Effects and side effects of a percutaneous thermal lesion of the dorsal root ganglion in patients with cervical pain syndrome

Safety of Conventional and Pulsed Radiofrequency Lesions of the Dorsal Root Entry Zone Complex (DREZC) for Interventional Pain Management: A Systematic Review

Study Design

Systematic literature review.

Objective

The goal of this systematic review is to assess the clinical safety and potential complications of conventional and pulsed radiofrequency ablations targeting dorsal root entry zone complex (DREZC) components in the treatment of chronic pain.

Background

There is a growing popularity for the use of radiofrequency ablation (RFA) techniques targeting DREZC components by pain management physicians for an increasing variety of indications. To date, we lack a systematic review to describe the safety and the type of complications associated with these procedures.

Methods

This was a systematic literature review. This systematic search was limited to peer-reviewed literature using “radiofrequency ablation” as a search keyword using PubMed’s database for manuscripts published between inception and December 2020. Abstracts that involved the application of radiofrequency currents, of any modality, to DREZC components for the treatment of pain were included for full-text review. Search was limited to original data describing clinical outcomes following RFA performed for pain indications only, involving the DREZC components outlined above, in human subjects, and written in English. The primary outcomes were complications associated with conventional RFA and pulsed radiofrequency ablation (PRF). Complications were categorized as type 1 (persistent neurological deficits or other serious adverse events, defined as any event that resulted in permanent of prolonged injury; type 2 (transient neuritis or neurological deficits, or other non-neurological non-minor adverse event); type 3 (minor adverse events (e.g., headache, soreness, bruising, etc.).

Results

Of the 62 selected manuscripts totaling 3157 patients, there were zero serious adverse events or persistent neurological deficits reported. A total of 36 (1.14%) transient neurological deficits, cases of transient neuritis, or non-minor adverse events like uncomplicated pneumothorax were reported. A total of 113 (3.58%) minor adverse events were reported (bruising, transient site soreness, headache).

Conclusions

This systematic review indicates that the use of RFA lesion of the DREZC for interventional pain management is very safe. There were no serious adverse effects with a sizable sample of randomized controlled trial (RCT), prospective observational, and retrospective studies.

Transforaminal Epidural Injection of Local Anesthetic and Dorsal Root Ganglion Pulsed Radiofrequency Treatment in Lumbar Radicular Pain: A Randomized, Triple-Blind, Active-Control Trial

BACKGROUND

Lumbar radicular pain (LRP) results from inflammation and irritation of lumbar spinal nerves and the dorsal root ganglion (DRG).

METHODS

Our study is a prospective, triple-blind, randomized, activecontrol trial (CTRI/2016/02/006666) comparing transforaminal epidural local anesthetic (LA) injection and pulsed radiofrequency treatment of DRG in patients with chronic LRP. Patients with LRP after failed conservative management for >3 months received selective diagnostic nerve root block with 1 mL 2% lidocaine. Fifty patients showing positive responses were divided into groups of 25 each. The LA group received transforaminal epidural injection of 1 mL 0.5% bupivacaine. The lumbar pulsed radiofrequency (LPRF) group received transforaminal epidural injection of 1 mL 0.5% bupivacaine with 3 cycles of pulsed radiofrequency of the DRG for 180 seconds RESULTS: Both groups were compared by observing pain intensity on a 0- to 100-point VAS and improvement in functional status by the Oswestry Disability Index (ODI version 2.0) at 2 weeks and 1, 2, 3, and 6 months. All baseline variables were comparable between the 2 groups. Statistically significant reduction in both outcomes was seen in the LPRF group compared to the LA group from 2 weeks to 6 months. One hundred percent of patients in the LPRF group had a ≥20- point decrease in VAS and significant percentage reduction in ODI at all time intervals up to 6 months, whereas it was seen in 80% and 28% of patients in the LA group at 3 and 6 months, respectively. No complications were seen in any patients CONCLUSION: Pulsed radiofrequency of the DRG applied for longer duration results in long-term pain relief and improvement in the functional quality of life in patients with chronic LRP.

The Dorsal Root Ganglion as a Therapeutic Target for Chronic Pain

Chronic neuropathic pain is a widespread problem with negative personal and societal consequences. Despite considerable clinical neuroscience research, the goal of developing effective, reliable, and durable treatments has remained elusive. The critical role played by the dorsal root ganglion (DRG) in the induction and maintenance of chronic pain has been largely overlooked in these efforts, however. It may be that, by targeting this site, robust new options for pain management will be revealed. This review summarizes recent advances in the knowledge base for DRG-targeted treatments for neuropathic pain:• Pharmacological options including the chemical targeting of voltage-dependent calcium channels, transient receptor potential channels, neurotrophin production, potentiation of opioid transduction pathways, and excitatory glutamate receptors.• Ablation or modulation of the DRG via continuous thermal radiofrequency and pulsed radiofrequency treatments.• Implanted electrical neurostimulator technologies.• Interventions involving the modification of DRG cellular function at the genetic level by using viral vectors and gene silencing methods.

A comprehensive review of pulsed radiofrequency in the treatment of pain associated with different spinal conditions

OBJECTIVE

The objective of this review was to evaluate the efficacy of pulsed radiofrequency (PRF) treatment of pain associated with different spinal conditions. The mechanisms of action and biological effects are shortly discussed to provide the scientific basis for this radiofrequency modality.

METHODS

We systematically searched for clinical studies on spinal clinical conditions using PRF. We searched the MEDLINE (PubMed) database. We classified the information in one table focusing on randomized controlled trials (RCTs) and other types of studies. Date of last electronic search was October 2016.

RESULTS

We found four RCTs that evaluated the efficacy of PRF on cervical radicular pain and five observational studies. Two trials and three observational studies were conducted in patients with facet pain. For disc-related pathology, we found one RCT with PRF applied intradiscally and three RCTs for dorsal root ganglia PRF modulation lumbosacral radicular pain. For sacroiliac joint pain, spondylolisthesis, malignancies and other minor spinal pathology, limited studies were conducted.

CONCLUSION

From the available evidence, the use of PRF to the dorsal root ganglion in cervical radicular pain is compelling. With regard to its lumbosacral counterpart, the use of PRF cannot be similarly advocated in view of the absence of standardization of PRF parameters, enrolment criteria and different methods in reporting results; but, the evidence is interesting. The use of PRF in lumbar facet pain was found to be less effective than conventional RF techniques. For the other different spinal conditions, we need further studies to assess the effectiveness of PRF. Advances in knowledge: The use of PRF in lumbar facet pain was found to be less effective than conventional RF techniques. For the other different spinal conditions, we need further studies to assess the effectiveness of PRF.

