What is the function of arachnoid villi?

Function of Arachnoid Villi

Arachnoid villi serve as one of two major pathways for cerebrospinal fluid (CSF) drainage from the subarachnoid space into the venous circulation, sharing this responsibility approximately equally with extracranial lymphatic pathways. 1, 2

Primary Functions

CSF Absorption and Drainage

• Arachnoid villi are microscopic projections of arachnoid tissue that extend into the dural venous sinuses, particularly the superior sagittal sinus, facilitating CSF drainage from the subarachnoid space into the venous system 3, 4
• In rats, arachnoid villi contribute approximately 50% of CSF tracer clearance from the cranial vault, with extracranial lymphatic pathways accounting for the other 50% 2
• The total CSF outflow system demonstrates functional duality, with both arachnoid villi and lymphatic routes working in concert 1

Protein and Metabolic Waste Clearance

• Arachnoid villi may play a particularly important role in absorbing CSF proteins and waste products of neuronal metabolism under normal physiological conditions 5
• Under pathological conditions, other CSF absorption sites can compensate for protein clearance when arachnoid villi function is compromised 5

Developmental and Anatomical Considerations

Maturation Timeline

• Microscopic arachnoid villi are present in preterm infants, though arachnoid granulations (larger aggregations of villi, also called Pacchionian granulations) are not yet mature at this stage 3
• Arachnoid villi produce imprints on skull bones early in development, appearing when bony sprouts receive their thin compact bone covering 6
• Initially, villi connect to diploic veins rather than dural sinuses; connection to sinuses develops later through short channels as villi agglomerate into larger formations 6

Clinical Pathophysiology

Hydrocephalus Development

• Fibrosis of arachnoid granulations, combined with meningeal fibrosis and subependymal gliosis, impairs CSF resorption and contributes to posthemorrhagic hydrocephalus (PHH) in preterm infants 3
• In cryptococcal meningitis, elevated intracranial pressure results partly from interference with CSF reabsorption at arachnoid villi, caused by high levels of fungal polysaccharide antigen or excessive organism growth 3
• Elevated biomarkers (TGF-β1 and PC1NP) in neonates with symptomatic PHH are consistent with impaired absorption due to arachnoid granulation fibrosis 3

Structural Complications

• Structural changes in arachnoid villi can produce thrombophlebitis and external hydrocephalus, particularly in children 6
• These anatomical relationships are clinically important for neurologists, neurosurgeons, and otolaryngologists managing CSF dynamics disorders 6

Contemporary Understanding

Evolving Concepts

• The traditional view that arachnoid villi serve as the major outflow route has been increasingly challenged, with accumulating evidence showing significant CSF drainage through lymphatic vessels along cranial and spinal nerves 4
• Current evidence supports a dual system where both arachnoid villi and lymphatic pathways contribute substantially to CSF drainage, rather than one pathway dominating 1, 2
• The exact contribution of each pathway may vary by species, degree of maturation, and pathological conditions 1