The subarachnoid space is the cerebrospinal fluid-filled space between the arachnoid mater and the pia mater, continuous from the brain to the sacral level. It contains CSF, trabeculae connecting the arachnoid to the pia, and the spinal nerve roots from the conus medullaris downward as the cauda equina. The lumbar cistern (below L2) is the widest part and the target for lumbar puncture.
The subarachnoid space is accessed by lumbar puncture (LP) at L3-L4 or L4-L5, below the conus medullaris, to sample CSF for meningitis, subarachnoid haemorrhage, and demyelinating disease diagnosis. Post-LP headache (post-dural puncture headache) from CSF leak through the dural hole is reduced by using smaller gauge pencil-point needles. Intrathecal drug delivery systems (baclofen, morphine, chemotherapy) access the subarachnoid space via implanted catheters. Subarachnoid haemorrhage from aneurysm rupture fills the basal cisterns and sulci with blood.
Dural puncture during lumbar puncture allows CSF to leak from the subarachnoid space, reducing intracranial pressure and causing orthostatic headache (worse sitting or standing, relieved lying flat); epidural blood patch restores subarachnoid space pressure by tamponading the dural hole from the epidural side.
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