Percutaneous Cervical Nucleoplasty vs. Pulsed Radio Frequency of the Dorsal Root Ganglion in Patients with Contained Cervical Disk Herniation; A Prospective, Randomized Controlled Trial

BACKGROUND

Cervical neck pain is often caused by cervical disk pathology and may cause severe symptoms and disability. Surgeons and patients are increasingly aware of postsurgery-related complications. This stimulated the clinical usage of minimally invasive treatments such as percutaneous nucleoplasty (PCN) and pulsed radio frequency (PRF). However, scientific evidence on both treatments is limited.

OBJECTIVE

Our objective was to evaluate the efficacy of PCN compared to PRF in patients with contained cervical disk herniation.

METHODS

A prospective randomized clinical trial was conducted including 34 patients with radicular pain due to a single contained cervical disk herniation who were treated with either PCN or PRF. Demographic data were collected, and the Medical Outcomes Study 12-Item Short Form (SF-12) Health Survey, visual analog scale (VAS), and the Neck Disability Index (NDI) were completed 1, 2, and 3 months after treatment. Treatment satisfaction and complications were recorded.

RESULTS

In the PCN group (n = 17, mean age 52.4 years, 10 female/7 male), patients were treated at C5 to C6 (8 cases) or C6 to C7 (9 cases). In the PRF group (n = 17, mean age 49.5 years, 8 female/9 male), patients were treated at C3 to C4 (1 case), C5 to C6 (10 cases), or C6 to C7 (6 cases). At 3 months, mean pain VAS improved significantly from baseline in the PCN group (mean improvement: 43.4 points) and in the PRF group (34.0 points). However, improvement in 1 group was not superior compared to the other group (P = 0.48). No serious complications were reported.

CONCLUSION

Within 3 months, both PCN and PRF show significant pain improvement in patients with contained cervical disk herniation, but none is superior to the other. Both treatment options appear to be effective and safe in regular clinical practice.

Comparative Effectivenesses of Pulsed Radiofrequency and Transforaminal Steroid Injection for Radicular Pain due to Disc Herniation: a Prospective Randomized Trial

Transforaminal Epidural steroid injections (TFESI) have been widely adopted to alleviate and control radicular pain in accord with current guidelines. However, sometimes repeated steroid injections have adverse effects, and thus, this prospective randomized trial was undertaken to compare the effectivenesses of pulsed radiofrequency (PRF) administered to a targeted dorsal root ganglion (DRG) and TFESI for the treatment of radicular pain due to disc herniation. Subjects were recruited when first proved unsuccessful (defined as a score of > 4 on a visual analogue scale (VAS; 0-10 mm) and of > 30% according to the Oswestry Disability Index (ODI) or the Neck Disability Index (NDI)). Forty-four patients that met the inclusion criteria were enrolled. The 38 subjects were randomly assigned to receive either PRF (PRF group; n = 19) or additional TFESI (TFESI group; n = 19) and were then followed for 2, 4, 8, and 12 weeks. To evaluate pain intensity were assessed by VAS. ODI and NDI were applied to evaluate functional disability. Mean VAS scores for cervical and lumbar radicular pain were significantly lower 12 weeks after treatment in both study groups. NDI and ODI scores also declined after treatment. However, no statistically significant difference was observed between the PRF and TFESI groups in terms of VAS, ODI, or NDI scores at any time during follow-up. PRF administered to a DRG might be as effective as TFESI in terms of attenuating radicular pain caused by disc herniation, and its use would avoid the adverse effects of steroid.

Photobiomodulation of the dorsal root ganglion for the treatment of low back pain: A pilot study

BACKGROUND AND OBJECTIVE

Chronic low back pain is a worldwide public health issue with high socioeconomic impact. The aim of this study was to determine the efficacy of laser irradiation of the dorsal root ganglion of the second lumbar spinal nerve for chronic axial low back pain compared to lidocaine injection and radiofrequency treatment.

STUDY DESIGN/MATERIALS AND METHODS

Twenty-eight patients were randomly divided into three treatment groups: lidocaine injection, radiofrequency, or laser. The second intervertebral foramen between the second and third lumbar vertebrae was accessed by percutaneous needle puncture bilaterally, guided by fluoroscopy. In the local anesthetic group, injection of 1 ml lidocaine without epinephrine was applied through a 20-gauge (G20) Quincke tip spinal needle inserted in the second lumbar intervertebral foramen. In the radiofrequency group, the probe (150 mm long with a 5 mm active tip) was directed through a G20 needle placed in the second lumbar intervertebral foramen and neuromodulation was done with a radiofrequency of Cosman G4® in pulses of 20 ms with wash-out period of 480 ms, for 300 seconds at 42°C. A single treatment was used. In the laser treatment group, a continuous wave, 808 nm wavelength diode laser (Photon Lase III® DCM, Brazil), with an output power of 100 mW was used for a single treatment. An 18 gauge needle was placed in the second lumbar intervertebral foramen guided by fluoroscopy. Light was delivered through a 600 µm optical fiber placed in the G18 needle. The tip of the fiber extended 5 mm beyond the tip of the needle in the second lumbar intervertebral foramen. The beam spot size was 0.003 cm(2) , irradiance = 35W/cm(2) , exposure time = 84 seconds, energy density = 2800J/cm(2) , total energy was 8.4 J. The low back pain score was assessed by the visual analog scale (VAS) and Pain Relief Scale (PRS) pre, post procedure and in 1 month follow up. Temperature was measured using a digital thermometer.

RESULTS

All patients in the local anesthetic and laser treatment groups reported a pain reduction of at least 50% immediately post-procedure and 10 out of 11 patients in the radiofrequency group reported a pain reduction of at least 50%. At 1 month post-treatment, the laser treatment group had the greatest number of patients who reported more than 50% pain relief based on PRS (7 out of 10 patients) while only 2 out of 7 patients and 3 out of 11 patients in the lidocaine and radiofrequency treatment groups respectively reported more than a 50% pain relief.

