The low incidence of significant adverse neurologic outcomes prevents definitive conclusions regarding these complications after regional anesthesia. The difficulties involved in investigating these low frequency, zero tolerance, events can be appreciated by the fact that the relative safety of using nerve stimulation versus a paresthetic technique has never been adequately resolved, despite decades of debate. Although the relatively high incidence of transient postoperative neurologic symptoms after regional block can be used to assess the relative risk, 9 the validity of these symptoms as a surrogate marker of significant injury is speculative. Although substantive data concerning significant injury are lacking, a large retrospective study recently reported five seizures and three nerve injuries in 3,290 patients undergoing peripheral nerve blocks guided by nerve stimulation, but no such events in 2,146 patients undergoing similar blocks guided by both nerve stimulation and ultrasound. 10 There are obviously substantial limitations to such retrospective reviews. Nonetheless, these data, and the published and unpublished experience with ultrasound in this setting, fail to raise alarm and instead imply greater safety by the addition of ultrasound imaging. Whether this is indeed true is obviously of great interest. Dr. Cory's letter raises important questions regarding the potential impact of beam intensity on neurologic outcomes after regional anesthesia. Beam intensity is only one of the numerous differences between ultrasound guidance and other approaches to regional blockade that could impact safety, all of which mandate rigorous investigation.
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