Since the 1950s, electrical currents have been used to create predictable thermal lesions; however, the use of radiofrequency for intractable pain did not appear in the literature until the 1970s. The basic premise involves the passage of radiofrequency currents through an electrode that is placed nearby a nociceptive pathway to interrupt the pain impulses. The thermal energy creates a predictable area of tissue destruction that is targeted to contain the nerves responsible for transmitting and/or modulating pain sensation. Today, there are variations of thermal ablative procedures, stemming from the discussed basic premise. These include pulsed radiofrequency ablation (PRF), water-cooled radiofrequency ablation (WCRF), and cryoneurolysis (CN).
Thermal energy is typically applied near or on the peripheral nerve origins along the levels of the spinal cord and is most often performed for chronic back and neck pain syndromes. It has been used for facet joint pain treatment, targeting the medial branch of the primary dorsal ramus. Additionally, it has been applied for discogenic back pain, targeting the ramus communicans. Further targets include the dorsal root ganglia for radicular back pain and lateral branch neve for sacroiliac disease. Less frequently, the literature demonstrates success in facial pain syndromes, thoracic pain, and anterior/posterior pelvic pain syndromes as well.
In a randomized study of dorsal root lesioning by Slappendel et al., lesions made at 40 degrees Celsius and 67 degrees Celsius showed no clinical significance. The authors surmised that perhaps the electrical currents, as opposed to the temperature, led to the clinical benefits. This prompted the development of pulsed radiofrequency (PRF). They theorized that neurodestructive temperatures could be avoided using higher voltage radiofrequency currents in a pulsatile manner. This allowed time for the heat to dissipate, minimizing the risk of thermal tissue injury. Lab studies showed evidence of neural stress and cellular substructure damage after PRF application. However, later studies established that a slow response time of temperature-measuring devices could not dependably exclude the possibility of brief temperature spikes, deflating the role of electrical currents causing tissue injury. Currently, no clear evidence of pain pathway disruption in response to solely high-frequency electrical current exists. It is held that there is a combined role of electrical and thermal destruction that leads to the clinical benefits observed. Due to the perceived safety and clinical efficacy of PRF, it has grown in popularity and utility over conventional radiofrequency ablation. PRF has been used to treat the dorsal root ganglion at all spinal levels in multiple pain syndromes including radicular pain, discogenic pain, facetogenic pain, post-herpetic neuralgia, post-amputation pain, and post inguinal herniorrhaphy pain. It also can be applied to various locations along with more peripheral nerves including medial branch nerve, suprascapular nerve, intercostal nerve, and pudendal nerve. This allows for treatment of syndromes that span from shoulder pain to meralgia paresthetica. Other non-intuitive uses include the splanchnic nerves for pancreatic pain and dorsal penile nerves for premature ejaculation. Its utility also reaches to the central nervous system and autonomic ganglia for trigeminal neuralgia, the sphenopalatine ganglion for head and neck pain, and the lumbar sympathetic chain for complex regional pain syndrome.
Another variation of radiofrequency ablation is water-cooled radiofrequency (WCRF) ablation. This technique was adopted from previous methods in cardiac electrophysiology and tumor ablation. While the premise remains largely the same, in WCRF a multichannel electrode is cooled by the continuous flow of water. This active cooling measure prevents the electrode itself from attaining the high tissue temperatures. Consequently, this allows continuous flow of the RF current to create a larger thermal lesion. Additionally, the WCRF lesion creates a characteristic lesion with a relatively cooler immediate spherical area, called an isotherm, around the probe. Following, there is a hotter isotherm surrounding that, with sequentially lower temperature isotherms following as distance from the probe is increased. Like conventional radiofrequency ablation, lesion size is also dependent on probe size, electrode temperature, and duration of the current. Other factors that can contribute to the size and shape of the lesion include active and passive heat sinks. Active heat sinks include blood flow in the epidural venous plexus and cerebrospinal fluid flow in the thecal sac. Passive heat sinks include muscular and boney structures. Currently, the use of WCRF is limited to clinical presentations in which the pain generator is considered to have numerous and variable sources of innervation. There are two distinct forms of WCRF techniques. Monopolar is used for sacroiliac joint dysfunction, and the bipolar technique is used for discogenic pain. Due to its ability to distribute thermal energy to larger areas, WCRF may be effective where more conventional forms of neuroablation have failed.
In the realm of thermal neurolytic therapy, there is an alternative technique known as cryoneurolysis. Benefits of this modality are that it is not associated with neuroma formation or hyperalgesia, which can be aspects of surgical sectioning, radiofrequency ablation, or chemical neurolysis. The mechanism behind cryoneurolysis appears to stem from the damage to the vasa nervorum and subsequent endoneurial edema, pressure, and consequent axonal destruction. Nerves regenerate at a rate of about 1 to 1.5 millimeters per week from spared connective tissue elements and Schwann cell basal laminal. The duration of analgesia depends on the time taken by the proximal axons to reinnervate their targeted tissues and typically ranges from weeks to months. The described use of cryoneurolysis in the literature is most prevalent for the treatment of post-thoracotomy pain. This experience led to its utilization in other chronic pain syndromes to include trigeminal neuralgia, atypical facial pain, spinal and extremity pains, abdominal pain syndromes, and atypical perineal pain. The wide breadth of utility of cryoablation seems to be limited only by the experience of the proceduralist.
