Arachnoid granulations

Arachnoid granulations are small protrusions of the arachnoid mater into the dural venous sinuses, particularly the superior sagittal sinus. They serve as the primary sites for cerebrospinal fluid (CSF) absorption into the venous system. Arachnoid granulations act as pressure-sensitive, one-way valves that permit CSF to exit the subarachnoid space when CSF pressure exceeds venous pressure, helping regulate intracranial pressure and CSF volume.

Structure

• Composition: Aggregates of arachnoid villi — microscopic finger-like extensions of the arachnoid mater
• Appearance: Macroscopic arachnoid granulations appear as small, whitish nodules on the inner surface of the dura
• Size: Typically 1–5 mm in diameter but can enlarge with age
• Histology: Covered by flattened arachnoid cells with a connective tissue core; lined by endothelial cells continuous with the venous sinus endothelium

Location

• Most common site: Superior sagittal sinus, especially near its lateral lacunae
• Other locations: Transverse sinus, straight sinus, cavernous sinus, and spinal dural venous sinuses
• Scalloping effect: Prominent granulations may indent the inner surface of the skull (granular foveolae), visible on radiographs or CT scans

Function

• CSF absorption: Allow CSF to pass from the subarachnoid space into the venous blood of dural sinuses
• One-way valve mechanism: Permit CSF outflow when subarachnoid pressure exceeds venous pressure, preventing backflow
• Volume regulation: Help maintain a constant CSF volume and intracranial pressure

Mechanism of CSF Transfer

CSF flows through the arachnoid granulations via transcellular channels or intercellular gaps in the endothelial lining. This bulk flow mechanism is pressure-dependent and does not rely on active transport. Some small solutes may pass freely, while larger molecules are selectively restricted.

Development

Arachnoid villi begin forming during infancy but become more prominent and functional after the first year of life. They enlarge with age and may calcify or increase in number in older adults. In early fetal development, CSF absorption occurs through alternative lymphatic routes before arachnoid villi mature.

Clinical Significance

• CSF flow obstruction: Impaired function or blockage of granulations (e.g., due to hemorrhage or infection) can lead to communicating hydrocephalus
• Arachnoid granulation hypertrophy: Enlarged granulations may mimic pathological lesions (e.g., dural sinus thrombosis or tumor) on imaging but are typically benign
• Leptomeningeal disease: Tumor or infection involving the arachnoid and pia may interfere with CSF absorption through the granulations
• Idiopathic intracranial hypertension: Dysfunctional CSF absorption is one proposed mechanism behind this condition

Imaging

• CT scan: Enlarged arachnoid granulations may appear as hypodense areas within the dural sinuses, particularly in the superior sagittal sinus
• MRI: Arachnoid granulations appear as CSF-isointense nodules on T1 and T2 images; they do not enhance with contrast
• Differentiation: Important to distinguish from venous sinus thrombosis, meningiomas, or dural metastases