Author

Khalid Malik, MD
Assistant Professor
Department of Anesthesiology
Northwestern University Feinberg School of Medicine
Chicago, IL

Introduction

During conventional radiofrequency (CRF) application for pain control, a thermal lesion is created adjacent to a nociceptive pathway to interrupt the pain impulses which aims to provide the necessary pain relief. The radiofrequency (RF) currents are applied through an electrode placed in the vicinity of the target neural structure and the passage of RF currents imparts energy to the surrounding tissues, raising their temperature, while the electrode itself is heated only passively during the thermal lesioning. Temperatures above 45°C are known to be neurodestructive,¹ therefore during CRF application tissue temperatures are typically raised to well above this range; typically to 80°C to 90°C. Thermal lesions at these high temperatures cause injury to nerve fibers of all types,² with the risk of motor weakness, local neuritis and deafferentation pain. To avoid these sequelae lower temperatures have been arbitrarily selected in many studies of the dorsal root ganglia (DRG) lesioning.³ In one such study, no difference in clinical results was found between CRF applied at 40°C or 67°C.⁴  The authors of this study concluded that factors other than heat may have determined the clinical outcomes and proposed possible role of electrical energy in causing neuronal dysfunction.

Pulsed radiofrequency (PRF) was introduced to maximize the delivery of electrical energy, by using higher voltages, while concomitantly obviating thermal tissue injury by precluding the tissue temperatures from rising above the neurodestructive range (42°C).⁵ These conflicting goals were achieved by applying RF currents in a pulsatile manner, to allow time for the heat to dissipate in between the RF pulses. During typical PRF application RF currents are applied for 20 milliseconds, at 2 Hertz, for a total duration of 120 seconds, with the maximum electrode temperature of 42°C. Sluijter et al.⁵ in their first publication on PRF described that high-density electrical currents generated at the electrode tip were capable of stressing the cellular membranes and caused altered cell function and cellular injury. Later, a combined role of electrical and thermal tissue injury from PRF application was suggested.^6,7  The latter view was based on the assumption that slow response time of temperature measuring devices used during PRF may not exclude the possibility of high temperature spikes and the likelihood of thermal tissue injury. As electrical rather than thermal injury is considered the source of neuronal dysfunction during PRF application, precise approximation of the electrode parallel to the target nerve, unlike CRF, is considered unnecessary.

Clinical Applications

PRF has been widely used for the treatment of several painful and also for some non-painful conditions.⁸ PRF of the various neuronal and non-neural structures performed in the clinical practice along with the various syndromes treated are listed below,

• DRG for cervical, thoracic and lumbar radicular pain (RP), postherpetic neuralgia (PHN), neuropathic spinal pain, herniated intervertebral disc, post-amputation stump pain, failed back surgery syndrome and chronic inguinal herniorrhaphy pain.
• Medial branches (MBs) of the dorsal spinal rami for facet syndrome (FS) and whiplash injury.
• Gasserian ganglion (GG) for Trigeminal neuralgia (TN).
• Suprascapular nerve (SSN) for rotator cuff tears and shoulder pain.
• Sacroiliac joint and lateral branches of the dorsal sacral rami for sacroiliac joint dysfunction.
• Intervertebral disc for discogenic pain.
• Sphenopalatine ganglion for head, neck and facial pain.
• Intercostal nerves for post-surgical thoracic pain and PHN.
• Lateral femoral cutaneous nerve for meralgia paresthetica.
• Myofascial trigger points for myofascial pain.
• Scar neuromas for post-surgical scar pain.
• Pudendal nerve for pudendal neuralgia.
• Dorsal penile nerves for premature ejaculation.
• Spermatic cord for chronic testicular pain.
• Splanchnic nerves for chronic benign pancreatic pain.
• Sciatic nerve for phantom limb pain.
• Intra articular application for arthrogenic pain
• Branches of obturator and femoral nerves for hip joint pain.
• Lumbar sympathetic chain for CRPS.
• Glossopharyngeal nerve for glossopharyngeal neuralgia.
• Greater occipital nerve (GON) for occipital neuralgia (ON).
• Genitofemoral, ilioinguinal and iliohypogastric nerves for groin pain and orchialgia.