There are relatively few contraindications for radiofrequency ablation. Absolute contraindications include patient refusal, increased intracranial pressure, and local infection. Because of the proximity to the spinal column for many of the procedures, sound clinical judgment and standard of care must be taken when confronted with anticoagulation medications and bleeding diathesis. Typically, ASRA (American Society of Regional Anesthesia and Pain Medicine) guidelines are followed. A brief review of when to stop common anticoagulation therapies includes aspirin for primary prophylaxis (6 days), clopidogrel (7 days), apixaban (3-5 days), rivaroxaban (3 days), warfarin (5 days), and intravenous heparin (4 hours). Coagulation studies also should be reviewed as appropriate. Relative contraindications include bacteremia and aberrant congenital or surgical anatomy. Because these are elective procedures, it is imperative to weigh risks and benefits and document patient agreement and understanding.
Like many procedures, required equipment starts with adequate space to support necessary material and personnel. Large items include a procedure table/bed that will comfortably support the patient while minimizing positioning injury, fluoroscopic imaging equipment, and a table to place surgical instruments sterilely. Monitors to assess the patient’s oxygenation, ventilation, circulation, and temperature should be used, especially if sedation is considered. Further, if deep sedation or general anesthesia is required, a certified anesthesia provider must be present. For the procedure itself, introducer needles, catheters with electrodes, and an equipment interface are used. Emergency equipment such as supplemental oxygen, suction, and code cart should be nearby and accessible.
Thermal nerve ablative techniques should be performed by highly trained specialists with experience in performing spinal procedures under fluoroscopy. Typically, these are board-certified, fellowship-trained pain medicine physicians with backgrounds in anesthesiology physical medicine and rehabilitation (PM&R), family medicine, neurology, emergency medicine, and psychiatry. Like many other procedures, support staff often includes a circulating nurse to assist with equipment and patient support. Additionally, a radiology technician supports with radiographic imaging.
The basic technique involves catheter-guided radiofrequency currents through an electrode placed nearby a nociceptive pathway to interrupt the pain impulses. This is done under fluoroscopic guidance. Interestingly, the tissue around the electrode is heated by the currents, whereas the electrode itself is heated passively by the surrounding tissue. During conventional radiofrequency technique, once the desired temperature is reached, the current is switched off. The current is then switched on to maintain the tissue temperature at a predetermined set point. The cycling between on-and-off currents maintains the selected tissue temperature. At temperatures above 45 degrees Celsius, nerve tissue begins to be destroyed; however, care is taken not to raise temperatures above to the point of tissue gas formation (80 to 90 degrees Celsius). While early studies suggested the selective destruction of unmyelinated C- and A-delta fibers at certain temperatures, further data revealed undifferentiating destruction of all nerve fibers during radiofrequency application. To avoid thermal injury to sensory and motor fibers, the utility of high-temperature radiofrequency ablation has generally been limited to facet denervation; however, an arbitrary range of 55 to 70 degrees Celsius is used for dorsal root ganglia lesioning. During PRF, radiofrequency, currents are cycled for 20 milliseconds, at 2 Hz, for 120 seconds. The voltage is controlled so that the highest temperature remains below 42 degrees Celsius.
Presently, the use of WCRF is limited to clinical presentations in which the pain generator is considered to have several sources of innervation. There are two basic techniques of WCRF. The monopolar technique is used for sacroiliac joint dysfunction, and the bipolar technique is used for discogenic pain. For sacroiliac joint dysfunction, at 17-gauge electrode with a 4 millimeter active tip was used to apply 150 seconds of current with temperature target of 60 degrees Celsius. Due to a larger projected lesion size, the introducer needle is distanced 8 to 10 millimeters from the lateral edge of the posterior sacral foramen. To avoid injury to the segmental spinal nerves, WCFR is not typically used on the lumbar dorsal rami, and conventional radiofrequency is used instead. For discogenic pain, bipolar WCRF is applied to the posterior-lateral disc annulus. In this case, two 17-gauge introducer needles are placed with electrode temperature raised to 55 degrees Celsius over 11 minutes and temperature sustained for an additional 4 minutes.
