The epidural space is a very narrow space that is situated immediately outside the spinal fluid, and separated from it by a special membrane called the ‘dura mater’ or dural membrane (hence the name epi-dural, outside the dura).
The basic anatomy is as follows: the extension from the brain into the back is the spinal cord, in which millions of nerve fibres are running upward, connecting the body to the brain, or downward, connecting the brain to the body. Around the whole length of the spinal cord is spinal fluid, which is held together by the dural membrane. This membrane forms a very narrow, long sac in which the spinal cord and the spinal fluid is housed, and runs from the skull all the way down to the sacrum.
This whole structure sits in the spinal canal, which is a long, narrow tunnel between the skull and the sacrum, inside the spinal column. The very small space found over the length of the back between the dural membrane and the inside of the bony spinal canal, is called the epidural space.
When the nerves leave the spinal cord, they have to go through the spinal fluid, through the dural ‘sac’ and through the epidural space, and then through the side openings in the spinal column. From here they run their different courses to the organs or limbs they innervate (supply). The nerves transmitting signals from the body to the spinal cord and brain follow the same route, but in opposite directions.
By injecting local anaesthetic into the epidural space, nerve signals can be blocked when they run through this space, so that no more signals from the affected body area can reach the spinal cord or the brain, nor can signals from the brain and spinal cord reach the body.
Reviewed by Prof CL Odendal, senior specialist at the department of anaesthesiology at the University of the Free State, April 2010.
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