Nerve injury discharge and spontaneous discharge arising from the injury site may be responsible for the development of persistent postoperative pain. Suter et al.  1suggested that this is an unlikely scenario. Using the rat spared nerve injury model they demonstrated that sciatic nerve block lasting at least 6 days does not prevent the development of allodynia or hyperalgesia after block resolution. We studied the effect of long-lasting nerve block in the model of hyperalgesia that can be viewed as a spared nerve injury model.2Our experimental results seem to agree with the main conclusion reached by Suter et al.  with an important exception.

Our methodology was close to that of Suter et al.  but had the following major differences. The first is related to the method of spared nerve injury. We transected the saphenous nerve and measured hyperalgesia in the sciatic nerve territory, whereas Suter et al.  sectioned the tibial and common peroneal nerves and studied the consequences of the injury in the sural and saphenous nerve territories. The second difference is related to the agent used to achieve nerve block and the duration of the block. We induced long-lasting nerve block with N-butyl tetracaine, an agent that combines local anesthetic and neurolytic properties and provides complete nerve block for more than 2 weeks.3Suter et al.  produced blockade with bupivacaine microspheres for at least 6 days.

The above letter was sent to the authors of the referenced report. The authors did not feel that a response was required.—Michael M. Todd, Editor-in-Chief

Our experiments demonstrated that long-lasting blockade of the saphenous nerve did not prevent late (1 to 3 weeks) hyperalgesia in the sciatic nerve territory caused by the saphenous nerve transection. In this regard, our results agree with those of Suter et al.  However, we found that in approximately 1 to 2 weeks the saphenous nerve blockade alone caused hyperalgesia in the sciatic skin territory. The effect of blockade on early hyperalgesia was obvious. Long-lasting block completely prevented it for the first 24 h and significantly reduced the degree of hyperalgesia for almost a week. In the Suter et al.  study, blockade of the sciatic nerve reduced the early changes in mechanical threshold in the saphenous skin territory; however, it was statistically significant only on day 7 after surgery. Thus, Suter et al.  had an indication of at least some preventive effect of the block on early hyperalgesia. In conclusion, we agree with the authors that peripheral long-term nerve blockade probably has no detectable effect on late hyperalgesia, but that does not include early hyperalgesia.

* Brigham and Women's Hospital, Boston, Massachusetts.

Suter MR, Papaloizos M, Berde CB, Woolf CJ, Gilliard N, Spahn DR, Decosterd I: Development of neuropathic pain in the rat spared nerve injury model is not prevented by a peripheral nerve block. Anesthesiology 2003; 99:1402–8
Kissin I, Lee SS, Bradley EL Jr: Hyperalgesia caused by nerve transection: Long-lasting block prevents early hyperalgesia in the receptive field of the surviving nerve. Anesth Analg 1999; 89:1475–81
Wang GK, Vladimirov M, Quan C, Mok WM, Thalhammer JG, Anthony DC: N-butyl tetracaine as a neurolytic agent for ultralong sciatic nerve block. Anesthesiology 1996; 85:1386–94