Comparing the injectate spread and nerve involvement between different injectate volumes for ultrasound-guided greater occipital nerve block at the C2 level: a cadaveric evaluation.

Purpose: The spread patterns between different injectate volumes have not yet been investigated in ultrasound-guided greater occipital nerve (GON) block at the C2 level. This cadaveric study was undertaken to compare the spread pattern and nerve involvements of different volumes of dye using this technique. Materials and methods: After randomization, ultrasound-guided GON blocks with I or 5 mL dye solution were performed at the C2 level on the right or left side of five fresh cadavers. The suboccipital regions were dissected, and nerve involvement was investigated. Results: Ten injections were successfully completed. In all cases of 5 mL dye, we observed the deeply stained posterior neck muscles, including the suboccipital triangle space. The suboccipital and third occipital nerves, in addition to GONs, were consistently stained when 5-mL dye was used in all injections (100%). Although all GONs were successfully stained in the 1-mL dye cases, three of five injections (60%) concomitantly stained the third occipital nerves. Conclusion: The clinical efficacy of this technique using the 5-mL injectate seems unlikely to arise from the blockade of GON alone. Instead, its efficacy likely arises from the blockade of most nerves originating from the dorsal ramus of the upper cervical spinal nerve at the suboccipital area. Even using 1 mL, of injectate may not guarantee blockade of the GON alone.