Cutaneous Field Stimulation (CFS)
Mechanism of Action
The human nervous system features a variety of nerve fibers, each carrying a different type of sensation to the spinal cord, which, in turn, forwards messages to the brain. Of particular interest are A beta (Aβ) fibers, A delta (Aδ) fibers, and C fibers. Aβ fibers transmit non-painful mechanical sensations, such as those generated from rubbing and touching. Aδ fibers transmit sharp, intense, immediate pain that signals that injury is taking place and that some action is urgently needed to evade the source of injury. C fibers transmit the dull, aching, long-term pain that is the hallmark of chronic pain syndromes. Both Aβ and Aδ fibers are myelinated, so they transmit signals to the spinal cord very quickly. C fibers are unmyelinated, so their signals travel slowly compared to the Aβ and Aδ fibers.
TENS and gate theory
Conventional TENS works according to gate theory, which posits that activation of the non-nociceptive (non-painful) Aβ fibers interferes with the pain signals coming from C fibers. This interference takes place in the spinal cord. Human beings use the gate-theory mechanism intuitively: for example, when people accidentally hit their elbows (activating the fast pain-transmitting Aδ fibers), they intuitively rub the site of the injury, activating the faster Aβ fibers to reduce the perception of pain.
The importance of signal speed
A lesson learned from gate theory is that signals travelling along fast-conducting nerves will reach the spinal cord before signals traveling along slow-conducting nerves, allowing them to block the transmission of the slower signals to brain. This blocking mechanism reduces the perception of pain. The mechanism of TENS is to activate Aβ fibers to block signals coming from relatively slow C fibers.
Limitations of TENS
TENS works best for general pain—pain from healthy nerve fibers reporting injuries in the body to the spinal cord and brain. For this reason, TENS is best for short-term pain. The effects of TENS generally last for the duration of stimulation therapy and for 15 or 20 minutes after therapy is stopped. For chronic pain, it isn’t practical to deliver TENS therapy continuously to prevent the recurrence of pain.
The importance of spatial distribution
Even if Aβ fibers are transmitting signals to the spinal column faster than the C fibers carrying chronic pain signals, they might not be reaching the right parts of the spinal cord to block the C fibers. Nerve fibers enter the spinal cord through nerve roots and then synapse (join) branches of nerves above and below the nerve roots. Aβ fibers and C fibers branch up and down the spinal column for approximately the same distance (Figure 1). A portion of the C fibers may have branches that extend above the Aβ fibers. C fibers connecting to the spinal cord above the Aβ fibers activated by TENS will not be blocked by the Aβ fibers, so the spinal cord will transmit their pain signals to the brain.
How CFS is Different
Cutaneous field stimulation (CFS) is a technique for stimulating Aδ fibers, based on the ground-breaking research of Jens Schouenborg, PhD, of Lund University in Sweden.
CFS activates Aδ fibers instead of Aβ fibers. Aδ fibers aren’t as fast as Aβ fibers, but, as myelinated fibers, they are still much faster than the C fibers that transmit chronic pain. Aδ fibers are more effective than Aβ fibers at blocking signals from C fibers, for a number of reasons. First, Aβ fibers and C fibers both connect to the spinal cord at approximately the same depth of the cord (laminae II and III); however, Aδ fibers penetrate the spinal cord across several layers (laminae I through IV), as shown in Figure 2. Furthermore, Aδ fibers cover a wider area of the spinal cord. Whereas Aβ fibers branch up and down the spinal cord approximately the same distance from the nerve root as the problematic C fibers, Aδ fibers branch up and down the spinal cord much further than C fibers. With CFS, it is less likely that branches of C fibers will extend above the activated Aδ fibers, so more of the chronic pain signals are blocked (Figure 3).
In effect, there is a hierarchy in the processing of signals in the spinal cord. The spinal cord prioritizes signals from Aδ fibers over signals from C fibers. Since Aδ signals transmit urgent messages about injuries to the body, they signal the brain to act quickly to take evasive action. As long as there are signals coming from Aδ fibers, the spinal cord will suppress signals from C fibers.
CFS’s 14 electrodes, firing one a time, generate a field of low-level Aδ nerve activation over the area where the patient feels chronic pain (Figure 4). The barrage of Aδ signals travels to the spinal cord, where the wide distribution of Aδ signals covers the narrower distribution of C-fiber signals, and the spinal cord prioritizes the relatively comfortable Aδ signals over those of the painful C-fiber signals.
Furthermore, the asynchronous stimulation of the Aδ fibers by CFS has a long-term depressive (LTD) effect on the perception of local C-fiber activity. This effect occurs in the spinal cord and provides pain relief for 3 –6 hours after therapy has stopped.