CONCLUSION

Laser irradiation caused an immediate decrease in low back pain post-procedure similar to pain reduction caused by lidocaine injection. Both lidocaine injection and laser irradiation were more effective than radiofrequency treatment for immediate and longer term (1 month post-treatment) chronic back pain. Lasers Surg. Med. 48:653-659, 2016. © 2016 Wiley Periodicals, Inc.

Pulsed radiofrequency applied to the dorsal root ganglia for treatment of post-stroke complex regional pain syndrome: A case series

STUDY OBJECTIVE

Complex regional pain syndrome is a painful and disabling syndrome where the patient presents with neuropathic pain, edema, or vasomotor or pseudomotor abnormalities that are often refractory to treatment. Complex regional pain syndrome type 1 may occurs in stroke patients. Radiofrequency is a therapeutic modality that has been used for years for diseases associated with neuropathic pain.

DESIGN

Case series report.

SETTING

Selcuk University Hospital.

PATIENTS

A 69-year-old woman and a 48-year-old women who suffered post-stroke complex regional pain syndrome type 1.

INTERVENTIONS

Pulsed radiofrequency current application to the cervical dorsal root ganglia.

MEASUREMENTS

Pain reduction.

MAIN RESULTS

The patients had complete resolution of their symptoms, which was maintained at 10 and 5 months of follow-up.

CONCLUSIONS

These cases illustrates that pulsed radiofrequency applied to cervical dorsal root ganglia might play a significant role in multi-modal approach of complex regional pain syndrome type 1 management after stroke. Further randomized, controlled studies are needed to support this argument.

Stellate ganglion pulsed radiofrequency ablation for stretch induced complex regional pain syndrome type II

Complex regional pain syndrome (CRPS) following injury or nerve damage, as its name signifies, is a challenging entity, and its successful management requires a multidisciplinary approach. It not only manifests as severe pain, but also gives rise to functional disability, lack of sleep, lack of enjoyment of life and poor quality of life. Various pain interventional techniques have been described in the literature for the management of CRPS ranging from sympathetic blocks to spinal cord stimulator. A 34-year-old liver transplant donor, who developed position-induced right upper limb neuropathic pain suggestive of CRPS type II was managed initially with medications and later with stellate ganglion block under fluoroscopic guidance at cervical C7 position. Following an initial significant improvement in pain and allodynia, which was transient, a pulsed radiofrequency ablation of stellate ganglion was performed successfully to provide prolonged and sustained pain relief, which persisted up to 14 months of follow-up.

The C5 root dermatome enlarges and modulates hand pain in total brachial plexus palsy

Hand pain is a major complaint in 80% of the patients with complete brachial plexus palsy; and, in 80% of these patients, the C5 root is ruptured and the C6-T1 roots avulsed from the spinal cord. It has been suggested that pain in brachial plexus injuries may not arise from avulsed roots, but rather from ruptured roots. Traditionally the C5 root dermatome does not extend to the hand. We have hypothesized that in total lesions of the brachial plexus the C5 root dermatome expands, reaching the hand. In 20 patients with confirmed C5 root rupture and C6-T1 root avulsion, we investigated the distribution of C5 root paresthesia six to eight weeks after grafting. After cervical percussion in search of Tinel’s sign, maps related to reported paresthesia were drawn on the affected limb. We observed that paresthesia following C5 root percussion reached the hands and fingers, dermatomes linked to the C6 and C8 roots. Immediately after percussion, for a few seconds, 14 patients who complained of pain during examination reported the augmentation of numbness and pain resolution. After brachial plexus injury, the C5 root dermatome expands and modulates hand pain.

Percutaneous pulsed radiofrequency treatment of the cervical dorsal root ganglion in the treatment of chronic cervical pain syndromes: a clinical audit

Cervicogenic headache and cervicobrachialgia are frequent diagnoses of chronic cervical pain. After failure of conservative treatment, an interventional approach may be indicated in the absence of any indication for causal surgical treatment. The pulsed radiofrequency (PRF) technique exposes the nerve to a high-frequency electric field while the temperature of the electrode tip does not exceed 42°C. This method is thought to be nondestructive and almost free of neurologic side effects and complications. Our extended pilot study was performed to confirm the perceived efficacy of PRF for short- and long-term relief of chronic cervical pain. We carried out a clinical audit of the first 18 patients treated with PRF at the cervical dorsal root ganglion. An independent evaluator reviewed the medical records. Patients with good clinical results at 8 weeks were evaluated for long-term effect (> 6 months), based on a 7-point Likert scale. Thirteen patients (72%) showed short-term clinical success (≥ 50% pain relief). Mean follow-up was 19.4 months (SD 8.9 months), maximum 2.5 years. The duration of satisfactory pain relief (6 or 7 on the Likert scale) varied between 2 and over 30 months, with a mean duration of 9.2 months (SD 11.2 months). Kaplan-Meier analysis illustrated that 50% of patients experienced success 3 months after treatment. We could not identify predictive variables for clinical outcome. None of the patients reported post-treatment neuritis or other adverse events. To our knowledge, this is the first documented series of chronic cervical pain syndromes treated with PRF. Satisfactory pain relief of at least 50% was achieved in 13 of 18 (72%) patients at 8 weeks. More than one year after treatment, six patients (33%) continue to rate treatment outcome as good or very good. No side effects were reported. j.

A systematic review: current and future directions of dorsal root ganglion therapeutics to treat chronic pain

OBJECTIVE

The purpose of the study was to systematically review the historical therapeutics for chronic pain care directed at the dorsal root ganglion (DRG) and to identify future trends and upcoming treatment strategies.

METHODS

A literature search on bibliographic resources, including EMBASE, PubMed Cochrane Database of Systemic Reviews from literature published from 1966 to December 1, 2012 to identify studies and treatments directed at the DRG to treat chronic pain, and was limited to the English language. Case series, case reports, and preclinical work were excluded. Information on emerging technologies and pharmacologics were captured separately, as they did not meet the inclusion criteria.

RESULTS

The literature review yielded three current clinical treatment strategies: ganglionectomy, conventional radiofrequency treatment of the dorsal root ganglion, and pulsed radiofrequency treatment of the DRG. Seven studies were identified utilizing ganglionectomy, 14 for conventional radiofrequency, and 16 for pulsed radiofrequency. Electrical stimulation and novel therapeutic delivery strategies have been proposed and are in development.

CONCLUSIONS

Despite a robust understanding of the DRG and its importance in acute nociception, as well as the development and maintenance of chronic pain, relatively poor evidence exists regarding current therapeutic strategies. Novel therapies like electrical and pharmacologic strategies are on the horizon, and more prospective study is required to better qualify the role of the DRG in chronic pain care.

