RADIOFREQUENCY LESIONING IN CHRONIC PAIN
Radiofrequency Lesioning in Chronic Pain
Neuro-ablative techniques in clinical practice are directed to block the nerve impulse carrying pain sensation to/ or inside the central nervous system to relief pain. This can be done through:
1. Surgical destruction.
2. Chemical destruction.
3. Cryocoagulation.
4. Lesioning with electric current, induction heating.
5. Focused ultrasound.
Physics Of Radiofrequency
The circuit consists of an active electrode, which delivers the current; a method for measuring tissue temperature (thermistor or thermocouple); a radiofrequency generator; and a passive electrode with a large surface area. Current in the region of the active electrode generates heat. The heat generated is a function of the amount of current per unit area (current density) that flows in the region of the electrode. The active electrode itself does not generate heat but is heated as a result of local tissue warming. The current flows from the active to the passive electrode. Because of the much greater surface area of the passive electrode, the current density is much less. Therefore, heating and tissue damage do not usually occur at the passive electrode.
Heating of the active electrode is an important safety feature of this system, because tissue damage is related to the temperature generated. The newer electrodes have a low thermal coefficient, meaning that the electrode absorbs heat well and heats rapidly, leading to a faster response and improved safety of the system. Excessive heating causes more diffuse and permanent tissue damage. It is possible to boil tissues, and these tissues may then adhere to the electrode and be avulsed when the electrode is removed. The thermocouple lends itself better to miniaturization than the thermistor and is therefore more widely used.
Most electrodes are available in a number of sizes and lengths. Both reusable and disposable needles are used. Most have varying lengths of the exposed tip, and the electrode must be selected for the desired purpose. For example, an 18-gauge reusable electrode with a 2-mm exposed tip is suitable for radiofrequency denervation of the trigeminal nerve, whereas a 22-gauge electrode with a 4-mm exposed tip is appropriate for lumbar facet denervation. In some neurosurgical procedures, curved needles are used to generate eccentric lesions.
Lesion Characteristics
It is critical to control lesion size. The size and consistency of the lesion are governed by four major factors:
1. Temperature generated: At higher temperatures, the local tissue reaction is greater.
2. Rate of thermal equilibrium: If there is more rapid equilibrium between tissues, the lesion is more uniform. Conversely, if there is slow and incomplete equilibrium, the lesion is erratic. Usually thermal equilibrium is complete by 60 seconds. The lesion size initially rises exponentially with time but becomes independent of time after approximately 30 seconds (=90 seconds) .
3. Electrode size and configuration: Larger electrodes generate larger lesions. For example, an 18-gauge electrode generates a lesion with a radius of 2.2 mm, whereas a 22-gauge electrode generates a radius of only 1.9 mm. Larger electrodes generate bigger lesions but at the expense of more tissue trauma on insertion, unwanted neural destruction, and larger reversible zones.
4. Local tissue characteristics: Lesions in tissues in contact with tissues of low electrical resistance such as blood and cerebrospinal fluid may be reduced or irregular in size and shape. Blood may also act as a heat sink, removing heat from the area and thereby limiting local tissue temperature rise and lesion size.
The size of the lesion does not correlate well with either the time or the power used, because the temperature generated depends on tissue characteristics. The lesion generated is usually an inverted cone. In vitro evidence suggests that the lesion radius is maximal at the part of the exposed electrode farthest from the tip. The actual tip may not even be incorporated in the lesion, and this has important clinical implications. Nerves in contact with the tip may be only partially blocked, and an electrode placed tangential to the nerve generates a more effective lesion. The effect on tissues depends on the temperature generated. Above 45°C, irreversible tissue injury occurs. Between 42° and 45°C, temporary neural blockade occurs. In general, the larger the lesion, the larger is the zone of reversibility.
The histologic appearance of lesions generated by radiofrequency is one of local tissue burn. Nerve architecture is destroyed. After the lesion is created, wallerian degeneration becomes apparent. The perineurium may also be destroyed. In radiofrequency lesioning, unlike cryoneurolysis, neuroma formation is possible. Clinically, it appears that there is relative selectivity for small, unmyelinated fibers at lower temperatures. Therefore, by limiting the temperature, it may be possible to damage pain fibers selectively.
Technique
Minimal sedation is used so that the patient can participate fully and accurately report the stimuli. All of the equipment, particularly the cables and thermocouple or thermistor, is checked before beginning the procedure. The lesion parameters, especially the maximal temperature, are preset. The radiofrequency probe must be of the appropriate size and length for the needle. In particular, the correct exposed tip length, needle diameter, and length are critical in improving the efficacy of the procedure and reducing the risk of inadvertent tissue injury. Fluoroscopy is mandatory.
The most common reason for failure to generate a lesion is a poor electrical connection, usually related to cable damage. Occasionally, the insulation on the active electrode is disrupted, with a subsequent reduction of current density and poor lesion generation. This may also lead to lesions further up along the shaft in other tissues traversed by the needle. Poor connection may also occur at the passive electrode, leading to poor conduction or even local tissue burning. The newer machines all measure impedance and can help isolate the source of the problem. Very high impedance (>2000 Ω) suggests electrical disconnection, whereas very low impedance ( ................
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