Hadn’t heard about this before:
[...] she felt a sudden pop in her hamstring. “It felt like a guitar string had been plucked and it had broken,” said Ms. Toussaint, who is now 45.
An intense burning sensation followed; it felt as if her leg had been doused in gasoline and set on fire, she said. The next day, the college athletics trainer determined that she had pulled her hamstring. But even years later, the pain would not subside. It migrated to her other leg, leaving her bedridden for nearly a decade, and overtook her vocal cords, leaving her temporarily mute.
All the while, doctors puzzled over and even doubted her mysterious condition.
Ms. Toussaint now knows that she is among an estimated one million Americans living with complex regional pain syndrome, a nerve disorder formerly known as reflex sympathetic dystrophy syndrome. For patients with the disorder, a trauma as mild as a fractured wrist or a twisted ankle can cause the nerves to misfire, so much so that intense pain messages are constantly sent to the brain.
Interestingly, neural stimulation only provides a short-term benefit with eventual adaptation. In some cases, ketamine administration (enough to put the patients in a temporary coma) has completely stopped the pain. Ketamine is an anesthetic (although it has been known to actually stimulate circulation at certain doses) with well-known psychedelic properties. It is also a non-competitive NMDA antagonist that is often used in conjunction with traditional opiods for an analgesic effect.
I wonder if this effect is simply due to the interaction with the NMDA receptor or is something more complex. (For example, the analgesic effects of ketamine when combined with a opiods seem unrelated.)
Here’s a link to the original paper in the journal Pain, which suggests that CRPS patients have suffered damage to small-diameter PNS nociceptive fibers.