The epidural space lies between the walls of the vertebral canal and the meninges. The meninges are composed of three distinct layers (dura, arachnoid, and pia mater) that are continuous cephalad with the cranial meninges. Dura mater, the outermost layer, extends from the foramen magnum to S2 in adults where it fuses with the filum terminale. The innermost layer of dura mater is highly vascular and serves as the principal route for elimination of drugs in the epidural and subarachnoid space. The arachnoid mater lies deep in the dura and serves as a tight barrier, separating the spinal cord from the epidural space. A potential space exists between the dura and arachnoid mater. The pia mater is the deepest layer of the spinal meninges and adheres to the spinal cord. The subarachnoid space lies between the arachnoid and pia mater, and contains the cerebrospinal fluid (CSF).
The CSF is produced by the choroid plexus and cerebral and spinal capillaries at a rate of 25 mL/h. In an adult, the CSF volume is approximately 100–150 mL. The entire volume of CSF is replaced every 4–6 h as it is removed through the spinal nerve roots and in the sagittal sinus.
Access to the subarachnoid space is accomplished using the spinal needles. The outside diameter of the needle determines the gauge of the needle. Smaller gauge needles lower the risk of postdural puncture headaches but can be difficult to introduce and are often deflected by the interspinous ligaments. Insertion of spinal needles smaller than 22 gauges is often accomplished with the use of an introducer to pass through the supraspinous ligament. Inner stylets prevent plugging of the needle with skin or epidural fat, and subsequent introduction of these substances into the subarachnoid space.
Patient position is critical for successful spinal anesthesia. For obese patients or patients with otherwise difficult anatomy, the sitting position is useful in identifying the midline landmarks of spinous processes. This position can also be useful in restricting spinal anesthesia to more caudal dermatomes when using a hyperbaric local anesthetic. Similarly, the lateral decubitus position can be used to localize a spinal block to one side when bilateral anesthesia is not required for an operation or procedure, limiting the side effects of spinal anesthesia. The spinal canal narrows above L2, so insertion of a spinal needle above L2-L3 is generally avoided to decrease the risk of spinal cord injury.
The midline approach for access to the subarachnoid space starts with identification of the desired level. Once local anesthesia has been accomplished, the introducer needle is inserted at the top of the vertebral body that forms the lower border of the intended interspace. The introducer should be angled slightly cephalad to avoid the spinous process of the superior vertebra. As the spinal needle is introduced, it will cross the skin, subcutaneous tissue, supraspinous ligament, interspinous ...