An epidural is a common method of pain relief, particularly known for its use during childbirth and certain surgical procedures. The technique involves delivering medication into a specific area near the spinal cord to block pain signals. The tool used to perform this procedure is the epidural needle, a specialized instrument designed for safety and precision, differing significantly from a standard hypodermic needle.
The Specialized Design of the Epidural Needle
The tool used to access the correct anatomical space is the Tuohy needle, named after its inventor, Dr. Edward Tuohy. Unlike standard injection needles, the epidural needle is relatively long, often measuring 3 to 5 inches. This length is necessary to reach the epidural space through the layers of tissue and ligaments in the back.
The needle has a wider diameter, typically 16- to 18-gauge, which is thick compared to standard needles. This larger size is required because the epidural needle is a hollow guide for a thin, flexible catheter that must pass through its center. The most distinct feature is the tip, which is not sharply pointed.
Instead, the Tuohy needle has a curved, rounded, or blunt bevel at the end. This curved tip is engineered to push aside dense tissues and ligaments rather than cutting them. This design allows the practitioner to better perceive resistance as the needle advances, reducing the risk of accidentally puncturing the dura mater, the membrane surrounding the spinal cord. The shaft of the needle is marked with centimeter graduations to ensure accurate depth awareness.
Placement and Mechanism of Epidural Anesthesia
The target for the epidural needle is the epidural space, located just outside the dura mater, the protective sheath encasing the spinal cord. This space is filled with small blood vessels, fat, and connective tissue, surrounding the spinal nerves as they exit the column. The space is narrow, sometimes only 5 to 6 millimeters in the lumbar region where epidurals are most frequently performed.
Once the needle tip is correctly positioned, a local anesthetic medication is delivered into this space. The medication then bathes the spinal nerve roots, which transmit pain signals. By blocking the electrical impulses in these nerves, the epidural provides pain relief.
The local anesthetic works by blocking sodium channels in the nerves, preventing the transmission of sensation to the brain. Smaller nerve fibers responsible for temperature and pain are blocked first, followed by larger fibers responsible for motor function. This mechanism allows for a significant reduction in pain without necessarily causing complete motor paralysis, though some temporary weakness in the legs is common.
The Epidural Delivery System and Insertion Process
The epidural procedure begins with the patient positioned, often sitting up or lying on their side, to help the practitioner locate the correct intervertebral space. After cleaning the area, a small injection of local anesthetic is given into the skin and underlying tissues to numb the insertion site, making the placement of the larger epidural needle more comfortable.
The practitioner then carefully advances the specialized Tuohy needle through the skin and various ligaments of the back. To confirm the needle has reached the target, a specific technique called “loss of resistance” (LOR) is used. This involves attaching a syringe filled with saline or air to the needle and gently applying pressure to the plunger.
Loss of Resistance Confirmation
As the needle passes through the dense ligamentum flavum, the final ligament before the epidural space, the practitioner feels a distinct, sudden loss of resistance on the plunger. This change confirms the needle tip is in the correct space.
Catheter Placement
Once placement is confirmed, the large Tuohy needle is not left in place. Instead, a very thin, flexible plastic tube called an epidural catheter is threaded through the hollow center of the needle.
The catheter is advanced a few centimeters into the epidural space, and the Tuohy needle is then completely removed. The thin, soft catheter remains secured to the patient’s back with tape. This allows for the continuous or intermittent delivery of medication from an external pump, providing ongoing pain relief for the duration required.