Changes in pain behavior and glial activation in the spinal dorsal horn after pulsed radiofrequency current administration to the dorsal root ganglion in a rat model of lumbar disc herniation: laboratory investigation

OBJECT

Herniated discs can induce sciatica by mechanical compression and/or chemical irritation caused by proinflammatory cytokines. Using immunohistochemistry methods in the dorsal horn of a rat model of lumbar disc herniation, the authors investigated the effects of pulsed radiofrequency (PRF) current administration to the dorsal root ganglion (DRG) on pain-related behavior and activation of microglia, astrocytes, and mitogen-activated protein kinase.

METHODS

A total of 33 Sprague-Dawley rats were randomly assigned to either a sham-operated group (n = 10) or a nucleus pulposus (NP)-exposed group (n = 23). Rats in the NP-exposed group were further subdivided into NP exposed with sham stimulation (NP+sham stimulation, n = 10), NP exposed with PRF (NP+PRF, n = 10), or euthanasia 10 days after NP exposure (n = 3). The DRGs in the NP+PRF rats were exposed to PRF waves (2 Hz) for 120 seconds at 45 V on postoperative Day 10. Rats were tested for mechanical allodynia 10 days after surgery and at 8 hours, 1 day, 3 days, 10 days, 20 days, and 40 days after PRF administration. Immunohistochemical staining of astrocytes (glial fibrillary acidic protein), microglia (OX-42), and phosphorylated extracellular signal-regulated kinases (pERKs) in the spinal dorsal horn was performed at 41 days after PRF administration.

RESULTS

Starting at 8 hours after PRF administration, mechanical withdrawal thresholds dramatically increased; this response persisted for 40 days (p < 0.05). After PRF administration, immunohistochemical expressions of OX-42 and pERK in the spinal dorsal horn were quantitatively reduced (p < 0.05).

CONCLUSIONS

Pulsed radiofrequency administration to the DRG reduced mechanical allodynia and downregulated microglia activity and pERK expression in the spinal dorsal horn of a rat model of lumbar disc herniation.

Dorsal root ganglion - a potential new therapeutic target for neuropathic pain

A regional approach can protect our patients from often unacceptable adverse effects produced by systematically applied drugs. Regional therapeutic approaches, as well as interventions at the level of the peripheral nervous system and particularly the dorsal root ganglion (DRG), represent an alternative to the systemic application of therapeutic agents. This article provides an overview of DRG anatomical peculiarities, explains why the DRG is an important therapeutic target, and how animal models of targeted drug delivery can help us in the translation of basic research into clinical practice.

Radiofrequency thermoneurolysis for the treatment of Morton's neuroma

Pedal neuroma is a common disorder. The authors undertook a review of 32 feet in 29 patients with a symptomatic neuroma treated between January 2007 and January 2010 to evaluate the effectiveness of radiofrequency thermoneurolysis therapy in alleviating symptoms. Overall relief of symptoms was rated as complete by 24 (83%) patients, with 5 patients experiencing minimal to no relief. Two patients were lost to follow-up after 1 month, 2 patients opted for no further intervention, and 1 patient went to open resection of the neuroma. Average follow-up was 13 months and total recovery time was 2 days. Complications included 1 foot with cellulitis treated by a course of oral antibiotics. The results of this retrospective study indicate radiofrequency thermoneurolysis therapy is a safe, effective, and minimally invasive alternative treatment for symptomatic neuromas of the foot.

Radio Frequency Ablation in Drug Resistant Chemotherapy-induced Peripheral Neuropathy: A Case Report and Review of Literature

Chemotherapy-induced peripheral neuropathy (CIPN) is a frequently encountered complication. It can result from a host of agents. Various modalities of treatment have been advocated, of which a novel method is radio frequency ablation. A 63-year-old male, a case of carcinoma prostrate with bone metastases, presented with tingling and numbness in right upper limb. He was given morphine, gabapentin and later switched to pregabalin, but medications provided only minor relief. Initially he was given stellate ganglion block, then radiofrequency ablation of dorsal root ganglion was done, but it failed to provide complete relief. Pulsed radiofrequency ablation (PRF) was then done for 90 seconds; two cycles each in both ulnar and median nerve. After the procedure the patient showed improvement in symptoms within four to five hours and 80% relief in symptoms. We conclude that PRF can be used for the treatment of drug resistant CIPN.

Pathological effects and motor response threshold changes following radiofrequency application at various distances from the L-5 nerve root: an experimental study

OBJECT

Radiofrequency (RF) ablation is a minimally invasive technique often used percutaneously in the treatment of many conditions such as spasticity, pain, and osteoid osteoma. The purpose of this study was to assess the value of motor response threshold (MRT) as an indirect indicator of the RF generator's electrode to nerve distance, and to evaluate the effects of RF at various distances from a nervous structure.

METHODS

The L-5 nerve root was studied in 102 Sprague-Dawley rats (sham contralateral side). Motor response thresholds at 0, 2, 4, 5, and 6 mm from the nerve root were assessed before and after RF application for 2 minutes at 80° C on Days 0 and 7. Radiofrequency was applied 0, 2, 4, 5, and 6 mm away from L-5 and with the addition of interposed cortical bone. The effects of RF application on MRT were studied, and subsequent nerve injury was evaluated using light microscopy pathological examination.

RESULTS

There is a significant correlation between MRT and the distance between the electrode tip and L-5, with MRT less than 0.5 V when the electrode was in direct contact with the root. Electrical and pathological changes following RF application were more pronounced at 0 mm, with worsening seen on Day 7. Radiofrequency at 2 and 4 mm produced fewer electrical and histological deleterious effects on the nerve on Days 0 and 7, with an obvious improvement on Day 7. At 5 mm, electrical and histological abnormalities were minimal on Day 0 and were fully reversible on Day 7. At 6 mm and with interposed cortical bone, MRT and pathological findings were unchanged on Days 0 and 7.

CONCLUSIONS

The MRT proved to be a useful and reliable tool in decreasing nerve morbidity following RF ablation in animals and may be used in humans for the same purpose. It serves as an indirect indicator of the proximity of the RF generator's electrode tip to any adjacent motor nervous structure. A minimum safe distance of 5 mm between the electrode tip and the nerve is required to avoid irreversible nerve injury, unless a bony wall is interposed between them, thus serving as a nerve shield. In medical conditions that require RF ablation of the nerve, such as spasticity and pain, the MRT must be lower than 0.5 V. When a nerve lesion is to be avoided such as in cases of osteoid osteoma, an MRT higher than 2.5 V is considered safe, reflecting a distance greater than 5 mm.

