On the heels of our article earlier this month about peripheral nerve stimulation (PNS), we had the opportunity to talk with one of the world’s leading authorities on the subject: Giancarlo Barolat, M.D. A neurosurgeon based at Denver’s Presbyterian/St. Luke’s Hospital, Dr. Barolat has been using PNS and other forms of neuromodulation to treat chronic pain for about 40 years.
He’s now a consultant on the next-generation PNS technology being developed by Ripple Neuro, which is specifically designed to control residual limb pain. Dr. Barolat offered a handful of insights about how the new PNS systems work, how they improve upon current forms of neuromodulation, and what the next steps are on the road to better treatment for phantom pain.
1. PNS has the potential to offer greater precision than other forms of neuromodulation.
“Neuromodulation refers to any treatment that regulates the nervous system through implanted devices,” says Barolat. “The devices can be implanted in the brain, the spinal cord, or the peripheral nerves”—generally the sciatic nerve in lower-limb amputees, and the radial, median, and ulnar nerves in upper-limb amputees.
Currently, most neuromodulation treatments for phantom pain involve spinal cord implants rather than peripheral nerve devices. PNS devices lie closer to the perceived source of phantom pain (the residual limb, rather than the spine) than other neuromodulation devices, so they promise greater precision and more comprehensive relief. It’s like the difference between dumping a bucket of water onto a campfire, versus dropping fire retardant from an air tanker once the forest is ablaze.
“I can use neuromodulation to treat phantom limb pain today,” Barolat says. “I treat a lot of patients who have amputation pain. Hopefully this new type of device will give me better results than what I can obtain today.”
2. Next-gen PNS technologies will wrap directly around the peripheral nerves.
Current PNS systems operate through small, threaded electrodes that don’t make direct contact with the peripheral nerves. The emerging technology will operate through cuffs that wrap around the peripheral nerve and remain in place permanently, a la pacemakers.
When this type of implant was initially tried decades ago, says Barolot, the nerves swelled and were constrained by the cuffs, resulting in nerve damage. “That’s why that modality was abandoned,” he explains. “But now they can make cuffs that will expand with the nerve, so there is renewed interest in it.” There are also multiple cuff sizes that can be fitted to the nerve(s) during implantation, adding another layer of precision.
3. New PNS systems will target subsets of peripheral nerves.
Today’s PNS devices operate on an all-or-nothing basis, Barolat says. “We can simulate the whole nerve or do nothing,” he says. “The current technology is not selective for the smaller fibers that make up the nerve.” Specifically, the existing systems aren’t capable of differentiating between sensory fibers (which convey information related to touch, temperature, pressure, and so forth), motor fibers (which interact with muscles), and nociceptors (which convey pain signals). New PNS systems will allow stimulation to be delivered to subsets of fibers within the nerves, allowing for infinitely greater precision than current technology can achieve.
“It’s very sophisticated,” Barolat says, “because it can be made to sense the electrical signals coming from the nervous system. So it can recognize the pain signal and set the stimulation patterns accordingly. That’s critical, because it’s more advantageous if the system can self-regulate.”
4. Emerging PNS devices will provide two-way communication.
Since they’re capable of distinguishing between different types of nerve fibers, next-gen PNS devices can gather input from sensory and motor fibers to improve prosthetic control and function. “The beauty of these devices is they can be integrated to control many kinds of signals,” says Barolat. “They not only stimulate, they allow you to sense. They might allow you to move the muscles.” While their primary purpose remains phantom pain control, Barolat is excited to see what broader neurological gains might be possible with the new PNS devices.
5. The future hasn’t quite arrived—but it’s closing in.
Before new PNS therapies can gain wide application, here’s what has to happen. First, Barolat will conduct cadaver studies to perfect the surgical technique for implanting the devices. Then comes an initial clinical trial involving fewer than a dozen patients, followed by a series of larger trials involving broader and more diverse populations. A final study for FDA approval is probably at least two years away.
Even then, says Barolat, it will take time to gain acceptance within the medical community. “Neuromodulation has been around for decades,” he notes, “but a lot of doctors aren’t really aware of it, and many insurance companies still tend to think of it as an experimental treatment.” He routinely battles insurance companies for coverage of neuromodulation treatments that already have FDA approval, so it will be that much harder to win coverage for a brand-new approach.
“There is a lot of intertia,” he says. “And it’s true that neuromodulation might not work unless the treatment is overseen by people who really know what they’re doing. There aren’t many people who have that knowledge.”