Clinical Efficacy

The majority of studies of PRF use have been observational in nature and reported its efficacy.⁸ Currently there are 17 controlled trials of PRF use which have evaluated its efficacy for RP, FS, shoulder pain, cervicogenic headache, PHN and TN.^5,9-24

Radicular Pain: There are five trials of PRF application to the affected DRG for RP, 4 in lumbar and 1 in cervical region.^5,9-12 The first trial of PRF use by Sluijter et al. reported its efficacy, in a group of 36 patients with RP, compared to CRF at 42°C at 6 weeks.⁵ This trial was limited by its size, vaguely described study population, lacked adequate randomization and blinding, had inadequate outcome measures (only patient satisfaction scores were used), reported only the short term results and lacked a placebo group. It also compared PRF to non-standard CRF (CRF at 42°C) not routinely used in clinical practice. A trial of 76 patients with lumbar RP compared PRF to combined PRF and CRF, applied to the DRG, and reported no advantage of added CRF.⁹ This trial also used a non-conventional form of CRF, not routinely used in clinical practice, CRF to maximum tolerated temperature that created a burning sensation in the low back to the foot; average temperatures was 54°C for 60 seconds; range not provided. One trial of 100 patients with back pain, with or without RP, compared PRF/DRG to electro-acupuncture and conservative treatment and reported relative efficacy of the PRF treatment.¹⁰ This trial also had significant limitations which included poorly defined study population, no description of the number and the level of DRG treated, no diagnostic nerve blocks performed prior to the DRG lesioning, unknown nature of the treatment provided in the conservatively treated group, poor description of blinding or randomization techniques and the assessment of only the short–term results. There are two randomized, double-blinded trials of PRF/DRG comparing it to sham treatment, one in cervical and one in lumbar region.^11,12 Even though these two trials are reasonable well conducted, both are small in size, report short-term results and report only the modest efficacy of PRF relative to the placebo. Consequently, based on the above evidence currently there is only little support of PRF/DRG for the treatment of RP in the literature.  

Facet Syndrome: There are 3 trials of PRF application to the MBs for FS, all in lumbar region.^13-15 A trial of 60 patients with FS compared PRF, CRF and sham treatment in 3 equal groups and reported equal and superior efficacy of CRF and PRF compared to the sham treatment but only at 6 hours after the treatment.¹³ While the patients in the CRF group continued to experience superior pain relief and improved function at 6 and 12 months, the patients in the PRF and sham treated groups had similar outcomes at these time points. This trial reported lack of PRF efficacy compared to CRF for lumbar FS. Interestingly, another similar trial (of 50 patients) compared only PRF to CRF (it lacked a placebo group) and reported no difference between the two groups at 3 months—equal efficacy of PRF and CRF was reported.¹⁴ This trial however was limited by a high dropout rate (results of only 26 patients, 13 patients in each group, were available for analysis), short-term results at 3 months, and insufficiently described study methods. One trial of 80 patients, with relatively well described study methods, compared PRF to MBs injections (steroid and local anesthetic) and reported relative efficacy of PRF for up to 6 months.¹⁵ As a result with two well conducted trials^13,15 reporting opposing results the efficacy of PRF of MBs for lumbar FS remains unclear. No such trials, of PRF of MBs for treatment of FS, exist in cervical region.