Pulsed radiofrequency treatment in interventional pain management: mechanisms and potential indications-a review

Background

The objective of this review is to evaluate the efficacy of Pulsed Radiofrequency (PRF) treatment in chronic pain management in randomized clinical trials (RCTs) and well-designed observational studies. The physics, mechanisms of action, and biological effects are discussed to provide the scientific basis for this promising modality.

Methods

We systematically searched for clinical studies on PRF. We searched the MEDLINE (PubMed) and EMBASE database, using the free text terms: pulsed radiofrequency, radio frequency, radiation, isothermal radiofrequency, and combination of these. We classified the information in two tables, one focusing only on RCTs, and another, containing prospective studies. Date of last electronic search was 30 May 2010. The methodological quality of the presented reports was scored using the original criteria proposed by Jadad et al.

Findings

We found six RCTs that evaluated the efficacy of PRF, one against corticosteroid injection, one against sham intervention, and the rest against conventional RF thermocoagulation. Two trials were conducted in patients with lower back pain due to lumbar zygapophyseal joint pain, one in cervical radicular pain, one in lumbosacral radicular pain, one in trigeminal neuralgia, and another in chronic shoulder pain.

Conclusion

From the available evidence, the use of PRF to the dorsal root ganglion in cervical radicular pain is compelling. With regards to its lumbosacral counterpart, the use of PRF cannot be similarly advocated in view of the methodological quality of the included study. PRF application to the supracapular nerve was found to be as efficacious as intra-articular corticosteroid in patients with chronic shoulder pain. The use of PRF in lumbar facet arthropathy and trigeminal neuralgia was found to be less effective than conventional RF thermocoagulation techniques.

11. Lumbosacral radicular pain

Lumbosacral radicular pain is characterized by a radiating pain in one or more lumbar or sacral dermatomes; it may or may not be accompanied by other radicular irritation symptoms and/or symptoms of decreased function. The annual prevalence in the general population, described as low back pain with leg pain traveling below the knee, varied from 9.9% to 25%, which means that it is presumably the most commonly occurring form of neuropathic pain. The patient's history may give a suggestion of lumbosacral radicular pain. The best known clinical investigation is the straight-leg raising test. Final diagnosis is made based on a combination of clinical examination and potentially additional tests. Medical imaging studies are indicated to exclude possible serious pathologies and to confirm the affected level in patients suffering lumbosacral radicular pain for longer than 3 months. Magnetic resonance imaging is preferred. Selective diagnostic blocks help confirming the affected level. There is controversy concerning the effectiveness of conservative management (physical therapy, exercise) and pharmacological treatment. When conservative treatment fails, in subacute lumbosacral radicular pain under the level L3 as the result of a contained herniation, transforaminal corticosteroid administration is recommended (2 B+). In chronic lumbosacral radicular pain, (pulsed) radiofrequency treatment adjacent to the spinal ganglion (DRG) can be considered (2 C+). For refractory lumbosacral radicular pain, adhesiolysis and epiduroscopy can be considered (2 B+/-), preferentially study-related. In patients with a therapy-resistant radicular pain in the context of a Failed Back Surgery Syndrome, spinal cord stimulation is recommended (2 A+). This treatment should be performed in specialized centers.

Radiofrequência na dor crônica

Radiofrequência (RF) é uma técnica minimamente invasiva alvo-seletiva e tem sido usada durante muitos anos para o tratamento de diferentes doenças, como dor lombar crônica, neuralgia trigeminal e outros. Trata-se de uma corrente elétrica alternada com frequência oscilatória de 500.000 hz, que flui através de um eletrodo introduzido percutaneamente. O calor é formado ao redor do eletrodo porque o tecido age como um resistor. Essa técnica pode, portanto, ser usada para causar lesões em tecidos nervosos no tratamento de dor crônica. O objetivo desta revisão é abordar alguns aspectos importantes do mecanismo e evolução da radiofrequência na dor crônica. Serão abordados os aspectos básicos da Física e o mecanismo de ação da radiofrequência, método que tem sido usado para tratar dores crônicas de diferentes etiologias, além da evolução com o advento da radiofrequência pulsátil. O uso da radiofrequência no manejo da dor crônica é uma ferramenta útil em diferentes condições dolorosas e tem sido usada com sucesso por mais de 25 anos. Seu mecanismo de ação ainda não foi totalmente elucidado. A radiofrequência pulsátil é, em particular, uma técnica minimamente destrutiva e poderá ser uma alternativa à forma convencional de tratamento para a dor por radiofrequência.

Radiofrequency and pulsed radiofrequency treatment of chronic pain syndromes: the available evidence

There are currently 6 reviews on (pulsed) radiofrequency (RF) for the management of spinal pain. Two reviews on interventional pain management techniques in general also discuss RF. The outcomes of those reviews depend on the type of studies included and the opinion of the reviewers, which may result in different evidence levels. Radiofrequency denervation at the cervical and lumbar level has produced the most solid evidence. The differences in treatment outcome registered in the 5 randomized controlled trials (RCTs) regarding lumbar facet denervation can be attributed to differences in patient selection and/or inappropriate technique. There is not sufficient evidence supporting the use of RF facet denervation for the management of cervicogenic headache. The studies examining the management of cervical radicular pain suggest a comparable efficacy for RF and pulsed RF (PRF). The PRF treatment is supposed to be safer and therefore should be preferred. The superiority of RF treatment adjacent to the lumbar dorsal root ganglion for the management of lumbar radicular pain has not been demonstrated in an RCT. Information regarding RF treatment of sacroiliac joint pain is accumulating. No randomized sham-controlled trials on the value of RF treatment of the Gasserian ganglion for the management of idiopathic trigeminal neuralgia have been published. One RCT indicates superiority of RF over PRF for the management of idiopathic trigeminal neuralgia. Future research to confirm or deny the efficacy of (P)RF should be conducted in carefully selected patient populations. The tests used for patient inclusion in such a trial could potentially help the clinician in selecting patients for this type of treatment. The value of PRF treatment of the peripheral nerves also needs to be confirmed in well-designed trials.

