Pathophysiology
Clinical meaning
The epidural space is a potential space located between the ligamentum flavum and the dura mater within the vertebral canal. It extends from the foramen magnum to the sacral hiatus and contains loose areolar connective tissue, fat, lymphatics, spinal nerve roots, and an extensive venous plexus known as the Batson plexus. The epidural space is accessed most commonly in the lumbar region (L2-L4 or L3-L5) for obstetric analgesia because the spinal cord terminates at L1-L2 (the conus medullaris), reducing the risk of direct cord injury. An epidural catheter is threaded into this space, allowing continuous infusion or intermittent bolus administration of local anesthetic agents and opioids. Local anesthetics such as bupivacaine work by blocking voltage-gated sodium channels in nerve fibers, preventing depolarization and the propagation of pain impulses along sensory nerve roots. Different nerve fiber types have varying susceptibility to local anesthetic blockade: small unmyelinated C-fibers (carrying dull, aching pain) and small myelinated A-delta fibers (carrying sharp pain) are blocked first, followed by sympathetic fibers (causing vasodilation and potential hypotension), then motor fibers (which ideally should be preserved to allow...