Cerebrospinal Fluid (CSF)

Cerebrospinal fluid (CSF) is a clear, colorless fluid that circulates within the ventricles of the brain, the central canal of the spinal cord, and the subarachnoid space surrounding the brain and spinal cord. It serves multiple vital roles, including mechanical protection, metabolic support, and waste clearance. CSF is continuously produced, circulated, and reabsorbed to maintain stable intracranial pressure and central nervous system (CNS) homeostasis.

Production

CSF is primarily produced by the choroid plexus, a specialized vascular structure located in the lateral ventricles, third ventricle, and fourth ventricle. A small amount is also formed by the ependymal lining and through interstitial fluid exchange from brain parenchyma.

• Main sites of production:
  • Lateral ventricles (via choroid plexus)
  • Third ventricle (via choroid plexus in roof)
  • Fourth ventricle (posterior part of roof)
• Total daily production: Approximately 500 mL per day
• Total volume at any given time: Around 150 mL in adults

Circulation Pathway

1. CSF is produced in the lateral ventricles
2. Flows through the interventricular foramina (foramina of Monro) into the third ventricle
3. Passes through the cerebral aqueduct (of Sylvius) into the fourth ventricle
4. Exits the ventricular system via:
   • Median aperture (foramen of Magendie)
   • Two lateral apertures (foramina of Luschka)
5. Enters the subarachnoid space around the brain and spinal cord
6. CSF is absorbed into the venous system through arachnoid granulations into the superior sagittal sinus

Functions

• Mechanical protection: Acts as a cushion, protecting the brain and spinal cord from trauma and sudden movements
• Buoyancy: Reduces the effective weight of the brain, preventing compression of inferior structures
• Chemical stability: Helps regulate extracellular ion balance and pH in the CNS
• Waste removal: Facilitates the clearance of metabolic waste and toxins through the glymphatic system
• Intracranial pressure regulation: Maintains stable pressure by adjusting CSF volume and absorption

Composition

• Appearance: Clear, colorless
• pH: Approximately 7.33
• Glucose: 45–80 mg/dL (typically 2/3 of serum glucose)
• Protein: 15–45 mg/dL
• White blood cells: 0–5 cells/μL (primarily lymphocytes)
• Red blood cells: Normally absent

Absorption

CSF is primarily reabsorbed through arachnoid granulations (villi) into the superior sagittal sinus. These projections of arachnoid mater act as one-way valves that permit CSF flow into the venous system when CSF pressure exceeds venous pressure. Minor absorption also occurs via lymphatics and perineural sheaths around cranial and spinal nerves.

Blood-CSF Barrier

The blood-CSF barrier is formed by tight junctions between choroid plexus epithelial cells. It regulates the exchange of substances between blood and CSF, maintaining CNS chemical stability. This barrier is distinct from the blood-brain barrier, which exists at cerebral capillaries.

Clinical Significance

• Hydrocephalus: Abnormal accumulation of CSF due to impaired flow (non-communicating) or absorption (communicating); leads to ventricular enlargement and increased intracranial pressure
• CSF leak: Loss of CSF through a dural defect may lead to orthostatic headaches, commonly seen after lumbar puncture or head trauma
• Meningitis: Infection of the meninges leads to inflammatory changes in CSF; diagnosis confirmed via lumbar puncture with elevated WBC, protein, and reduced glucose
• Subarachnoid hemorrhage: Presence of blood in the CSF, often due to ruptured aneurysm; CSF may appear xanthochromic after several hours
• Idiopathic intracranial hypertension: Elevated CSF pressure without identifiable cause; may present with papilledema and headache

Diagnostic Sampling – Lumbar Puncture

• Typical site: Between L3–L4 or L4–L5 vertebral levels, below the conus medullaris
• Indications: Suspected CNS infection, subarachnoid hemorrhage, inflammatory disorders, or to measure intracranial pressure
• Contraindications: Raised intracranial pressure with mass effect (risk of herniation), coagulopathy, local skin infection

Imaging and CSF Flow Studies

• MRI: Can detect hydrocephalus, aqueductal stenosis, or CSF leaks; cine MRI used to assess CSF flow dynamics
• CT: Often used in emergency settings to evaluate ventricular size and possible hemorrhage
• Radionuclide cisternography: Used to evaluate CSF leaks or normal pressure hydrocephalus