Path of Least Resistance: Multilevel Epidural Spread Following Large Volume Intertransverse Process Injection

To evaluate paraspinal injectate spread following intertransverse process (ITP) block using 10 mL versus 20 mL in cadaveric models.

Prospective.

An anatomy laboratory at an academic medical institution.

Adult cadavers (6 traditionally embalmed, 2 GreenMBalmed).

Under ultrasound guidance, 0.1% methylene blue dye was injected into the T3-4 or T7-8 ITP space using the mid-transverse process technique. A total of 12 injections were performed, with 6 injections per volume.

In 8 adult cadavers, retropleural dissection and multilevel vertebral corpectomy assessed dye spread to the intercostal nerves, paravertebral space, and anterior/posterior epidural spaces. Paravertebral spread occurred in 11 of 12 injections. With 10 mL, the median spread was 2 levels in the paravertebral space and 1.5 levels in the intercostal space. With 20 mL, the median spread was 2 levels in both spaces. Epidural spread (anterior and/or posterior) occurred in 5 of 6 injections in each volume group. However, multilevel epidural spread (≥3 levels) occurred in 3 of 6 injections with 20 mL and 0 of 6 with 10 mL. With 20 mL, anterior epidural spread ranged 0 to 5 levels and posterior epidural spread ranged 0 to 7 levels, reflecting greater variability and occasional extensive spread. No statistically significant between-volume differences in median spread were detected.

These findings support anatomical continuity between the ITP and epidural spaces. Compared with 10 mL, 20 mL ITP injections showed greater variability and occasional extensive epidural spread, which may increase the risk of sympathectomy-related effects such as hypotension.