Pulsed radiofrequency of sural nerve with the aid of ultrasound may result in resolution of chronic foot pain: A case report and review of literature

Dear Editor, Ultrasound may result in better efficacy of pulsed radiofrequency (PRF) due to visualization of the targeted nerve and needle tip. We report one such patient. A 47-year-old female presented with right chronic foot pain (CFP) of unknown etiology from 12 years starting from the lateral aspect of the foot to the plantar region without sensorimotor deficit, diurnal, and postural variation, and no specific aggravating and relieving factors. Nerve conduction studies were normal. A numeric rating scale (NRS) pain score of 6/10 was reported with oral medications of duloxetine, pregabalin, and amitriptyline, physiotherapy, corrective footwear, and lifestyle modifications. Sural nerve (SN) PRF was planned for long relief. A linear ultrasound probe (Sonosite 5-10 mHz ©) identified SN as a small hyperechoic structure proximal to the lateral malleolus (LM), lateral to the lateral saphenous vein (LSV), and superficial to the deep fascia [Figure 1]. After a favorable diagnostic block, two cycles of PRF at 45 V, 42° were performed for 120 seconds cycle each with a 20 gauge 5 cm needle, 5 mm active tip (Halyard©) after negative motor stimulation with 2 Hz 2 mV and positive sensory stimulation at 50 Hz at 0.7 V after injecting 0.5 ml lignocaine 2%. NRS 0-1 was reported within 24 hours of the procedure. Pain NRS remained 0-2 for a year with oral pregabalin 75 mg once a day. The above procedure was repeated at one year when pain NRS was 5-6. The patient is pain free and on regular follow-up, and has consented to this report.Figure 1: The sural nerve close to the saphenous vein (SSV). The needle location close to the nerve is marked with triangular white solid arrows. PBM – peroneus brevis muscleFoot pain is an unpleasant sensory and emotional experience following perceived damage to any tissue distal to the tibia or fibula. CFP is characteristically diffuse, spreads beyond the original site of injury, and exhibits a nonlinear relationship between nociception and pain intensity. Different nerve blocks, like posterior tibial, sural, medial and inferior calcaneal, superficial peroneal, and lateral plantar nerve blocks, have been tried for CFP. Colored Doppler ultrasound aids in the identification of a saphenous vein located between the posterior border of LM and the Achilles tendon at the ankle. SN is located lateral to LSV and superficial to the deep fascia and is identified with an internal fascicular pattern of hypoechoic nerve fascicles surrounded by the hyperechoic connective tissue.[1] A linear ultrasound probe is used with an in-plane or out-of-plane needle approach [Figure 2].Figure 2: Probe position for in-plane needling to Saphenous nerveA literature search describes eight SN PRF in five reports for CFP with a duration of less than 5 years of which seven are landmark-guided and one with ultrasound[1-5] [Table 1]. In the present patient, CFP of 12 years was probably resolved due to an effective procedure with the aid of ultrasound.Table 1: Case summary of sural nerve blocksWe conclude that for CFP, SN PRF with ultrasound-guided guidance may increase its efficacy due to precise location and needle tip visualization. Consent has been taken from the patient for publication of this report. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.