The “erector spinae” comprises a group of muscles including the iliocostalis, longissimus, and spinalis muscles.
They run bilaterally from the skull to the pelvis and sacral region, and from the spinous to the transverse processes, extending to the ribs.
The muscles change their size and profile during their craniocaudal course alongside the spine. As part of the “core” muscles, one of their main functions is to stabilize the spine.
The erector spinae plane nerve block (ESPB) is a recently introduced technique, and clear indications are still not well defined. Likewise, the mechanism of action is not fully understood; some studies suggest that an anterior diffusion of the local anesthetic into the paravertebral space could be one of the explanations, although an interfascial spread toward the posterior rami of spinal nerves is probably the main mechanisms of action.
After selecting the target transverse process for the nerve block, place the transducer in a paramedian sagittal orientation, approximately 2cm away from the midline (spinous processes), and try to vizualize the transverse process.
At higher thoracic levels, e.g., above T5; trapezius, rhomboid major and erector spinae muscles can be identified as three layers superficial to the transverse processes. In the lower and mid-thoracic levels, only the trapezius and erector spinae muscles can be seen.
If the transducer is placed too medial, the thoracic laminae will be visualized as flat hyperechoic lines. To fix: Slowly slide the transducer laterally.
When the transducer is placed too lateral, ribs will be visualized as rounded acoustic shadows with an intermediate hyperechoic pleural line. To fix: Slowly slide the transducer medially.
Insert the needle in-plane from a cranial to caudad direction until the needle tip contacts the transverse process.
Inject 1-3 mL of local anesthetic to confirm proper injection plane by visualization of a spread deep to the erector spinae muscles and superficial to the transverse process.
Complete the nerve block with 20-30 mL of local anesthetic.