Dorsal root ganglia (DRG), which contain the cell bodies of primary afferent neurons transmitting sensory information from the periphery to the central nervous system, play a key role in the pathogenesis of chronic pain syndromes caused by spinal pathology and peripheral nerve injury. The evidence supporting a primary role for the DRG in chronic pain states has led to the growing use of treatment directed at dorsal root ganglion. Pulsed radiofrequency of the dorsal root ganglion (PRF-DRG) is a potentially attractive alternative to epidural steroid injection in the treatment of chronic lumbar radicular pain. It is target-specific and avoids the use of steroids, thereby eliminating their potential side effects.
The DRG has a pivotal role in the pathophysiology of radicular pain. Pulsed radiofrequency treatment of the lumbar DRG in patients suffering lumbosacral radicular pain can reduce pain and the need for analgesic medication for a sustained length of time. PRF is based on neuromodulation on dorsal root ganglion; therapeutic principles are not fully understood, but the technique has shown promising results in many fields of application, including chronic post-surgical thoracic pain, chronic low back pain and chronic radicular pain. The fluoroscopy guidance allows an accurate location of target ganglion, ensuring high safety and efficacy profile of the whole procedure.
Lumbosacral radicular syndrome 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, lies between 9.9% and 25%. Also, the point prevalence (4.6–13.4%) and lifetime prevalence (1.2–43%) are very high, which means that lumbosacral radicular pain is presumably the most commonly occurring form of neuropathic pain.
Acute lumbosacral radicular pain completely or partially resolves in 60% of the patients within 12 weeks of onset. However, about 30% of the patients are still suffering from pain after 3 months to 1 year.
Dorsal root ganglia are large collections of neurons on the dorsal spinal roots. Ganglia are usually located in the intervertebral foramina, immediately lateral to the perforation of the dura mater by the roots. The first and second cervical ganglia lie on the vertebral arches of the atlas and axis, respectively. The sacral ganglia lie inside the vertebral canal, and the coccygeal ganglion usually lies within the dura mater.
Following peripheral nerve injury, ion channel modulation occurs leading to nociceptor sensitization, expansion of receptive fields, diminished central inhibition, increased neuronal excitability in the spinal cord and reorganization in the dorsal horn. Ion channel alterations that are in part responsible for these effects include proliferation of voltage-dependent sodium channels in the DRG, down regulation of voltage-gated potassium channels and increased expression of the calcium channel alpha-2 delta-1 subunit.
A growing body of literature supports a relationship between peripheral nerve injury and sympathetic sprouting in the DRG. Hyperexcitability and ectopic firing occur not only at the site of injury, but also in DRG cell bodies The end result of these changes is peripheral and central sensitization, manifesting as spontaneous pain, hyperalgesia, and allodynia.
Pulsed radiofrequency (PRF) was developed as a modification of the well-known radiofrequency ablation treatment. In conventional radiofrequency ablation, a high frequency alternating current is used to produce coagulative necrosis of the target nerve tissue without any selectivity for nociceptive fibers. However, in PRF, a current in short (20 msec) high voltage bursts is followed by silent phases (480 msec) which allow for heat dissemination, keeping the target tissue temperature controlled below 42°C. The mechanisms via which PRF causes analgesia are still not clearly understood, but laboratory experiments have highlighted some possible ways in which it might act, including its effects on neuropathic pain. Clinical use of PRF has been expanding, despite there being limited evidence of clinical efficacy in the form of randomized controlled trials (RCTs).
Chronic lumbar radicular (CLR) pain is a term used to describe neuropathic pain symptoms in the distribution of a particular lumbar nerve root due to disc protrusion, spinal stenosis, facet hypertrophy, or fibrosis after previous surgery. The pathophysiology of CLR pain involves mechanical, inflammatory, and immunologic factors that affect the function of the dorsal root ganglion (DRG).
The procedure is usually done on a daycase basis. The procedure is performed under fluoroscopic guidance to ensure accuracy of needle placement. A special RF needle is positioned close to the dorsal root ganglion at the appropriate level and sensory stimulation with 50 Hz is applied to confirm accurate positioning of the needle. Motor stimulation at 2 Hz is used to determine a threshold 1.5–2.0 times greater than the sensory threshold to avoid placement near the anterior nerve root. Pulsed radiofrequency is applied for 10 minutes at 45V with tissue temperature maintained at 42°C. Small amount of local anaesthetic is injected around the DRG at the end of the procedure.
Pain relief from the procedure may last between 6 months to 2 years, though this may not always be the case. Patients need to be aware that the outcome of the procedure is variable and they may not receive the desired benefits. Similarly, they must be aware of the transient nature of the therapeutic benefits and that they may need repeated injections.
Complications are rare with pulsed radio frequency treatments. However, as with any procedure, there are inherent risks including failure of procedure, bruising, infection, hepatoma and nerve injury. The procedure can cause a temporary flare of pain and any benefit from the procedure may not be evident for up to 8 weeks following treatment.