Study shows postoperative nerve blocks do not cause hyper-sensitization

Nerve blocks – local anesthetic injections that block pain signals – are a valuable opioid-sparing technique for controlling pain after surgery. However, some patients experience temporary sharp increases in pain as the nerve block wears off. This phenomenon, often termed "rebound pain," does not result from nerve hypersensitivity to painful testing (e.g. heat, pressure, punctate pain), reports a healthy volunteer study in the August issue of Anesthesiology, the peer-reviewed medical journal of the American Society of Anesthesiologists (ASA). 

More likely, the perceived increase in pain is caused by the "unmasking" of surgical pain as the nerve block wears off, suggests author Yun-Yun K. Chen, M.D., of Brigham and Women's Hospital, Boston. "Concerns about rebound pain should not deter clinicians from using nerve blocks for postoperative pain control. Our findings show no evidence of heightened sensitization - a key driver of acute and chronic pain - suggesting that rebound pain is more likely the natural return of surgical discomfort as the block wears off, rather than an alteration in the nerve after local anesthetic blockade." 

Nerve blocks are a form of regional anesthesia with many benefits in controlling postoperative pain, including reducing the need for opioids. However, up to half of patients experience severe worsening pain after the nerve block wears off, sometimes resulting in emergency department visits or readmission. It has been unclear whether this reflects the return of untreated surgical pain or whether it may result from a hypersensitivity effect caused by the nerve block. 

To gain insight into the mechanism of rebound pain, Dr. Chen and colleagues performed an experimental study in 40 healthy volunteers. In one randomly selected arm, all participants received a single-injection axillary brachial plexus block: a type of nerve block routinely used for surgery on the hand and arm. The other arm received no nerve block. 

For several hours during and after the nerve block began to wear off, participants underwent tests to evaluate responses to various painful stimuli using quantitative sensory testing of pain: pinprick, pressure, and heat. The researchers assessed the development of hypersensitivity by comparing pain responses between the block and control arms. 

At no point during the process of block resolution (from time of motor function return out to 3 hours after) was there evidence of rebound pain in the arms undergoing nerve blocks. The heat stimulus needed to produce even mild pain – one point on a 0-10-point scale – showed little or no difference between arms. Responses to the other pain stimuli were similar: no increase in sensitivity from baseline was detected in the previously blocked arm. 

Overall, the results showed "no evidence of significant sensitization" in any of the pain tests – including measures of central sensitization (pain processing by the central nervous system). This is an important consideration because central sensitization is thought to be a key step in the development of chronic pain. "These findings suggest that the phenomenon of rebound pain is more likely driven by the surgical pain that emerges once the block is resolving," the researchers wrote. 

The researchers note some limitations of their study – especially that the participants did not undergo surgery and thus did not experience the expected pain and healing responses after a surgical incision. 

Our study provides reassurance that nerve blocks are safe and effective for pain control after surgery."

Dr. Yun-Yun K. Chen, M.D., Brigham and Women's Hospital, Boston

The researchers discuss strategies to prevent or lessen the impact of rebound pain, highlighting the role of multimodal anesthesia targeting different pain pathways, patient education around "reasonable expectations" for postoperative pain control, and emphasis on anticipating nerve block resolution and appropriately timing pain medication as it approaches. 

In an accompanying editorial, Benedict Alter, M.D., Ph.D., of the University of Pittsburgh, drew attention to the critical distinction between nociception – nerve transmission of pain impulses – and the complex and individual experience of pain. Dr. Alter highlights the "critical and ongoing" efforts to reduce rebound pain after surgery. He concluded: "A better understanding of uncontrolled pain after resolution of nerve blockade will improve postoperative pain management and overall surgical outcomes."