The epidural space is a ‘potential space’ that contains fatty tissue, nerve fibers and blood vessels; and is located between the bony vertebral canal and the outer surface of the dura mater. The vertebral column is stabilized by ligaments. The ligamentum flavum is the structure through which the epidural needle and catheter must pass when being inserted to reach the epidural space. The epidural space contains fat which surrounds and pads the spinal cord. This fat functions as a ‘depot’ for opioids and local anesthetics when these medications are administered for epidural analgesia. Opioids administered into the epidural space diffuse across the meninges and CSF to receptors in the dorsal horn of the spinal cord.
Moving towards the spinal cord from the epidural space are the membranes or meninges that cover the spinal cord. They are: 1) the dura mater; 2) the arachnoid mater; and 3) the pia mater which adheres tightly to the spinal cord and brain. The intrathecal space (also known as the subarachnoid space) is the space that lies between the arachnoid and pia mater, and contains the cerebral spinal fluid (CSF). Opioids delivered intrathecally diffuse through the CSF into the dorsal horn of the spinal cord, but because the site of delivery is closer to the spinal cord, only 1/10th the amount of opioid is required intrathecally compared with epidural route of delivery.