Shoulder Pain: Of the 4 trials of PRF to SSN for shoulder pain, 3 compared PRF to another treatment modality of uncertain efficacy, and one compared it to sham treatment.^16-19 A single blinded study of 50 patients, in 2 equal groups, compared intra-articular steroid injections to fluoroscopically directed PRF of the SSN and reported relative efficacy of the intra-articular injections at 3 months.¹⁶ A trial of 40 patients, in 2 equal groups, compared transcutaneous electrically nerve stimulation (TENS) applied to the shoulder joint (for 20 minutes five times a week for 20 sessions) and fluoroscopically directed PRF of the SSN. At 3 months, no difference in outcomes was reported between the groups.¹⁷ A trial of 60 patients (42 completed the study) compared 12 weeks of physical therapy alone to 12 weeks of physical therapy and single ultrasound guided PRF of the SSN and reported the relative efficacy of combined treatment at 12 weeks.¹⁸ The only trial comparing PRF of SSN with similarly performed sham procedure (lidocaine injection) reported similar efficacy of PRF and placebo at 6 months.¹⁹ This trial was limited by its size (only 13 patients available for analysis at 6 months), slow recruitment (22 patients recruited over 3 years) and a high dropout rate (9 of the 22 patients dropped out of the study at 6 months) and its results were therefore inconclusive. Consequently, with 3 trials reporting variable efficacy of PRF of SSN compared to disparate pain relieving modalities of indeterminate significance and one severely limited trial reporting its lack of efficacy, the true value of PRF of SSN for shoulder pain remains essentially unknown.  

Cervicogenic Headache: Only one trial evaluated the efficacy of PRF of GON in patients with cervicogenic headache. In this trial, 30 patients in 2 equal groups, received either PRF or local anesthetic blocks of the GON and it reported no difference between the groups at 3 and 9 months.²⁰ The trial was limited by its small size and the non-blinded investigators.

Postherpetic Neuralgia: Only one trial evaluated the efficacy of PRF in patients with PHN in thoracic dermatomes.²¹ In this trial, 96 patients in 2 equal groups, received either weekly PRF or sham treatments of the intercostal nerves (using fluoroscopic guidance) for 3 weeks and the patients were followed for 6 months (only 4 patients dropped out of the study). The trial appeared well conducted and even though the authors concluded that PRF was effective compared to the sham treatment the results documented were conflicting—even though the SF-36 scores for bodily pain were significantly lower for up to 6 months, the VAS pain scores were significantly lower for only 14 days.    

Trigeminal Neuralgia: There are 3 trials of PRF application to GG for the treatment of TN. A double-blinded, randomized trial of 40 patients with TN reported lack of PRF efficacy compared to CRF at 3 months and recommended against its.²² Another randomized, double-blinded trial of 60 patients (in three equal groups) compared CRF at 75°C for 2-3 minutes, CRF at 75°C for 4-5 minutes and PRF for 10 minutes combined with CRF at 75°C for 2-3 minutes and reported no difference between the groups at 1 year.²³ A third, randomized and double-blinded, trial of 60 patients, in 2 equal groups, (53 completed the study) compared standard-voltage (36.30±5.57) with high-voltage (71.52±7.97) PRF, applied for 4 minutes. It reported relative efficacy of the higher voltage PRF.²⁴ Consequently, all the trials of PRF of GG for TN evaluated various combinations of PRF and CRF, they did not contain a sham or conservatively treated group and reported disparate results. The exact efficacy of PRF of GG for TN therefore remains essentially untested.

Complications

Apart from transient local post-procedural discomfort, no significant or long-term complications from the use of either PRF have been reported in the literature.⁸

Summary

PRF is used extensively in clinical practice for a range of painful and even some non-painful syndromes. Yet, its exact mechanism of action is vague and based on the variable efficacy reported in a range of clinical trials, the true value of its clinical use remains ambiguous.

COOLED RADIOFREQUENCY

Introduction

Water-cooled Radiofrequency (WC-RF) or cooled radiofrequency is applied by using a specialized multichannel electrode that is actively cooled by the continuous flow of water at ambient temperature. Active cooling of the electrode prevents it from acquiring the high surrounding tissue temperatures and allows continued flow of the RF current. Consequently, heating of larger tissue volume is possible and a larger thermal lesion is created as a result.^25,26   Similar to CRF lesion, the size of WC-RF lesion depends on the probe size, electrode temperature, tissue properties and the duration of RF current application. For example, if 50°C isotherm is considered a lesion’s edge, by using an 18 gauge electrode with 6 mm active tip, heated to 55°C to 60°C, for 150 seconds, a thermal lesion of 8 to 10 mm in diameter would be created.^25,26 The larger area of neural destruction accomplished with WC-RF increases the probability of successful denervation of a pain generator with abundant and variable nociceptive innervation.

