Cerebrospinal Fluid (CSF)
The clear fluid surrounding the brain and spinal cord that provides cushioning, nutrient delivery, and waste removal. CSF analysis can reveal biomarkers of neurological disease. Intrathecal administration delivers drugs directly into the CSF to bypass the blood-brain barrier.
Technical Context
CSF is produced primarily by choroid plexus epithelium (approximately 500 mL/day, with total volume approximately 140 mL — complete turnover 3-4 times daily). Composition: clear, colourless, protein-poor (0.2-0.4 g/L vs 60-80 g/L in plasma), glucose approximately 60% of plasma, and low cell count (<5 cells/μL). CSF functions: mechanical protection (cushioning the brain), buoyancy (reducing effective brain weight from 1400g to approximately 50g), chemical stability (maintaining stable ionic environment for neural signalling), and waste clearance (glymphatic system — CSF flows through perivascular spaces, exchanging with interstitial fluid, clearing metabolic waste including amyloid-β during sleep). CSF analysis is clinically valuable: protein elevation (infection, inflammation), glucose decrease (bacterial meningitis), oligoclonal bands (MS), Aβ42 decrease + phospho-tau increase (Alzheimer's), and cytology (CNS malignancy). Intrathecal drug administration delivers directly into CSF, achieving high CNS concentrations with minimal systemic exposure.