Overview

Microscopic arachnoid villi enlarge with age into macroscopic granulations (Pacchionian bodies). They push through the meningeal dura into the lumen of the superior sagittal sinus and its lateral venous lacunae, and to a lesser extent into the transverse and other sinuses and along the spinal nerve root sleeves. Each consists of a core of subarachnoid space and arachnoid trabeculae, covered by arachnoid cells and an arachnoid cap cell layer, then a thin endothelium of the sinus.

CSF moves into the venous blood by bulk flow, through transcellular vacuoles and channels, whenever CSF pressure exceeds venous sinus pressure. When the pressure gradient reverses, the granulations collapse, so blood does not flow back into the subarachnoid space. Large granulations can erode the inner table of the skull, leaving granular foveolae, small pits near the sagittal suture that are visible on dry skulls and imaging.

Location: Mainly along the superior sagittal sinus and its lateral lacunae near the vertex; also the transverse sinuses and spinal nerve root sleeves.

Anatomy

Histology: Arachnoid cells and collagen trabeculae enclosing extensions of the subarachnoid space; capped by arachnoid cap cells (origin of meningiomas) beneath the sinus endothelium.

Function

Return of CSF to the venous circulation
One-way valve preventing reflux of blood into the subarachnoid space
Pressure-dependent regulation of CSF volume

Clinical significance

Communicating hydrocephalus

After meningitis or subarachnoid haemorrhage, protein, blood and inflammatory fibrosis can clog the granulations. CSF then flows freely out of the ventricles but cannot be absorbed, and all four ventricles enlarge.

Venous sinus thrombosis and raised ICP

Thrombosis or stenosis of the superior sagittal or transverse sinuses raises venous pressure. This removes the gradient for CSF absorption, raising ICP and causing papilloedema.

Test yourself

Where is CSF chiefly returned to the venous blood?

Choroid plexus capillaries
Internal vertebral venous plexus
Ependymal cells of the central canal
Arachnoid granulations into the dural venous sinuses

Answer: Arachnoid granulations into the dural venous sinuses

Arachnoid granulations, especially along the superior sagittal sinus, act as pressure-dependent, one-way valves into the dural venous sinuses. Some CSF also drains along cranial and spinal nerve sheaths to lymphatics. Scarring of the granulations after meningitis or SAH causes communicating hydrocephalus.