The two distinct techniques of WC-RF application in clinical use include monopolar and bipolar WC-RF lesioning. A monopolar lesion is created by using a 17 gauge WC-RF electrode with RF current applied for 150 seconds and the tip temperature raised to 60°C. Unipolar WC-RF is used mainly for the treatment of sacroiliac joint dysfunction (SJD) and occasionally it has been applied to other superficial nerves for the treatment of certain neuralgias. For the treatment of SJD it is applied to dorsal L4, L5 rami and S1 to S3 lateral branches and either two or three monopolar lesions are created.^27,28 Due to the larger lesion created compared to the CRF lesion the WC-RF electrode is placed at a distance from the target nerve—8 to 10 mm from the lateral edge of posterior sacral foramen.³   There are two reports of monopolar WC-RF application to other superficial neural structures—scar neuroma and greater occipital nerve.^29,30 Bipolar WC-RF is used primarily for the treatment of discogenic pain and the application is known commercially as intradiscal biacuplasty (IDB). It entails placement of two 17 gauge WC-RF electrodes in the postero-lateral disc annulus and the electrode temperature is raised to 55°C for 16 minutes.³¹

Clinical Application

In addition to the isolated reports of its use for neuromas and superficial nerves,^29,30 WC-RF is currently being used primarily, when the potential pain generator has abundant and variable innervation, for SJD and discogenic pain.^27,28,31

Clinical Efficacy

There are 2 trials of WC-RF use for the treatment of SJD.^27,32 The first is a blinded trial of 28 patients, with typical SJD symptoms and positive response to a single diagnostic sacroiliac joint injection.²⁷ The patients were randomized, in 2 equal groups, to receive WC-RF (L4–L5 primary dorsal rami and S1to S3 lateral branches) and sham treatment (placebo denervation). At 1-month, 11/14 (79%) patients in the RF group and 2/14 patients (14%) in the placebo group had successful outcomes (50% pain relief and significant functional improvement). The second trial, also a blinded and randomized, was comprised of 51 patients with SJD symptoms and positive response to dual lateral branch blocks.³² The patients, randomized in a 2:1 ratio (34 patients in the RF and 17 in the sham group), received either WC-RF (applied to S1–S3 lateral branches and L5 dorsal ramus) or sham treatment (identically performed except RF application). Un-blinding occurred at 3 months and the patients were allowed to crossover to the RF group; there were no dropouts at 3 months. Significant improvements in pain (NRS, SF-36BP-Bodily Pain), physical function (SF-36PF-Physical Functioning), disability (ODI), quality of life, and treatment success (Global Perceived Effect) was reported for the treatment group at 3 months. Hence, the two well conducted trials reported the short-term efficacy of WC-RF relative to placebo for SJD. The only trial of WC-RF use for discogenic low back pain is a double-blinded trial of 59 well selected patients with chronic low back pain with positive pain reproduced on provocative discography.³³ The patients were randomized to receive IDB to the affected discs (n=27—one level n=16, two levels n=11) and sham treatment (n=30—procedure performed identically except the probe was not placed in the disc and RF energy not delivered). The patients in the IDB group exhibited significant improvements in physical function (SF-36), pain (NRS), disability (ODI), and reduced opioid usage for up to 6 months. Therefore this one well conducted trial reported the efficacy of IDB for selected patients with discogenic low back pain compared to placebo.

Complications

Except a report of third degree skin burn,³⁴ no significant complications from WC-RF use are reported in the literature.

Summary

WC-RF is a relatively new pain relieving modality with well-defined mechanism of action and a clear efficacy demonstrated in few well conducted trials. Even though currently its use is limited primarily to discogenic pain and SJD it may have broader clinical applications.²⁹

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