Skip to Main Content




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 ...

Want remote access to your institution's subscription?

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.


Create a Free MyAccess Profile

* Required Fields

Note: If you have registered for a MyAccess profile on any of the Access sites, you can use the same MyAccess login credentials across all sites.

Passwords must be between 6 and 40 characters long (no whitespace), cannot contain characters #, &, and must contain:
  • at least one lowercase letter
  • at least one uppercase letter
  • at least one digit

Benefits of a MyAccess Profile:

  • Remote access to the site off-campus on any device
  • Notification of new content via custom alerts
  • Bookmark your favorite content such as chapters, figures, tables, videos, cases and more
  • Save and download images to PowerPoint
  • Self-Assessment quizzes saved for quick review
  • Custom Curriculum access for both instructors and learners

Subscription Options

AccessAnesthesiology Full Site: One-Year Subscription

Connect to the full suite of AccessAnesthesiology content and resources including procedural videos, interactive self-assessment, real-life cases, 20+ textbooks, and more

$995 USD
Buy Now

Pay Per View: Timed Access to all of AccessAnesthesiology

24 Hour Subscription $34.95

Buy Now

48 Hour Subscription $54.95

Buy Now

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.