Background and Objectives The routes of distribution and barriers to flow of solutions in the epidural space are incompletely determined. This study examined macroscopic details of epidural injectate spread in postmortem humans by cryomicrotome imaging.

Methods Soon after death, 3 nonembalmed adult human subjects were injected with ink through epidural catheters inserted by standard techniques. Following freezing, microtome sectioning was performed to reveal anatomic features down to 100 μm. To control for effects of death, an adult baboon was injected during general anesthesia and subsequently examined in the same fashion.

Results Injected ink was readily evident and showed spread as rivulets through numerous small channels rather than as a unified advancing front. The fascia that extends laterally from the posterior longitudinal ligament is an important barrier restricting solution flow. Solution preferentially traveled along the nerve root sheath through the intervertebral foramen.

Conclusions Distribution of solution in the epidural space is nonuniform. Rather than a uniform advancing front, spread is directed among paths between structures according to pressures by which they are compressed. No structural barriers block flow through the intervertebral foramina or spinal canal other than the fascia of the posterior longitudinal ligament.