arachnoid granulation

**Anatomy:** Arachnoid granulations (Pacchionian granulations) are projections of arachnoid mater and subarachnoid space that penetrate the dura and bulge into the lumen of the dural venous sinuses. They are most numerous along the superior sagittal sinus and its lateral lacunae. They are covered by arachnoid cells and the venous endothelium. Microscopic forms are called arachnoid villi.

**Physiology:** - CSF is produced mainly by the choroid plexus (ependymal cells) at about 500 mL/day. Total volume is about 150 mL. - CSF flow: lateral ventricles → foramen of Monro → third ventricle → cerebral aqueduct (of Sylvius) → fourth ventricle → foramina of Luschka (lateral) and Magendie (median) → subarachnoid space → arachnoid granulations → dural venous sinuses. - Absorption is passive and pressure-dependent. The granulations act as one-way valves, so CSF flows into the venous blood when CSF pressure exceeds venous pressure (about 5 to 7 mmHg). Blood does not flow back into the CSF.

**Clinical significance:** - **Communicating (non-obstructive) hydrocephalus:** Reduced CSF reabsorption at the granulations raises ventricular size and ICP. Ventricles dilate, but there is no obstruction within the ventricular system. Common causes: - Post-subarachnoid hemorrhage, where blood products clog the granulations. - Post-meningitis, where scarring or inflammation impairs absorption. - Elevated venous sinus pressure, for example from cerebral venous sinus thrombosis. - **Normal pressure hydrocephalus:** Impaired CSF absorption causes the triad of "wet, wobbly, wacky" (urinary incontinence, gait apraxia/magnetic gait, dementia). Ventricles are enlarged, and opening pressure on LP is normal. It is treated with a VP shunt. - **Idiopathic intracranial hypertension:** Impaired CSF outflow and/or venous sinus stenosis is implicated. It presents with headache, papilledema, and vision changes in obese women of childbearing age, and with normal imaging. LP shows high opening pressure with normal CSF composition. Treatment is acetazolamide, weight loss, and sometimes shunting or optic nerve fenestration. - **Imaging:** Granulations appear as well-circumscribed, CSF-density filling defects in the dural sinuses on CT/MRI venography. They can mimic venous sinus thrombosis. They can also cause focal scalloping of the inner skull table. Calcified granulations are a benign finding. - **CSF leak:** Rarely, large granulations can erode bone and cause spontaneous CSF leak.

**High-yield pearls:** - Granulation absorption problem → communicating hydrocephalus (all ventricles enlarged). - Aqueductal stenosis, or a mass in the 4th ventricle → obstructive (non-communicating) hydrocephalus. - Hydrocephalus ex vacuo (e.g., Alzheimer disease) → ventricles enlarged from brain atrophy, with normal ICP. - Meninges layers, from outside to inside: dura → arachnoid → pia. The subarachnoid space lies between the arachnoid and pia and contains CSF and vessels. Bridging veins cross the subdural space, and their tearing causes subdural hematoma.