Have you ever wondered how epidurals provide such great pain relief in specific parts of the body? Why do they create areas of numbness in one area of the body, whereas other areas remain untouched? The answer lies in where the epidural medication is injected, being the epidural space.
Epidural Space Definition
The epidural space is a thin elongated potential space within the spinal canal located between two meningeal layers. Sharing the spinal canal in the central portion is the dural sac (containing cerebral spinal fluid and the spinal cord). The epidural space has numerous contents, including spinal nerves, epidural fat, dural sac, lymphatics, and blood vessels.
Where is the Epidural Space Located?
The epidural space is found within the spinal canal. It is a potential space that surrounds the dural sac.
The upper (superior) boundary is created by the fusion of the spinal and periosteal layers of the dura mater at the foramen magnum. It then extends downwards (inferiorly) in the spinal canal and ends at the sacrococcygeal membrane.
The front (anterior) boundary ends at the posterior longitudinal ligament of the spine and the vertebral bodies and intervertebral discs (this forms the front of the canal).
The back (posterior) boundary is defined by the ligamentum flavum, the capsule of facet joints, and laminae. (back of the canal).
The space’s side (lateral) boundary is formed by the vertebral pedicles and intervertebral foramina (sides of the canal).
What are its Contents?
The epidural space contains numerous contents, including:
- spinal nerves
- epidural fat
- dura sac
- epidural arteries
- epidural veins
The spinal nerves come off the spinal cord in pairs and travel between the dural sac and the neural foramina. They are numbered according to the cervical, thoracic, lumbar, or sacral vertebra.
The amount of epidural fat in the space varies proportionally to the rest of the body. The epidural fat is normally located between the vessels, nerves, and ligaments of the epidural space. The epidural fat tissue in the space is unencapsulated, meaning it generally does not create a mass effect on the spinal nerves and dural sac.
The role of the fat in the space is to provide a buffer to the pulsatile movements of the dural sac. This protects the nerve structure and assists in the movement of the dural sac over the periosteum of the spinal column during flexion and extension.
The dural sac is a membranous sac that contents include the cauda equina and spinal cord. It is located within the center of the epidural space. The dural sac extends down the spinal canal and ends at the second sacral vertebra level.
The lymphatics of the epidural space are located around the dural roots. Their function is to remove foreign materials from the subarachnoid and epidural spaces.
The epidural arteries are found in the lateral region of the space. Epidural veins, though, develop a plexus and can be involved in a bloody or traumatic epidural tap during epidural needle insertion.
How Deep is the Epidural Space?
The depth varies from person to person depending on the thickness of the various layers of tissue that are located between the epidural space and the skin of the back.
There are five layers of tissue that contribute to the depth. The layers of tissue consist of:
- Subcutaneous fat
- Supraspnous ligament
- Interspinous Ligament
- Ligamentum Flavum
A study of 210 patients examining the depth of the epidural space showed that the average depth was 4.77cm +/- 0.55 cm for males and 4.25cm +/- 0.55 cm for females. The depth to the epidural space ranged from 3.0-7.0 cm.
Factors that influence the depth of the epidural space include the patient’s body weight and height of the patient.
How do Anesthesiologists determine where the epidural space is?
A reliable way Anaesthesiologists identify the space is through a technique called “loss of resistance “(LOR).
This method utilizes normal saline or air in a syringe to locate the space, with both techniques being equally effective.
By applying continuous or intermittent pressure on the syringe, a change in pressure on the syringe becomes noticeable on entering the epidural space.
The change in pressure is because when the epidural needle makes contact with the ligamentum flavum, injection is very difficult due to its density. In contrast, injection into the epidural space has little resistance.
Injection of Epidural space steroids
Patients with radicular pain caused by nerve irritation may be treated with an epidural injection of steroids into the epidural space.
Once the corticosteroids are injected into the space, they provide strong anti-inflammatory actions on the spinal nerves, decreasing pain and improving the patient’s function.