Use of Pulsed Radiofrequency in Clinical Practice
Radiofrequency current was introduced as a modality to make therapeutic lesions at various target nerves throughout the body. Its mode of action has been presumed to be entirely due to its effect of producing thermal lesions in neural tissue. However, this mode of action has recently been challenged.1-22 This article summarizes the current state of development of pulsed RF lesioning in clinical practice. It describes in detail a technique for performance of a pulsed radiofrequency lesion of the dorsal root ganglion for treatment of radicular pain. Finally, it summarizes the current literature regarding efficacy.
The widespread use of RF current for treatment of spinal pain began in 1980 when Sluijter and Metha introduced a 22g cannula through which a thermocouple probe could be inserted.21 This allowed the procedure to be performed percutaneously with minimal discomfort. The mode of action of radiofrequency current was initially attributed to the thermocoagulation of nerve fibers. This was brought into question when Sluijter performed the first pulsed RF lesion in February of 1996. He suggested that the electromagnetic field rather than temperature was responsible for the analgesic effect.1 It was subsequently determined that the electric field in particular was more likely to be responsible for the effect.2 Since thermal destruction of nervous tissue takes place at 45°C, Sluijter was careful to choose lesioning parameters that would not produce a temperature greater than 42°C (38° is more typical). This meant that the dorsal root ganglion might be treated without risk of deafferentation pain.1,6 Traditional thermal lesioning of the dorsal root ganglion has also been associated with neuroma formation, allodynia and dysesthesias.3 Given the potential for nerve damage with heating of the dorsal root ganglion, pulsed RF (PRF) introduced a method of treatment for radicular pain that had potential for therapeutic efficacy without the attendant risks.
Pulsed RF is delivered in short (20msec) bursts twice per second followed by a quiet phase (lasting 480msec) in which no current is applied. This allows for heat dissipation thus keeping the tissue temperature below the neurodestructive threshold of 45°C. Pulsing the current also allows the power output of the generator to be substantially increased. The usual output of voltage in the continuous mode is 15-25 volts while a pulsed radiofrequency lesion is usually performed at 45 volts.21
The role of pulsed RF in clinical practice has been an issue of debate. The argument against its use has been that it is unnecessary due to the availability of continuous RF which already has scientifically validated studies showing its efficacy. This argument, while true for treatment of medial branches, does not hold up well when considering use of RF current for the treatment of radiculopathies and painful peripheral neuropathies. For both of these chronic and painful conditions, there is currently little to offer these patients. When one considers the benign nature of this treatment and its possibility of real relief, there is little reason not to offer it as a therapeutic option.
Pulsed RF can be used without concern of producing sensory or motor loss.1,3,6,7,21 Since there is no satisfactory surgical solution for patients with chronic radicular pain or peripheral neuropathy, this mode of treatment offers a potentially valuable alternative to maintaining patients on chronic opiate therapy. In addition, it may offer an advantage over epidural steroids when one considers the reports of devastating outcomes associated with their use.23-26
Pulsed RF was originally thought to be a totally non-destructive procedure. However, experimental work shows this is not the case.2 Convention holds that the word lesion should not be used when referring to a pulsed RF procedure. However, Cosman and Cosman measured heat bursts in the neurodestructive range occurring in a thin layer of tissue immediately surrounding the electrode.2 This evidence would dispute the currently held belief that pulsed RF does not cause a lesion. The dictionary defines the word lesion as “a localized pathological change in a bodily organ or tissue,” and Sluijter feels that a pulsed RF procedure clearly meets this criteria.27 However, neither he nor others who have looked at this issue feel that this small layer of cellular destruction fully explains the clinical effect produced by a pulsed RF procedure.2,18,27
The use of the pulsed mode in clinical practice has been slow in gaining widespread acceptance of this modality. This may be due to the paucity of evidence showing clear therapeutic advantage over placebo during the early years of its use. However, over the past five years, there have been several studies that demonstrate clear advantage over placebo. The elimination of the CPT code for the use of pulsed RF in 2005 has created yet another obstacle. However, there are still ethical and legal means of billing for the procedure and will be discussed later in the article.
Mechanism of Action
Initially, the effect of radiofrequency current was thought to be due solely to thermocoagulation of the fibers transmitting nociceptive stimuli to the spinal cord.6,18,22 Conventional heat radiofrequency causes non-selective thermal damage to the offending nerve by creating an electrical field between the small, uninsulated electrode needle tip (connected to a voltage generator) and a large inactive dispersion electrode. The current flow induces movement of ions within the tissue that alternate at the same frequency as the radiofrequency current.6,7,18,28 The result of this current flow is friction which produces heat in the tissues surrounding the electrode tip. The resultant lesion is spheroid in shape with the long axis parallel to the needle tip. The size of the lesion is fairly predictable and is based on tip temperature, lesion duration, needle diameter, length of the active tip, tissue vascularity, and heat conductivity of the surrounding tissue.6,7,28 This type of lesion is placed between the nociceptive focus and the spinal cord.13,16
The concept that tissue destruction was the means by which RF current produced its effect was re-evaluated in light of certain findings that were inconsistent with this theory.1-22 For example, it was known that heating of the dorsal root ganglion for the treatment of acute radicular pain due to a disc protrusion produced only a short term stunning effect on sensory fibers, while the pain relief seemed to last much longer. In addition, the application of heat to the dorsal root ganglion was distal (peripheral) to the nociceptive input. Thus, production of pain relief was not dependent on production of a lesion between the source of nociception and the spinal cord. These observations provided supporting evidence that led to the development of the pulsed radiofrequency procedure.