A magnetic resonance imaging study of local anesthetic spread in patients receiving an erector spinae plane block.

Purpose: Despite the popularity of the erector spinae plane (ESP) block, both the mechanism of the block and the extent of injectate spread is unclear. This study used magnetic resonance imaging (MRI) to evaluate the spread of local anesthetic injectate following ESP blocks in six patients with pain.

Methods: Six patients received a left-sided ultrasound-guided ESP block at the T10 level. The injectate contained 29.7 mL of 0.25% bupivacaine and 0.3 mL of gadolinium in the first patient, with an additional 5 mL (50 mg) of triamcinolone in the subsequent five patients. Sensory block to pinprick and cold as well as pain score (with 0 indicating no pain and 10 being maximum pain) were assessed 20 and 30 min respectively following the ESP block. MRI was performed one hour after the block.

Result: The injectate spread into the intercostal space and neural foramina in all six patients, but the extent of cephalocaudal spread was variable, with a median [interquartile range] spread of 9 [5-11] and 3 [2-6] levels for the intercostal space and neural foramina, respectively. The injectate also spread extensively within the erector spinae muscles. Spread to the epidural space was seen in two patients. Sensory block was achieved in both ventral and dorsal dermatomes in all patients, though the extent was variable.

Conclusions: Our study showed that the ESP block injectate consistently spread to the erector spinae muscles, neural foramina, and intercostal space. It was associated with sensory changes and pain relief in the dorsal and ventral thoracic and abdominal walls. Nevertheless, the extent of spread to the neural foramina and intercostal space, and the sensory block itself, was highly variable.