We evaluated a 67-year-old man with a 3-year history of posterior neck pain and headache with vertigo that resulted from a head injury sustained in an accident. The medical and family histories were negative for headaches. There were no significant findings on a brain magnetic resonance (MR) image immediately after the accident and a brain MR angiographic image 1 year after the accident. A neurosurgeon referred him to our pain clinic for pain control after confirming normal findings of a computed tomography scan of the temporal bone and cerebellopontine angle cistern.

The chief complaint was posterior neck pain which continued all day except while at work. There was a pain-relieving point in the posterior neck area that reduced the pain when he applied pressure with his finger. He rated his pain as 3-4/10 using a 0-10 numerical rating scale. The physical examination revealed that there was no mass-like lesion in the neck area and no pain related to neck motion. There was only degenerative changes in the simple X-ray image of cervical spine (Fig. 1). Vertigo with headaches began after the accident and occurred approximately 4 times a day and lasted 5 minutes. Nausea accompanied the vertigo, but there was no loss of consciousness. There were no aggravating or relieving factors related to the vertigo. He had tinnitus and fullness in his left ear.

With a diagnosis of cervicogenic headaches, he underwent C2 dorsal root ganglion and third occipital nerve blocks with 2 ml of 0.5% lidocaine and 10 mg of triamcinolone at each site guided by radiologic imaging. Infiltration of local anesthetics near the pain-relieving point was done. He was treated with medication for true vertigo with a diagnosis of Meniere's syndrome. Five days later, the headaches improved, but the posterior neck pain persisted. The injection of local anesthetics had a temporary effect. Thus, 10 days after the 1^st visit, after explaining the procedure, efficacy, and side effects of PRF, we placed the patient in a prone position. Under radiologic image guiding, the pain-relieving point in the soft tissue of the posterior neck was between the 2^nd and 3^rd cervical vertebrae, 1.5 cm from the midline. The 8-cm RF electrode with a 5-mm active tip was advanced to the pain-relieving point at a depth of 1 cm (Fig. 2). After clarifying the location of the pain-relieving point through test stimulation with 50 Hz and 0.5 mA that shows concordant pain as usual, we performed PRF for 120 seconds at 40℃. Immediately after the procedure, the patient rated his pain in the posterior neck as 0/10 using a 0-10 numerical rating scale. Eleven days after the PRF procedure, relief of headache and posterior neck pain was maintained with slight dizziness. The patient reported continued relief of headache and posterior neck pain by telephone follow-up for 5 months.

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Unlike conventional RF treatment with continuous thermal energy, PRF treatment using high-voltage energy (usually 45 V) at a 20-msec burst, followed by a 480-msec rest period at a less neurodestructive temperature not to exceed 42℃, has been reported as an effective pain treatment modality.

The mechanism of PRF without the significant heat-induced tissue damage is debatable. A popular theory is that the rapidly changing electronic fields produced by PRF alter the transmission of pain signal via a pathway involving c-Fos, Previous study has shown that PRF increased c-Fos expression in the dorsal horn, a response that was sustained as long as 7 days after treatment [6]. This result also hinted at the inhibition of excitatory C fibers and long term depression.

Since previous studies have reported the effects of PRF with a lower complication rate, the application of PRF is increasing, even on the non-nervous tissues. RF ablation applied to non-nervous tissue aims to destroy target tissues; however, the indication is very limited. It is necessary to maintain a temperature of 60-100℃ to achieve complete ablation of the soft tissue, especially tumor. For osteoid osteomas resulting in severe pain, tumors less than 15 mm in size are not appropriate for surgery and are generally treated with RFA. For metastatic bone tumors, the treatment of choice is radiotherapy. However, recurrent pain after radiotherapy could be treated with RFA [4]. The mechanism of pain relief after RFA is not well-understood. In a previous study, possible mechanisms of pain relief of tumors after RFA include the following: destruction of sensory nerves of periosteum, thus blocking the transmission of pain; decreased stimulation to sensory nerve endings by reduction of tumor volume; and direct destruction of tumor cells secreting cytokines, resulting in stimulation of nerve endings [7]. The application of RFA for soft tissue tumors has been limited because the safety and efficacy of RF in the treatment of soft tissue tumors has not been investigated systematically. However, there are some case reports involving patients with soft tissue tumors have been treated with RFA and shown a good response [8].

Tamini et al. treated nine patients with myofascial trigger points and scar neuromas [5]. In 8 of 9 patients, a 75-100% reduction of pain was achieved immediately after the PRF procedure at tender points. In 6 of 9 patients, pain relief was sustained over 6 months. The authors suggest that PRF of the trigger point could be a minimally invasive, less effective method to control pain. The patient with a more diffuse painful condition shows a less effective response. The effects of PRF have limits for the treatment field. This result is consistent with the previous theory that the neuromodulatory effect of PRF is derived from the voltage change of nervous cell membranes near the needle tip.

This patient had an unusual pain-relieving point measuring 1.5 × 1.5 cm that resulted in relief of the posterior neck pain with pressure. The pain-relieving point differed from a trigger point that results in severe pain with pressure. Trigger points are discrete, very sensitive areas of skeletal muscle that contain palpable, taut bands of muscle. At the point that it showed unusual pain aspect with pressure, we elected to use PRF in the treatment of focal pain having a pain-relieving point.

We performed PRF after clarifying the location of the pain-relieving point through test stimulation with 50 Hz and 0.5 mA that shows concordant pain as usual. We thought that the placement of electrode tip was in a subcutaneous tissue. Because, the 8 cm RF electrode with a 5 mm active tip was advanced to the pain-relieving point at a depth of 1 cm without bone touch or fascial click. Considering the formation of electric field by PRF electrode, PRF lesion could be located in front of electrode. Therefore, the possibility that PRF lesion could be placed at the fascia or periosteum cannot be excluded.

Although we have observed positive results from PRF treatment, knowledge of the precise mechanism and technique remains elusive. It is clear that the PRF procedure to soft tissues has limited theoretical background and clinical outcomes. Notwithstanding the shortcomings of this report, the positive response demonstrated through this review suggests that further systemic evaluation of this treatment approach is warranted.

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