Within the thecal sac, there are 3 layers of meningeal membranes: the outer, dura mater is tough and protective and this forms the outer part of the dural sac (in practice, the terms intrathecal and intradural are often used interchangeably).
Abnormalities outside this layer are described as epidural/peridural/extradural.
The middle layer, the arachnoid mater (so called due to the spider's web type appearance) is in direct contact with the cerebrospinal fluid, which bathes the entire spinal cord and brain.
Inflammation of this layer is termed arachnoiditis and scar tissue may be described as intrathecal (i.e. inside the thecal sac).
The inner layer is the pia mater.
The initial inflammation induces a process of scarring in the arachnoid membrane.
This may also affect the dura and the area outside the dura, so that epidural fibrosis and arachnoiditis often co-exist.
Alert: http://www.fda.gov/cdrh/safety/neurostim.html
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