Authors: Nicholas M. Bouli, MD
Yogi A. Patel
Neuromodulation has emerged as the dominant paradigm for the management of medically refractory pain, including the use of implanted pumps, chronic electrical stimulation systems, and more esoteric cellular and molecular approaches. Among these, spinal cord stimulation (SCS) has assumed a dominant position. The widespread adoption of SCS is largely due to the ease of implantation through percutaneous epidural approaches, which allows for implantation by pain practitioners in the rehabilitation and anesthesia fields. However, neurosurgical pain surgeons are beginning to recognize that chronic peripheral nerve stimulation (PNS) may provide a more effective and flexible approach to pain neuromodulation. This difference is most obvious with respect to craniofacial pain, which exists above spinal segments.
Despite recent history, PNS has existed for longer than spinal cord stimulation. Transcutaneous PNS was originally described by the ancient Roman physician Scribonius Largus with regard to the use of the Torpedo Fish in the therapy of headache. In 1859, Julius Althus described ulnar and sciatic stimulation for pain. Transcutaneous PNS continues in widespread use as TENS units. Modern PNS was introduced by Wall and Sweet, who implanted stimulators in their own infraorbital and ulnar nerves. Following on the tradition of these pioneers, contemporary neurosurgeons are pursuing innovation with off-label application of SCS devices for PNS, as well as in hypothesis-driven research in collaboration with engineers.
Contemporary clinical research in PNS largely constitutes reports of a series of patients implanted with SCS systems off-label through either percutaneous approaches or open surgery. Percutaneous implantation can be approached through “field stimulation,” in which electrodes are implanted directly below the dermis to stimulate cutaneous nerve fibers, or into close proximity to specific peripheral nerves. In field stimulation, multiple electrodes can be implanted to allow for current to run between electrodes covering large fields of cutaneous nerves. These fields can be sculpted to fit the regions of pain through the distribution of implanted electrodes.
Figure 1: Craniofacial PNS: Percutaneous supraorbital and infraorbital stimulator electrodes with a trigeminal root and ganglion stimulator placed with axiom stealth guidance.
Figure 2: Open PNS “sandwich technique” (courtesy of Milind Deogaonkar, OSU).
Figure 3: Advanced PNS electrode design.
Field stimulation has been the subject of recent trials by the major device companies. Percutaneous targeting of specific nerves can be achieved by implantation with known landmarks, as in the case of supra and infraorbital stimulation, or occipital nerve stimulation (Figure 1). Image guidance techniques can also be used to target specific nerves, as in the case of trigeminal root stimulation using frameless stereotaxy and fluoroscopy, or the use of ultrasound to guide implantation in soft tissue.
Open exposure of any peripheral nerve can be used to implant paddle-like leads. Implantation of paddle-like electrodes arrayed in a diagonal arrangement can allow for selectively activating fascicle bundles responsible for specific sensory distributions. Single electrodes can be implanted next to the nerve, or dual leads may be implanted in a “sandwich” configuration (Figure 2). Open PNS paddle systems provide extremely stable paresthesia fields, as migration is unusual and they are not plagued by the movement of the spinal cord within the canal. They also avoid off-target paresthesia as often encountered in SCS.
In collaboration with bioengineering at Georgia Tech, we have begun to explore nontraditional stimulation parameters as well as new electrode designs. Standard cuff electrodes are not optimal for obtaining high signal-to-noise ratio (SNR) recordings of neural activity or for stimulating specific subsets of axons in a nerve bundle. The Transneural Cuff1 (Figure 3) reduces the electrode-tissue interface impedance, provides high SNR nerve activity recordings, and lowers thresholds for electrical stimulation and block. Innovation can also be applied to the stimulation parameters used to achieve effective PNS. Kilohertz high-frequency electrical stimulation can be applied for treatment of chronic pain in ranges beyond those accessible with conventional internal pulse generators. These frequencies may prove more selective for pain transducing fibers and can potentially deliver analgesia without concomitant paresthesia.
Thus, innovation in PNS for more effective chronic pain management spans the space of neurovation, from off-label clinical series to hypothesis-driven experiments, and from extramurally funded research to entrepreneurial activity.
Development and studies related to evaluation of the
T-Cuff are ongoing and supported by Axion Biosystems.