Cooling techniques have been known to cause analgesia since ancient times; however, to get lasting neurolytic effects, a critical temperature of -20 degrees Celsius must be reached. In addition, the efficacy is dependent not only on temperature, but the duration of cryosectioning, probe size, the proximity of the probe to the target nerve, and the number of freeze cycles applied. The modern cryoprobe is a double lumen aluminum tube that links to a gas source of either carbon dioxide or nitrous oxide. The probe temperature target is typically between -50 and -70 degrees Celsius. The gas is delivered with 600 psi of pressure through the inner lumen and returns to the unit through the outer lumen. A dramatic drop in pressure, approximately 600 psi to 10 psi allows for gas expansion and a consequent ice ball formation around the probe tip. The cryoprobe size correlates with the size of the ice ball formed. A 14-gauge (2mm) probe forms a 5.5 millimeter ice ball, and an 18 gauge (1.4 mm) probe forms a 3.5 millimeter ice ball. To facilitate placement of the probe and offer skin protection from the probe, an introducer is typically used. Typically, a 12-gauge angiocatheter needle is used to introduce a 2 millimeter (14-gauge) probe, and a 14 to 16-gauge angiocatheter needle is used to introduce a 1.4 millimeter (18-gauge) probe. Because the location of target nerve is paramount to nerve disruption, most cryoprobes are furnished with a built-in nerve stimulator that allows for both sensory (100Hz) and motor (2Hz) testing. Further, gas flows must be accurately regulated. Inadequate gas flows are ineffective while excessive flows may lead to unwanted freezing along the probe length. Lastly, before removing the probe, the ice ball must be thawed. An unthawed ice ball can cause local injury and nerve avulsion if withdrawn prematurely.
Adverse effects of thermal lesioning can include bleeding, infection, needle placement induced nerve damage, placement, and burns caused by incorrect grounding pad placement. The most common complication is post-procedural discomfort, and auspiciously, is transient. Although there is a lowered risk of neuroma formation and nerve regeneration after cryoneurolysis, the most significantly reported complication is neuropathic pain. Alopecia and changes in pigmentation also have been reported and are particularly concerning when thermal lesions are performed in proximity to the face. Fortunately, adverse effects and complications of thermal neurolytic therapy are exceedingly rare. Risk and incidence can be mitigated by proper technique, radiographic guidance, sterile technique, appropriate pre-procedural checklists, and increased procedural skill.
Reviewing the clinical efficacy of the different treatment modalities can help elucidate an effective technique. Pulsed radiofrequency has been used most frequently for treatment of cervical or lumbar radicular pains. A study by Tanaka et al. reviewed this utility and has reported its efficacy. PRF has shown its value in the treatment of facet syndrome as well. One randomized controlled trial by Simopoulos et al. showed clinical efficacy of PRF in trigeminal neuralgia treatment, whereas, multiple case reports discussed use for shoulder pain. Other applications of PRF have been based on single case reports or case series. Even though the observational studies collectively support its efficacy, larger controlled studies are needed as current data shows variable efficacy for various reported conditions. Further, the reported effectiveness was largely short term. The application of water-cooled radiofrequency ablation has less data with one randomized controlled trial by Hacker et al. showing significantly lower pain and disability scores in those with the sacroiliac joint disease. Wright et al. provided a second retrospective look and reported its successful application in 27 patients. For WCRF in patients suffering from discogenic pain, one 15-patient case series reported clinical efficacy. Other case reports support its use as well but exemplify how clinical efficacy is still in its primary stages. As stated for cryoablation, most of its documented use is in post-thoracotomy pain with much data coming from the 1980s; however, in 2008, Ju et al. published a double-blind study of 107 patients that reported comparable pain control to the thoracic epidural. The authors, however, could not recommend the application of the technique due to increased incidence of neuropathic pain in the study group. Multiple reports of cryoablation of head, face, and neck pain have been published, with only one controlled trial by Zakrzewska showing adequate effectiveness without evidence of additional complications. Of three reviewed studies of applying cryoablation in postoperative pain after herniorrhaphy, only one study by Birkenmaier et al. reported reduced postoperative analgesic usage in the study group. The other two studies showed no statistical significance in analgesic usage or pain scores, and one of the two, a study by Hodor et al. and Wang, showed the augmented incidence of sensory disturbances in the treatment arm.
Chronic pain continues to be one of the most challenging diseases to treat. Because of its multifactorial nature, treatment is consequently multimodal to include medical, physical, and interventional therapies. As the medical society fights to reduce narcotic usage in the treatment of chronic pain, the application of interventional techniques will continue to be notable treatment options in the battle against intractable pain. Amongst those, radiofrequency ablation used for the right indications has clinically aided in symptomatic relief and reduced medication dependence in this intricate diagnosis.
Radiofrequency ablation is widely used to manage pain and several other disorders. It should be understood that this technique is just one method for pain control and it has its share of complications. It is not a substitute for other means of pain control but a complementary therapy. A multidisciplinary approach with input from nurses and pharmacists may help select patients who are most likely to benefit from this therapy. Once selected, the procedure requires interdisciplinary cooperation to be successful. This usually involves the surgeon, an ultrasound technician, and a nurse. The ultrasound technician will assist in placement and confirmation of placement. The nurse will assist in monitoring vital signs and reducing the patient's anxiety as they are usually awake during the procedure. The nurse should be prepared with any medications commonly used. The interprofessional team must be aware of complications expected and be prepared to deal with them. Further, members of the team must report concerns immediately to the operative surgeon performing the procedure. Post-treatment, nursing monitoring will continue. In addition, patient education is often necessary with concerns of the patient the nurse is unable to address reported to the surgeon for further discussion. The best outcomes with radiofrequency ablation occur with an interprofessional team approach. [Level 5]
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