An anesthetic that stops only pain

The trouble with nerve block anesthesia is that it blocks pain and motor function. But researchers this week reported progress on an anesthetic that only blunts pain.

Scientists from Children's Hospital Boston were studying substances called surfactants, which are used to help drugs spread more easily through tissue. While testing various surfactants along with the anesthetics QX-314 and QX-222 (types of lidocaine) they found the mixtures halted pain in the sciatic nerves of rats for up to seven hours but didn't impair the animals' movements. In one experiment, the rats were able to tolerate having their paws on a hot plate for long periods of time yet could still stand and maintain balance. The study was published online this week in the Proceedings of the National Academy of Sciences (www.pnas.org/content/106/17/7125.full.pdf+html?sidb3dc4239-9d81-4b65-86cc-4c757cbc1459).

The researchers don't know how the surfactants cause this unique response. It could be that surfactants are able to penetrate sensory nerves but not motor nerves. Surfactants can be toxic in high doses. So further studies will be needed in larger animals, and the research team will look for different types of substances that enhance the movement of drugs into tissues.

As for the question of why anesthetics that preserve motor function are even needed, the best example is childbirth. Such a nerve block would allow a woman to effectively use her muscles to push without feeling the pain.