cerebrospinal fluid

**Production:** Secreted by ependymal cells of the choroid plexus (mainly lateral ventricles, also 3rd and 4th) via Na+/K+-ATPase and carbonic anhydrase (acetazolamide decreases production). ~500 mL/day; total volume ~150 mL (≈30 mL in ventricles, remainder in subarachnoid space).

**Flow pathway:** Lateral ventricles → foramina of Monro (interventricular foramina) → 3rd ventricle → cerebral aqueduct (of Sylvius) → 4th ventricle → foramina of Luschka (lateral, paired) and Magendie (medial, single) → subarachnoid space (cisterns, e.g., cisterna magna) → reabsorbed by arachnoid granulations into the dural venous sinuses (superior sagittal sinus).

**Functions:** Mechanical cushioning/buoyancy (reduces effective brain weight from ~1400 g to ~50 g), maintenance of ICP, homeostasis of the extracellular chemical environment, nutrient delivery, waste removal (glymphatic system), and a route for neuroendocrine signaling.

**Barriers:** Blood-CSF barrier = tight junctions between choroid plexus epithelial cells. Blood-brain barrier = tight junctions of capillary endothelium, astrocyte foot processes, and basement membrane.

**Normal values (adult):** - Opening pressure: 70-180 mm H2O (<200-250 mm H2O in obese patients) - Appearance: clear, colorless - WBC: 0-5 cells/µL (lymphocytes/monocytes) - RBC: 0 - Protein: 15-45 mg/dL - Glucose: 45-80 mg/dL (~60% of serum glucose; get a simultaneous serum glucose)

**Classic CSF patterns:** - Bacterial meningitis: ↑↑ neutrophils (often >1000), ↑ protein, ↓ glucose, ↑ opening pressure. Organisms by age: neonates (GBS, E. coli, Listeria), children/adults (S. pneumoniae, N. meningitidis), >50 or immunocompromised (add Listeria). - Viral (aseptic) meningitis: ↑ lymphocytes, mildly ↑ protein, normal glucose (enterovirus most common; HSV PCR positive in HSV encephalitis, often with RBCs). - Fungal/TB meningitis: ↑ lymphocytes, ↑↑ protein, ↓ glucose (Cryptococcus: India ink, antigen; TB: AFB, high protein, basilar enhancement). - Guillain-Barré syndrome: albuminocytologic dissociation (↑ protein, normal cell count). - Multiple sclerosis: oligoclonal IgG bands, mildly ↑ protein, ↑ myelin basic protein. - Subarachnoid hemorrhage: xanthochromia (bilirubin from RBC breakdown, appears ~6-12 hr after bleed), RBCs that do NOT clear from tube 1 to tube 4 (vs. traumatic tap where RBCs clear). Next step after negative noncontrast CT within 6 hr is often CT angiography; LP if CT is delayed or inconclusive. - Idiopathic intracranial hypertension (pseudotumor cerebri): ↑ opening pressure with otherwise normal CSF; treat with weight loss, acetazolamide; therapeutic LP. - Normal pressure hydrocephalus: normal opening pressure; triad = "wet, wobbly, wacky" (urinary incontinence, gait apraxia, dementia); improves with large-volume LP or shunt.

**Hydrocephalus:** - Communicating (nonobstructive): impaired reabsorption at arachnoid granulations (e.g., post-SAH, meningitis); ventricles enlarged, flow through the system open. Includes "hydrocephalus ex vacuo" (brain atrophy, normal ICP, e.g., Alzheimer disease) which is not true hydrocephalus. - Noncommunicating (obstructive): blockage within the ventricular system (aqueductal stenosis, Chiari II malformation, Dandy-Walker malformation, tumors such as medulloblastoma/ependymoma). - Rare overproduction: choroid plexus papilloma.

**Lumbar puncture:** Needle passes through skin → supraspinous and interspinous ligaments → ligamentum flavum → epidural space → dura and arachnoid → subarachnoid space (at L3-L4 or L4-L5, at the level of the iliac crests/supracristal plane). The spinal cord ends at ~L1-L2 in adults (conus medullaris), so LP below this level is safe (L3-L4 or L4-L5). Contraindications: signs of ↑ ICP/mass effect (risk of herniation; get CT first), coagulopathy/thrombocytopenia, and local skin infection. Complications: post-LP headache (positional, worse upright, from CSF leak; treat with caffeine, fluids, epidural blood patch), infection, bleeding, herniation.

**Pearls:** CSF leak (rhinorrhea/otorrhea) after skull base fracture shows beta-2 transferrin and a "halo/ring sign." Choroid plexus also secretes CSF in the fourth ventricle; Chiari malformations can cause obstructive hydrocephalus and syringomyelia. Clinical signs of raised ICP: Cushing triad (hypertension, bradycardia, irregular respirations), papilledema, headache, vomiting.