Evaluating the Anatomic Spread of Selective Nerve Scalp Blocks Using Methylene Blue: A Cadaveric Analysis

The modern scalp block consists of local anesthesia injections that target the supraorbital, supratrochlear, zygomaticotemporal, auriculotemporal, and greater and lesser occipital nerves. Limited data exist on the local anesthetic spread that occurs with this technique. This study examines the extent of the spread of a scalp block using methylene blue in a cadaveric model.

A traditional landmark-based scalp block was performed on 6 unembalmed human cadavers using 25-G, 1.5-inch needles to inject 1 to 2 mL of methylene blue 0.1% at each nerve bilaterally; a total volume of 20 mL was injected. The cadavers were then dissected, and the spread of injectate was measured and recorded.

All the nerves required for analgesic coverage were appropriately stained by the injections, except for in 2 specimens where the lesser occipital nerve could not be identified. The zygomatic (stained in 2 of 8 specimens) and temporal (stained in 5 of 8 specimens) branches of the facial nerve were stained as a result of the zygomaticotemporal and/or auriculotemporal injections. Tracking from the zygomatic injection site was noted as far inferior as the temporalis muscle attachments on the mandible.

This cadaveric study confirms that the landmark-based technique for scalp blocks consistently stained all 6 nerves involved in scalp innervation. There was significant unintentional spread to the branches of the facial nerve when using the landmark technique.