The hyperthermic effect of a distal volar forearm nerve block: a possible treatment of acute digital frostbite injuries?

Background: The authors have observed that carpal tunnel surgery nerve blocks consisting of subfascial distal volar forearm injection of 10 cc of 1% lidocaine with epinephrine result in fingers that appear hyperemic, warm, and numb in both median and ulnar nerve distributions. The purposes of this study were to (1) determine whether forearm nerve blocks in patients undergoing carpal tunnel releases result in an objective increase in finger temperature, and (2) document the location and duration of finger anesthesia.

Methods: Thirty-nine patients undergoing unilateral carpal tunnel release were studied prospectively. An infrared thermometer was used to measure the temperature in the fingers of operative and nonoperative hands before and after injection of local anesthetic. The distal volar forearm block was performed using 10 cc of 1% lidocaine with 1:100,000 epinephrine deep to the forearm fascia between the median and ulnar nerves 1 cm proximal to the wrist crease. Before and after carpal tunnel release, bilateral finger temperatures were measured at hourly intervals. Statistical analysis included a one-sample test of proportions.

Results: The finger temperature of the operative hand was significantly warmer than the unoperated hand over the first 2 hours after the nerve block. Seventy-four percent of patients had a statistically significant increase in temperature. On average, the nerve block lasted 6.27 hours in the median nerve distribution and 5.78 hours in the ulnar nerve distribution.

Conclusions: Forearm nerve blocks produce a chemical sympathectomy that provides a significant increase in skin temperature as a result of vasodilatation in most patients. They also provide prolonged finger numbness. This could be of clinical benefit in patients with acute finger frostbite injuries.