Pulsed radiofrequency: a critical review of its efficacy

Pulsed radiofrequency is a growingly popular pain treatment modality. However, its clinical efficacy remains controversial. In this review, the available literature on pulsed radiofrequency is critically analysed to determine its clinical efficacy. Our search of the literature for pulsed radiofrequency yielded 341 citations; after reviewing the abstracts we found 51 relevant articles. There were 4 review articles: 44 articles pertained to the application of pulsed radiofrequency by an electrode placed in the vicinity of a neural structure. Of these only two were randomised controlled trials. Of the remaining 42 articles, one was a non-randomised controlled trial, three were prospective uncontrolled trials: there were six retrospective studies, 11 case reports, eight laboratory studies, two position papers, five editorials and seven items of correspondence, while one publication reported two studies. Three articles pertained to transcutaneous application of pulsed radiofrequency. Of the two randomised controlled trials, one reported efficacy of the pulsed radiofrequency while the other reported it to be ineffective. The majority of the uncontrolled and observational studies reported clinical efficacy of pulsed radiofrequency, however many of these studies had limitations. Further randomised controlled clinical trials are recommended in order for the practising pain physician to clearly understand the role of pulsed radiofrequency in the treatment of various chronic pain syndromes.

Irreversible spinal nerve injury from dorsal ramus radiofrequency neurotomy: a case report

Radiofrequency neurotomy (RFN) of the medial branches of the dorsal rami is a successful method of treating facet joint pain. Documented serious complications are rare. We discuss the case of a 33-year-old woman with low back pain (LBP) who sustained a right L5 nerve root injury during RFN. The patient had several months of axial LBP after a motor vehicle collision. She had no relief after anti-inflammatory medications, physical therapy, L5-S1 interlaminar epidural corticosteroid injections, and a right sacroiliac joint injection. She then received bilateral L3 and L4 medial branch and bilateral L5 dorsal ramus blocks with excellent temporary pain relief. Subsequently she underwent bilateral L3 and L4 medial branch and bilateral L5 dorsal ramus RFN. Afterward, she noticed new right leg pain and paresthesias extending throughout the L5 dermatome. Electromyography and magnetic resonance imaging were normal and she was diagnosed with a right L5 sensory radiculopathy. The right leg symptoms were unresponsive to multiple medications. After a successful trial with a spinal cord stimulator, she underwent permanent stimulator placement. Afterward, she had 90% relief of her right leg pain and discontinued all analgesics. Irreversible injury of nontarget nerves is a possible complication of RFN, and can be avoided by following proper procedural protocol.

Application of radiofrequency treatment in practical pain management: state of the art

Many therapeutic interventions for chronic pain are available, and decisions about optimal management are not easy to make. Radiofrequency (RF) treatment is classified as a percutaneous minimal invasive procedure for patients who do not respond to appropriate medical and physical therapy. Although RF treatment is widely used differences in current practice exist due to ongoing controversies.

Pulsed radiofrequency in lumbar radicular pain: clinical effects in various etiological groups

BACKGROUND

The purpose of this study was to evaluate the effectiveness of pulsed radiofrequency (PRF) applied to the lumbar dorsal root ganglion (DRG).

METHODS

A retrospective analysis of 54 consecutive patients who underwent 75 PRF procedures was performed. The patients were divided into three groups according to the etiology of the lesion (herniated disc [HD], spinal stenosis [SS], and failed back surgery syndrome [FBSS]). The analgesic efficacy of the technique was assessed using a 10-point Numeric Rating Scale (NRS) at baseline and, along with the Global Perceived Effect (GPE), at 30, 60, 90, and 180 days. The reduction in medications and the number of complications associated with the technique were assessed.

RESULTS

A decrease in the NRS score was observed in patients with HD (P < 0.05) and SS (P < 0.001), but not in those with FBSS. The GPE scores confirmed this finding. No complications were noted.

CONCLUSIONS

We observed that PRF of the DRG was significantly more efficacious in HD and SS than in FBSS patients. The application of PRF was not effective in FBSS.

Pulsed radiofrequency application in the treatment of chronic pain

The efficacy of pulsed radiofrequency (PRF) in the treatment of painful lumbosacral spondylosis has been reported. This case series reviews 22 consecutive patients presenting to clinic who had been previously treated with PRF with good results. Patients being prescribed opioids were excluded. During the PRF application, tissue temperature was limited to 43 degrees C. A minimum of 200 mA of current was delivered in each case. The minimum current (at 50 Hz) necessary to stimulate the involved nerve was recorded. The duration of time from PRF treatment until the patient requested a subsequent application was documented. The effective duration of PRF in patients treated for lumbosacral spondylosis ranged from 5 to 18 months (mean +/- SD: 9 +/- 3.7 months; n = 16). PRF applications to dorsal root ganglia were effective from 2 to 12 months (7 +/- 3.8 months; n = 8). Similar results were observed when PRF was applied to cervical medial branch nerves, one suprascapular nerve, and one stellate ganglion. The mean (50 Hz) sensory stimulation thresholds obtained before treatment ranged from 0.08 V to 0.14 V. In this select population of patients not receiving controlled substances, who had a favorable response to a previous PRF application, the duration of pain relief supports the use of PRF as an effective pain treatment.

The histopathological and electron-microscopic examination of the stereotactic pulsed radiofrequency and conventional radiofrequency thermocoagulation lesions in rat brain

OBJECTIVE

Neurodestructive procedures have been used for treating intractable pain for a long time. Pulsed radiofrequency (RF) is a newly defined energy type. Pulsed RF may be used in the treatment of patients with some pain syndromes in whom the pain could not be controlled by the alternative techniques. The objective of the present study was to examine the histological and electron microscopical changes in rat brain after pulsed RF application.

METHODS

Forty-five male rats were used in these experiments. Lesions were applied stereotactically to the target areas of the rat brains. Two different RF energy type were used as representative models of pulsed-RF and conventional-RF procedures. The rats were kept alive for 21 days and then killed. The effect of pulsed RF lesions on cerebral tissue ultrastructure was studied.

RESULTS

In the pulsed RF group, intracytoplasmic edema, clarity of the mitochondrial cristas and opening in the cell membrane pores were observed on the electron microscopic examination. In the conventional RF group, these findings were more prominent. In the pulsed RF group, the ratio of the effected neurons was 5.5% on light microscopic examination. In the conventional RF group, the ratio of the effected neurons was 14.26% and central necrosis was observed additionally.

