Currently Recommended TON Injectate Volumes Concomitantly Block the GON: Clinical Implications for Managing Cervicogenic Headache

Background: Headache (HA) is a significant cause of morbidity globally. Despite many available treatment options, HAs that are refractory to conservative management can be challenging to treat. Third occipital nerve (TON) and greater occipital nerve (GON) irritation are potential etiologic agents of primary and cervicogenic HAs that can be targeted using minimally invasive treatment options such as nerve blocks or radiofrequency ablation. However, a substantial number of patients that undergo radiofrequency ablation do not experience pain relief despite a positive diagnostic medial branch block (MBB). Objective: In this study, we investigate the underlying cause for the high rate of false positives associated with MBBs by evaluating injectate spread in cadaveric subjects. Study Design: Cadaveric study. Setting: Academic medical center. Methods: After obtaining exemption status from our Institutional Review Board, TON injections were performed on 5 preserved cadavers, a total of 10 TONs, using anatomic landmarks, partial dissection, and palpation to guide needle placement. Cadaveric dissections were performed to evaluate the location, vertical spread, and grossly observed injectate coating of the TON and GON for each quantity of methylene blue injectate, 0.3 mL and 0.5 mL, administered. Results: The average distance between the TON and GON at their respective foraminal exit points was 1.81 cm. The average vertical spread for 0.3 mL and 0.5 mL of methylene blue injectate was 2.02 + 0.35 cm and 3.26 + 0.48 cm when performing a TON block. When using 0.3 mL injectate, both the TON and GON were simultaneously coated 60% of the time. After increasing the injectate volume to 0.5 mL, both the TON and GON were simultaneously coated 100% of the time. Limitations: The cadaveric design of this study presents limitations when translating cadaveric findings to the clinical setting. Also, the small sample size limits its power and generalizability. Lastly, the potential for researcher bias exists as the investigators were not blinded. Conclusions: This study demonstrates that currently recommended injectate volumes for TON blocks may result in concomitant coating of the GON. Conventional radiofrequency ablation (RFA) of these nerves may not lesion both the TON and GON given its restrictive circumferential lesioning diameter of 5 – 7 mm. As such, interventionalists should consider performing radiofrequency ablation to both the TON and GON after a positive TON block. Key Words: Chronic pain, cervicogenic headache, third occipital nerve, greater occipital nerve, injectate spread, radiofrequency ablation