CSF

PRODUCTION AND FLOW: CSF is made by ependymal cells of the choroid plexus (~500 mL/day; total volume ~150 mL in adults, turned over about 3-4x/day). Production uses active secretion (Na+/K+-ATPase, carbonic anhydrase), so acetazolamide (carbonic anhydrase inhibitor) decreases CSF production. Flow: lateral ventricles → foramina of Monro → third ventricle → cerebral aqueduct (of Sylvius) → fourth ventricle → foramina of Luschka (lateral) and Magendie (median) → subarachnoid space → reabsorbed by arachnoid granulations into the dural venous sinuses (superior sagittal sinus).

BARRIERS: The blood-CSF barrier is formed by tight junctions between choroid plexus ependymal cells. The blood-brain barrier is formed by capillary endothelial tight junctions, astrocyte foot processes, and the basement membrane. Both limit passage of many substances.

FUNCTIONS: Buoyancy (reduces effective brain weight from ~1400 g to ~25 g), mechanical protection, chemical stability, nutrient delivery, and waste removal (glymphatic system, most active during sleep).

NORMAL VALUES (adult): Opening pressure 70-180 mm H2O (lumbar puncture, lateral decubitus position). WBC 0-5 cells/µL (lymphocytes/monocytes). Protein 15-45 mg/dL. Glucose about 60% of serum glucose (roughly 45-80 mg/dL). RBC: none. The fluid is clear and colorless.

LUMBAR PUNCTURE: Needle passes through skin, subcutaneous tissue, supraspinous and interspinous ligaments, ligamentum flavum, epidural space, dura, and arachnoid into the subarachnoid space. It is done at L3-L4 or L4-L5 (the line between iliac crests, the intercristal line, is at L4), below the conus medullaris (L1-L2 in adults). Contraindications: mass lesion or elevated ICP with risk of herniation (get imaging first), coagulopathy or thrombocytopenia, and skin infection at the site.

CSF PATTERNS: - Bacterial meningitis: High neutrophils (often >1000), high protein, low glucose, high opening pressure. Common organisms: S. pneumoniae, N. meningitidis (adults); GBS, E. coli, Listeria (neonates); Listeria also in the elderly and immunocompromised. - Viral (aseptic) meningitis: Lymphocytic pleocytosis, mildly high protein, normal glucose (enteroviruses most common; HSV-2 can cause recurrent Mollaret meningitis). - Fungal/TB meningitis: Lymphocytes, high protein, low glucose. Cryptococcus: India ink stain or cryptococcal antigen; very high opening pressure may require serial LPs. TB: basilar enhancement and cranial nerve palsies. - HSV encephalitis: Lymphocytes plus RBCs (temporal lobe hemorrhagic necrosis); diagnose with PCR and treat with IV acyclovir. - Subarachnoid hemorrhage: Xanthochromia (bilirubin from RBC breakdown, appears ~6-12 hours after bleed) and RBCs that do not clear from tube 1 to tube 4. CT head is the first test (most sensitive within 6 hours). - Guillain-Barré syndrome: Albuminocytologic dissociation (high protein with normal cell count). - Multiple sclerosis: Oligoclonal IgG bands, elevated IgG index, and increased myelin basic protein. - Idiopathic intracranial hypertension: High opening pressure with normal CSF composition (young obese women, papilledema, headache, vision changes). Treat with weight loss, acetazolamide, and serial LPs or shunting if severe. - Normal pressure hydrocephalus: Normal opening pressure; triad of "wet, wobbly, wacky" (urinary incontinence, gait apraxia, dementia). Ventriculomegaly with a normal CSF profile; treat with VP shunt. - Neurosyphilis: Positive VDRL (specific, not sensitive) in CSF. - Other: Elevated 14-3-3 protein in Creutzfeldt-Jakob disease (also RT-QuIC); low Aβ42 and high tau/phospho-tau in Alzheimer disease.

HYDROCEPHALUS: - Communicating (non-obstructive): Impaired reabsorption at arachnoid granulations (e.g., post-meningitis, SAH). Normal pressure hydrocephalus is a subtype. - Non-communicating (obstructive): Blockage within the ventricular system (e.g., aqueductal stenosis, Dandy-Walker malformation, posterior fossa tumors, Chiari II). - Hydrocephalus ex vacuo: Brain atrophy causes ventricle enlargement, but ICP is normal (e.g., Alzheimer disease, advanced HIV, Pick disease).

HERNIATION AND ICP: Elevated ICP causes Cushing triad (hypertension, bradycardia, irregular respirations). Herniation types: subfalcine (cingulate gyrus under falx; ACA compression), uncal/transtentorial (CN III palsy, "blown pupil," contralateral hemiparesis, Duret hemorrhages in the midbrain/pons), and tonsillar (cerebellar tonsils through foramen magnum; medullary compression, death).

CLINICAL PEARLS: Post-LP headache arises from CSF leak and is positional (worse upright); treat with fluids, caffeine, then epidural blood patch. CSF rhinorrhea or otorrhea after skull base fracture can be confirmed by beta-2 transferrin. In neonates, the choroid plexus is a site for papillomas (hydrocephalus from overproduction in children).