DISCUSSION

Pulsed RF caused ultrastructural changes in the neurons. The pulsed RF may possibly cause a depression on the cell membrane potential by opening the cell membrane pores and resulting in the ion entrance into the cell cytoplasm and intracytoplasmic edema. However, it seems that all these changes were reversible.

Percutaneous radiofrequency lesion adjacent to the dorsal root ganglion in the treatment of spasticity and pain in children with cerebral palsy

The use of percutaneous radiofrequency (RF) lesion adjacent to the dorsal root ganglion (RF-DRG) in the treatment of pain has been established for years. A relatively novel indication for RF-DRG treatment is spasticity in children with cerebral palsy. In this article the pathophysiology and management of spasticity is discussed with an emphasis on the role of RF-DRG. In the management of spasticity, RF-DRG could prove to be a little invasive treatment option with little adverse effects.

Pulsed radiofrequency: current clinical and biological literature available

OBJECTIVE

Pulsed radiofrequency, where short bursts of radiofrequency energy are applied to nervous tissue, has been used by pain practitioners as a non- or minimally neurodestructive technique, alternative to radiofrequency heat lesions. Clinical advantages and mechanisms of this treatment remain unclear. The objective of this study was to review current clinical and laboratory data.

DESIGN

We systematically searched the MEDLINE database (PubMed) and tables of contents of electronically available pain journals. Reference lists of relevant reports and international scientific pain congress abstract books were also hand searched. Only those reports on pulsed radiofrequency were withheld.

RESULTS

The final analysis yielded 58 reports on the clinical use of pulsed radiofrequency in different applications: 33 full publications and 25 abstracts. We also retrieved six basic science reports, five full publications, and one abstract.

CONCLUSIONS

The accumulation of these data shows that the use of pulsed radiofrequency generates an increasing interest of pain physicians for the management of a variety of pain syndromes. Although the mechanism of action has not been completely elucidated, laboratory reports suggest a genuine neurobiological phenomenon altering the pain signaling, which some have described as neuromodulatory. No side effects related to the pulsed radiofrequency technique were reported to date. Further research in the clinical and biological effects is justified.

Pulsed radiofrequency adjacent to the cervical dorsal root ganglion in chronic cervical radicular pain: a double blind sham controlled randomized clinical trial

Cervical radicular pain affects approximately 1 on 1000 adults per year. Although many treatment modalities are described in the literature, the available evidence for efficacy is not sufficient to allow definitive conclusions on the optimal therapy to be made. The effect of pulsed radiofrequency treatment for this type of patients was evaluated in a prospective audit that showed satisfactory pain relief for a mean period of 9.2 months, justifying a randomized sham controlled trial. Twenty-three patients, out of 256 screened, met the inclusion criteria and were randomly assigned in a double blind fashion to receive either pulsed radiofrequency or sham intervention. The evaluation was done by an independent observer. At 3 months the pulsed radiofrequency group showed a significantly better outcome with regard to the global perceived effect (>50% improvement) and visual analogue scale (20 point pain reduction). The quality of life scales also showed a positive trend in favor of the pulsed radiofrequency group, but significance was only reached in the SF-36 domain vitality at 3 months. The need for pain medication was significantly reduced in the pulsed radiofrequency group after six months. No complications were observed during the study period. These study results are in agreement with the findings of our previous clinical audit that pulsed radiofrequency treatment of the cervical dorsal root ganglion may provide pain relief for a limited number of carefully selected patients with chronic cervical radicular pain as assessed by clinical and neurological examination.

Selective Nerve Root Injections Can Predict Surgical Outcome For Lumbar and Cervical Radiculopathy

OBJECTIVE

Diagnostic selective nerve root injection (SNI) results were analyzed in 101 patients who underwent lumbar or cervical decompression for radiculopathy and compared to surgical outcome 1 year postoperatively. A comparison of surgical outcomes was also examined between magnetic resonance imaging (MRI) and SNI results.

RESULTS

Of the 101 patients, 91 (90%) had positive and 10 had negative SNI results at the level operated. Ninety-one percent of the patients with a positive SNI had good surgical outcomes, whereas 60% of the patients with a negative SNI had good outcomes. Of the patients with a positive MRI result, 87% had good surgical outcomes, whereas a similar percentage of the patients with a negative MRI (85%) had good surgical outcomes. When findings between SNI and MRI differed (n = 20), surgery at a level consistent with the SNI was more strongly associated with a good surgical outcome. Of the patients with a poor surgical outcome, surgery was most often performed at a level inconsistent with the SNI finding.

CONCLUSIONS

Our study found that a diagnostic SNI can safely and accurately discern the presence or absence of cervical or lumbar radiculopathy. The diagnostic SNI can persuade surgeons from operating on an initially suspicious, but incorrect, level of radiculopathy. In cases where MRI findings are equivocal, multilevel, and/or do not agree with the patient's symptoms, the result of a negative diagnostic SNI (ie, lack of presence of radiculopathy) becomes superior in predicting the absence of an offending lesion.

The histologic effects of pulsed and continuous radiofrequency lesions at 42 degrees C to rat dorsal root ganglion and sciatic nerve

STUDY DESIGN

Experimental histologic study of the effects of radiofrequency (RF) or convective heating of the rat dorsal root ganglion or sciatic nerve to 42 degrees C.

OBJECTIVE

To determine whether treatment causes neuropathologic changes in an effort to explore the mechanisms and safety of pulsed RF pain therapy.

SUMMARY OF BACKGROUND DATA

Clinical data suggest that low temperature pulsed RF energy delivered to the DRG is a safe and effective form of therapy for low back pain. However, the mechanism by which this treatment modifies pain is unclear.

METHODS

A total of 118 Sprague-Dawley rats were divided into five groups for different RF and thermal treatments. All treatments increased tissue temperature to 42 degrees C. Treatments of the DRG included pulsed RF, continuous RF, and conductive heat. The generator output was increased until 42 degrees C was obtained in the tissue and was then maintained for 120 seconds. As a positive control, some rat sciatic nerves were treated with continuous RF lesions at 80 degrees C. Animals were killed for histologic study at 2, 7, or 21 days after treatment. Tissue was fixed in gluteraldehyde and embedded in plastic resin for detailed light microscopic neuropathologic evaluation.

