Cerebrospinal Fluid
Section V. Case Studies in CSF (CSF hypotension, edema, hydrocephalus, Chiari malformations, and cerebral aqueduct stenosis).
CSF is not static. Anatomy and physiology are inexorably intertwined. CSF is constantly produced (as an ultrafiltrate of blood at the choroid plexus) and resorbed. Resorption ultimately occurs at the arachnoid granulations, lymphatics of the skull base and dura, the 'paravascular cast' of the glymphatic system via interstitial fluid, and the spinal canal.
With every heart beat, there is CSF systole. The influx of arterial blood can be conceptualized as a net pulsation, squeezing the brain and propelling CSF through the ventricular system to the spinal CSF spaces and cisterns ('intracranial systole'). There is instantaneous to-and-fro flow, which can manifest as flow voids on certain MRI sequences, and net bulk flow. There is instantaneous
The Monro-Kellie Hypothesis
The Intracranial Volume is essentially fixed in a Mature Intact Skull
After sutures are closed, the intracranial compartment has a fixed volume. To a first approximation, we can consider three internal components: brain, blood, and CSF.
Volume Compensation
If CSF volume decreases, the blood volume will commensurately increase as the brain volume is relatively fixed. The change in blood volume is more marked in venous structures compared to arteries.
Spontaneous Intracranial Hypotension
Spontaneous intracranial hypotension (SIH) arises from spinal CSF leaks. This results in CSF hypovolemia and potentially alterations in craniospinal CSF space compliance. There is resultant increased intracranial venous volume and 'brain sag' that account for findings seen on MRI exams of the brain. In distinction, leaks from the skull base typically present with CSF rhinorrhea, CSF otorrhea and/or ascending meningitis, but not SIH.
One method to estimate the likelihood of CSF leak based on brain MRI findings is calculation of the Bern score.
Quantitative Criteria for Bern SIH Score
The three quantitative measures for the Bern score include measurement of the suprasellar cistern (2 points), prepontine cistern (1 point), and mamillopontine distance (1 point). Although the precise sequence used for these measurements may vary, these essentially can be measured on a midline sagittal image.
Qualitative Criterion for Bern Score
Engorgement of the dural venous sinuses can be assessed on the mid-portion of the transverse sinus. A convex inferior border is indicative of engorgement (2 points). Engorgement can also be assessed with the superior sagittal sinus.
Qualitative Criteria of the Bern Score
The final two criteria are diffuse smooth pachymeningeal enhancement (2 points) and subdural fluid collections (1 point) (usually at the convexity and bilateral, but not shown here.
There are three major categories of spinal CSF leak:
Dural Tear.
Nerve Root Sleeve (often from a diverticulum)
CSF-venous fistula.
There are other less common causes of leak and the source of some leaks are never identified.
Complex diverticula
or contained collection with leak
Patient with suspected leak, high suspicion Bern score and SLEC - MRI spine (not shown). Lateral decubitus digital subtraction myelogram demonstrates a complex contained collection associated with a nerve root sleeve, but no definitive leak.
Complex diverticula
or contained collection with leak
3D image from a delayed myelogram demonstrates multiple nerve root sleeve diverticula at multiple levels; these are well depicted on delayed images and help confirm that all areas filling with contrast appear contained. Surgical exploration at L2-L3 found a leak however, with a dural defect and associated CSF venous fistula.
CSF-Venous Fistula
Lateral dynamic decubitus digital subtraction myelogram shows a complex CSF-venous fistula at the nerve root (green arrow) with a prominent draining vein (red arrows).
Choroid plexus papilloma with hydrocephalus
A lobulated choroid plexus papilloma is present in the atrium/posterior body of the left lateral ventricle. Hydrocephalus results from over-production of CSF.
