek the minimal current sufficient to stimulate a motor nerve (generally <0.5 mA) (11,12). Although there is only a single published laboratory study examining this application, the test has the potential to monitor a motor response in clinical settings where paresthesia cannot be reported. This technique has potential to make neuraxial procedures safer and should not be overlooked. Investigators in Europe have developed expertise in visualizing the epidural space with ultrasound (13,14). Real-time imaging with ultrasound may further enhance the safety of neuraxial procedures (15). While we agree with Dr. Rose (2) that further research is necessary to verify the “broadly held belief that epidural analgesia is associated with better postoperative analgesia,” we feel we also need to address more philosophical issues. For example, what is a reasonable risk? Furthermore, as Dr. Rose implies (2), how can we acquire a fully informed consent when we do not fully understand the nature of the risk? Research into the pathogenesis of spinal epidural hematoma formation, evolution, and incidence is desperately needed. Scott A. Lang Ban Tsui, MD, MSC, FRCP(C) Thomas Grau, MD

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