RESULTS

The methods used to heat the tissue to 42 degrees C caused no significant difference in pathology. However, subclinical changes included endoneurial edema caused by alterations in the function of the blood-nerve barrier, fibroblast activation, and collagen deposition. Tissue returned to normal conditions by 7 days in nerve and 21 days in the DRG. These minor structural changes observed at the light microscopic level in normal animals do not exclude the possibility that there would be nonstructural changes in gene expression or cytokine upregulation in injured tissue. Lesions at 80 degrees C caused consistent thermal injury characterized by Wallerian degeneration of nerve fibers.

CONCLUSIONS

The data support the hypothesis that pulsed RF treatment does not rely on thermal injury of neurologic tissue to achieve its effect.

Radiofrequency lesioning using two different time modalities for the treatment of lumbar discogenic pain: a randomized trial

STUDY DESIGN

A prospective randomized trial.

OBJECTIVE

To evaluate the efficacy of percutaneous intradiscal radiofrequency thermocoagulation by modifying the duration of heating, using two different time methods, for relieving pain and improving functional disability.

SUMMARY OF BACKGROUND DATA

Lumbar discogenic pain is the major problem in lumbar degenerative disc disease that percutaneous intradiscal radiofrequency thermocoagulation has been suggested for as a nonsurgical invasive treatment technique for lumbar discogenic pain. In a previous controlled study, this method was found to be ineffective with 8 weeks of follow-up.

MATERIALS AND METHODS

Sixty patients with chronic low back pain were selected for provocative discography to diagnose the discogenic pain and to locate the discs to be treated. From this group, 39 patients were randomly selected and divided into two groups. In the first group, treatment was performed for 120 seconds, and in the second group for 360 seconds, both at 80C. Patients were assessed with a visual analogue scale for pain relief and functional improvement. Evaluations were performed before, immediately after treatment, at 1 and 2 weeks, and at 1, 3, and 6 months after the procedure.

RESULTS

A total of 39 patients with positive provocative discographies were found to eligible for the study. There were no statistical differences in pain relief and functional improvement between two groups (P > 0.05). The immediate, 1-week and 2-week, and 1-month visual analogue scale (VAS) scores were decreased significantly in both groups when comparing them with the pretreatment scores (P < 0.05). However, the final values after 6 months were similar to those measured at the beginning of the study (P > 0.05).

CONCLUSION

Percutaneous intradiscal radiofrequency thermocoagulation has been suggested and performed to relieve discogenic pain. In the previous controlled study, no effective pain relief has been obtained. In this study, the authors increased the duration of radiofrequency thermocoagulation to improve the effectiveness of this method. Yet, the authors have not found any significant differences between the application of lesioning at two different times in percutaneous intradiscal radiofrequency thermocoagulation.

Radiofrequency procedures in chronic pain

Radiofrequency current is simply a tool used for creating discrete thermal lesions in neural pathways in order to interrupt transmission. In pain medicine, radiofrequency lesions have been used to interrupt nociceptive pathways at various sites. This is a palliative treatment not without complications, so its use should be limited to those patients with cancer pain or chronic non-cancer pain for whom conservative non-surgical therapies have been ineffective or intolerable. With the development of alternatives such as intrathecal opioid infusion and neuromodulation technologies, the number of patients considered for neuroablative therapy may dwindle. Nevertheless, there is evidence that radiofrequency neurotomy has an important role in the management of trigeminal neuralgia, nerve root avulsion and spinal pain. In this chapter the evidence for efficacy and safety is reviewed and interrogated with special emphasis on the available randomized controlled trails and systematic review.

Radiofrequency ablation for chronic pain control

With an increased knowledge of neural anatomy and technologic improvement, radiofrequency ablation (RFA) became an often-used technique for the pain control over an extended time period. Today, RFA is used safely for spinal pains of facet or discogenic origin, sympathetically maintained pain, and other pains of neural origin.

The morphological effects of a radio frequency lesion adjacent to the dorsal root ganglion (RF-DRG)--an experimental study in the goat

Radiofrequency (RF) lesions adjacent to the dorsal root ganglion (DRG) are increasingly used in the treatment of intractable chronic pain of spinal origin. Opinions differ on which nerve fibres are affected by these lesions. Morphological studies have been carried out to assess the effects of radiofrequency lesions on nervous tissue. Interpretation has been difficult, since most studies have been performed under circumstances which are not comparable to the clinical situation. This study was undertaken to investigate morphological effects of RF lesions as they develop in the normal clinical situation. In two goats 22 G 100 mm SMK electrodes with a 5 mm active tip (Radionics) were positioned posterior to the lumber dorsal root ganglia (DRG). Sixty-second 67;C lesions were made on one side. The contralateral side was used as sham operation. The goats were sacrificed 2 weeks after the procedure. The lesions were studied for size as well as for effects on proliferation and regeneration using Ki-67 (MIB-1). Lesions made inside the DRG (23) were 1.8-2.0 mm in size. In these lesions there was a total loss of myelinated fibres. In lesions made adjacent to the DRG there was a significantly higher MIB-1 labelling on the treated side as compared to the sham-treated side. An RF lesion inside the DRG destroys myelinated fibres. A lesion adjacent to the DRG increases MIB-1 activity, indicating proliferation and regeneration after 2 weeks, despite the fact that the lesion was made outside the ganglion.

Long-lasting analgesic effect of radiofrequency treatment of the lumbosacral dorsal root ganglion

OBJECT

The authors conducted a study to establish the benefit of radiofrequency (RF) treatment of the lumbosacral dorsal root ganglion (DRG) as a therapy to reduce symptomatic pain in patients with chronic spinal pain radiating to the leg.

METHODS

Two hundred seventy-nine patients were evaluated after undergoing their first RF treatment of the DRG. A four-point pain perception scale was used. Short-term effect was documented after 2 months. The influence of surgical history on outcome was examined by using chi-square analysis. The mean duration of analgesic effect was calculated by applying a probit survival analysis. Two months after undergoing RF treatment, 59% of patients reported satisfactory pain reduction. No serious adverse effects were noted. Surgical history was shown to have no significant effect on outcome. The long-term half-life time of pain reduction was 44.5 months.

CONCLUSIONS

The use of RF in the treatment of DRG appears to be a useful and safe therapy in patients with chronic spinal pain that radiates to the leg. The initial success rate is approximately 60%. It seems to lead to a time-limited effect on the target structure, and the mean duration of pain reduction is approximately 3.7 years. The mechanism of action remains unclear.