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.
Sensory innervation of the upper posterior thorax arises from the dorsal rami of the first cervical (C1) through the fifth lumbar (L5) nerves; while the ventral rami of the thoracic spinal nerves from T1-T12 continue as intercostal nerves innervating the anterolateral chest and abdominal wall.
ESSENTIAL FACT: The erector spinae plane 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.
With the transducer placed in a paramedian sagittal orientation at the level of T5, the trapezius, rhomboid, and erector spinae muscles appear as longitudinal hypoechoic structures superficial to the transverse processes that are visualized like squared hyperechoic rims with an acoustic shadow behind.