The use of electric current for pain management has a long history. As early as the second half of the 19th century, brain lesions were made in animals with the application of direct current, and empirical rules for quantifying lesion size based on current and time were developed. One of the first uses in humans was for the management of trigeminal neuralgia in 1931, when a direct current was delivered through a needle with a 10-mm uninsulated tip placed in the gasserian ganglion. This technique produced lesions with unpredictable size. The use of high-frequency electric current was later found to produce lesions with predictable size. Because frequencies of 300 to 500 kHz were also used in radio transmitters, the current was called radiofrequency (RF) current. Later, temperature monitoring was suggested to be the most important parameter to obtain a standardized lesion size.
In pain management, RF was first used for percutaneous lateral cordotomy to relieve unilateral pain in cancer patients. A few years later, RF treatment of trigeminal neuralgia was described. The first use of RF current for spinal pain was reported by Shealy, who performed RF lesioning of the medial branch for lumbar zygapophyseal joint pain. Another application for spinal pain was introduced by Uematsu, who described RF lesioning of the dorsal root ganglion (DRG).
At the end of the 1970s, percutaneous cordotomy and RF treatment of the gasserian ganglion were the only widely accepted RF procedures. A turning point came in 1980, when small-diameter electrodes, known as the Sluijter Mehta Kit (SMK) system, were introduced for the treatment of spinal pain. The system consists of a 22-gauge disposable cannula with a fine thermocouple probe inside for measurement of temperature. The smaller electrode size resulted in diminished discomfort during the procedure. Because there was less risk for mechanical injury to major nerve trunks, targets in the anterior spinal compartment were no longer off limits, and procedures such as RF lesions adjacent to the DRG (RF-DRG) and lesions of the communicating ramus and sympathetic chain became part of the treatment armamentarium.
Over the years the concept that the clinical effect of RF current was caused by the formation of heat had not been challenged. A selective effect of heat on thin nerve fibers was thought to interfere with the conduction of nociceptive stimuli.
There were several reasons why the role of heat was finally questioned. First, the classic concept presupposes a strict configuration: the RF lesion must be made in between the nociceptive focus and the central nervous system (CNS). Yet RF lesions were also successful when not performed between the nociceptive focus and the CNS. For example, in the treatment of acute radicular pain from a herniated disk, the electrode is placed distal to the nociceptive focus. Second, RF-DRG induces only transient sensory loss, which is possibly heat related, whereas the pain relief may be of much longer duration. Finally, the role of heat was also questioned by the publication that no differences in outcome were noted when two different tip temperatures (i.e., 40° C and 67° C) were applied. It is against this background that pulsed radiofrequency (PRF) treatment was developed. PRF delivers strong, fluctuating electric fields while the temperature effects are kept to a minimum. PRF was conceived as a novel, potentially safer mode of administration of RF energy. It can be specifically useful for treatments in which RF lesioning is not indicated, such as peripheral neuropathies, arthrogenic pain, painful trigger points, and application to the DRG in patients with neuropathy or radicular pain.
Radiofrequency Generator System
A modern RF lesion generator has the following functions:
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Continuous online measurement of impedance
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Nerve stimulation
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RF delivery mode
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Pulsed current delivery mode
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Monitoring of voltage, current, and wattage during the RF procedure
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Temperature monitoring
Electrical impedance is measured to confirm continuity of the electrical circuit. After placement of the needle under fluoroscopic control, nerve stimulation is performed to confirm proper position of the electrode. Stimulation is carried out at 50 Hz to ensure proximity of the electrode to sensory fibers; 2-Hz stimulation is performed to detect muscle contractions, which indicate that the position of the needle is too close to motor fibers. If an electrode is actually resting on a nerve, the minimum stimulation level required to produce a discharge is 0.25 V. At a distance of 1 cm from the nerve, 2 V would be required; thus the stimulation threshold is an indicator of the distance of the electrode from the nerve. Temperature is measured with a thermocouple electrode. A thermocouple electrode consists of a junction of two dissimilar metal elements and produces a voltage that is proportional to temperature.