Radiofrequency (RF) ablation is a minimally invasive percutaneous procedure that is thought to reduce pain by altering the transmission of pain impulses [4, 5]. Conventional RF ablation (CRFA) causes controlled tissue destruction by irreversible coagulative necrosis [6]. However, recent evidence demonstrates that CRFA provides only a transient sensory loss in contrast to a much longer duration of pain relief. Electric fields produced by RF current may induce changes in the nerve cells and alter pain processing mechanisms at various sites, particularly at the molecular level [4].

All RF ablation techniques involve the transfer of alternating RF current (450–1,200 kHz) through insulated needle electrodes. The electrode is insulated, except for 2–10 mm at the tip. RF needles tend to be smaller than cryoprobes (Fig. 16.2). The electrode is positioned close to the target using nerve stimulators and image guidance. In conventional RF neurotomy, the electrode is positioned parallel to the target as the electrode coagulates transversely. A generator produces an electric field concentrated at the uninsulated tip of the electrode. The transfer of energy generates molecular oscillations that produce ionic friction and heat within the tissues. Once the cells are heated above a certain temperature, controlled tissue destruction occurs causing a lesion surrounding the uninsulated tip [6]. Early studies claimed that RF coagulation destroyed Aδ and C fibers preventing nociception. However, subsequent studies have shown that RF coagulation is non-selective and disrupts all nerves [4–7].

Pulsed RF ablation (PRFA) was described by Slappendel et al. that compared the outcomes of cervical RF at the dorsal root ganglion (CRFA) in patients treated with lesions made at 40 °C to those at 67 °C [8]. While changing the temperature did not impact efficacy, it was believed that the RF current itself was therapeutic because of the overall electrical effects on the target nerve. The therapeutic effect of PRFA current is provided by applying brief bursts of RF energy and allowing the heat to dissipate at the target tissues, avoiding further damage to the nerve [9].

In PRFA, a current of 50,000 Hz is usually delivered in 20-msec pulses at a frequency of 2 per second (other protocols have been described). The electrode temperature is limited to 42 °C preventing any thermal lesion [8, 10]. The current is densest distal to the tip of the electrode and in contrast to CRFA, the electrode is applied perpendicular to the target nerve. Laboratory studies show that heating a nerve to a lower temperature (40–45 °C) causes reversible conduction blocks, but no pathologic lesion is produced [9, 11]. The distance of the electrode from the nerve target influences tissue damage in both CRFA and PRFA. Within 500 μm from the electrode, both CRFA and PRFA protocols produced tissue damage. Between 500 and 1,000 μm, tissue damage occurred with CRFA protocols, but not in PRFA protocols. Electron microscopy shows that ganglia treated with CRFA causes significant neuronal damage, whereas the ganglia treated with PRFA leaves nuclear membranes intact. However, studies demonstrating the efficacy of PRFA is limited as truly randomized clinical trials are lacking [8, 11].

Cooled RF ablation (cooled RFA) is a newer RF technique used to treat various pain syndromes. The mechanism of pain relief is similar to CRFA. An electrode is placed close to the target nerve and conduction is disrupted relieving the pain. Cooled RFA utilizes a specialized electrode which is actively cooled by a continuous flow of water at ambient temperatures. This prevents the electrode from acquiring high surrounding tissue temperatures and increases the overall exposure to the RF current, heating larger tissue volumes with a higher thermal lesion. Similar to conventional RF, the lesion size depends on the size of the probe, the electrode temperature, and the duration of RF current that is applied [12]. Perhaps by delivering larger amounts of RF current to the target nerve, cooled RFA can be used in treating pain syndromes where conventional RF is unsuccessful [13, 14].

Most painful stimuli from abdominal viscera are transmitted by unmyelinated C fibers found in muscle, periosteum, mesentery, peritoneum, and viscera. The pain is characterized as dull, cramping, burning, gnawing, and gradual in onset. Secondary autonomic effects such as sweating, restlessness, nausea, vomiting, perspiration, and pallor can accompany the visceral pain. Abdominal visceral nociceptors respond to both mechanical and chemical stimuli. Visceral pain tends to be midline as sensory afferents are sent to both sides of the spinal cord. It is also poorly localized since the innervation is multi-segmental and the number of nerve endings is minimal [15]. The afferent fibers that mediate abdominal visceral pain usually follow the distribution of the autonomic nervous system, and consequently, the autonomic ganglia are the main targets for pain relief.