Extradural neural axis compartment (EDNAC) exists from the tip of the coccyx all the way to the back of the globe, and yet it is relatively unknown as a concept. It is bounded externally by the periosteum of the vertebrae and sacrum inferiorly and the skull superiorly, and the visceral (meningeal) layer of the dura (dura propria/theca) internally.
The space contains veins (without valves), nerves and adipose tissue. The size of this space varies depending on the location. In the spine, the EDNAC is known as the epidural space and is capacious, filled with ample fat and epidural veins. Intracranially it is essentially non-existent in most parts as the two layers are closely apposed/fused. Where they separate the space is predominantly filled with veins forming the dural venous sinuses, including the cavernous sinus.
Anteriorly the space continues into the orbit with the dura propria forming the optic nerve sheath, and the periosteum flowing over the inner surface of the bones 1,2.
Understanding the relationship of the two layers of the dura is important:
- spinal epidural hematomas are within the EDNAC, whereas intracranial extradural hematomas are external to it (actually subperiosteal hematomas); note that intracranial hemorrhage within the EDNAC is exceedingly rare, sometimes known as intralaminar dural hematoma
- fat can be found anywhere in this space, accounting not only for the normal fatty epidural space in the spine and the orbital fat, but also not infrequent fatty falx and the presence of fat in the cavernous sinus