Blood-brain barrier physiology

1. Bedside vignette: bacterial meningitis, raised ICP, why dexamethasone#

A 4-year-old with pneumococcal meningitis is admitted septic, irritable, with photophobia and neck stiffness. CSF shows 8000 WBC/µL (90% neutrophils), glucose 1.2, protein 5.4 g/L. Imaging shows obliteration of basal cisterns and a small hydrocephalus. Antibiotics (ceftriaxone + vancomycin) are started immediately; dexamethasone 0.15 mg/kg every 6 h is started before or with the first dose of antibiotic.

Why dexamethasone? Bacterial lysis from antibiotic action releases cell-wall fragments that drive a brisk inflammatory response across the inflamed BBB, raising ICP and causing the hearing loss and neurological injury that complicate pneumococcal meningitis. Dexamethasone seals the inflammatory leak before the antibiotic-triggered cytokine surge. The adult European Dexamethasone trial and a Cochrane meta-analysis showed reduced death and hearing loss in adult pneumococcal meningitis; pediatric Hib data are clearer, pediatric pneumococcal data more mixed but supportive. 1 3 4

The bedside take is mechanistic: this is a BBB intervention timed against a BBB event. Steroids work here for the same reason they harm in TBI (CRASH): they alter a permeability response that is helpful in some contexts and not others.

2. The structure#

The barrier is built by continuous endothelium with tight junctions (claudin-5 and occludin), wrapped in astrocyte end-feet abluminally, with pericytes sitting in pockets along the basement membrane. The unit is collectively called the neurovascular unit, and the barrier function depends on all three cell types staying healthy.

3. What gets through#

• Free diffusion: O₂, CO₂, water, small lipid-soluble molecules (general anaesthetics, opioids, alcohol). This is why a CO₂ change reaches the cerebral interstitium in seconds, and why an arterial blood-gas drives intracranial chemistry.
• Active transport: glucose (GLUT1), large neutral amino acids (LAT1, the carrier shared by L-DOPA and methyldopa), monocarboxylates (lactate; MCT1, the carrier the ketogenic diet uses to feed the brain when GLUT1 is defective).
• Barrier-active efflux: P-glycoprotein, MRPs, BCRP; pumps that take certain drugs out of the brain capillaries back to the blood. The reason loperamide does not sedate.
• Excluded: most antibiotics, most chemotherapy, large ions in solution, plasma proteins.

4. Why it matters at the bedside#

The BBB changes every prescribing decision in neurocritical care.

• Mannitol works because its osmotic gradient is preserved across an intact BBB. With BBB disruption, mannitol redistributes, producing rebound oedema. 5
• Hypertonic saline has a higher reflection coefficient (~1.0 for Na⁺ vs ~0.9 for mannitol): it works even with mild BBB disruption, with less rebound. 2
• Glucocorticoids seal the BBB in vasogenic tumour oedema and in inflammatory leak of bacterial meningitis; they harm in TBI (CRASH), where the oedema is mixed cytotoxic + vasogenic. 1
• Antibiotic penetration drops dramatically without inflammation, rises with meningeal inflammation. Part of why empiric meningitis dosing is high. 3 4
• CO₂ and O₂ pass freely, which is why ventilator settings move CBF and intracranial chemistry within seconds (see CO₂ reactivity page).

Reflection coefficient: why HTS often beats mannitol after leak#

The reflection coefficient σ describes the fraction of an osmolar agent reflected by the BBB:

• σ = 1.0 (Na⁺): fully reflected; full osmotic gradient preserved.
• σ ≈ 0.9 (mannitol): mostly reflected, but some leak.
• σ ≈ 0.0 (urea, water): not reflected; no osmotic gradient.

After BBB disruption, mannitol's effective σ falls further; the osmotic gradient is short-lived; water can equilibrate into oedematous tissue ("rebound"). HTS retains a near-1.0 σ even in modest disruption; the gradient is preserved longer.

5. In injury#

• TBI: BBB disruption is biphasic, early (minutes) and delayed (days), peaking at 24–72 h.
• SAH: diffuse disruption from blood breakdown products.
• Stroke: within minutes in core, hours in penumbra; predicts haemorrhagic transformation risk.
• Sepsis: neuroinflammation opens the BBB diffusely; relevant for septic encephalopathy.
• Meningitis / encephalitis: localised disruption tracks the inflammatory front; the same disruption that lets pathogen breach the barrier lets antibiotic in.

6. Pitfalls#

• Treating all oedema with osmotherapy: vasogenic tumour oedema responds best to steroids; cytotoxic oedema to perfusion restoration; mixed TBI oedema to osmotherapy. Match the mechanism.
• Steroids in TBI: CRASH showed harm. Do not give methylprednisolone in TBI.
• Antibiotic dose reduction in meningitis "because the patient is improving": meningeal inflammation falls as the patient improves; antibiotic penetration falls with it. Maintain CSF-penetrating doses for the full course.
• Forgetting the BBB: most chemotherapy and many antibiotics that work systemically do not work in the brain. Choose agents that cross.
• Hypotonic IV fluids in the at-risk brain: free water crosses the BBB regardless; hyponatraemia worsens oedema. Maintain serum Na in the upper-normal range.

7. Combine with…#

• Modality: microdialysis: tissue chemistry on the brain side of the BBB.
• Modality: NIRS: regional tissue oximetry; affected by BBB-mediated water shifts.
• Foundation: cerebral metabolism: the substrate transport the BBB regulates.
• Foundation: CO₂ and O₂ reactivity: the gas exchange that crosses the BBB in seconds.
• Integration: meningitis and encephalitis: the BBB-as-target disease.

8. Anatomy of the BBB#

The endothelial cells are tightly joined by claudin-5 and occludin tight-junction proteins. Astrocyte end-feet ensheath the abluminal surface, and pericytes regulate vessel diameter and barrier integrity. The neurovascular unit is the integrated cell ensemble.

9. Transport mechanisms#

• GLUT1 carries glucose down its concentration gradient. Defective GLUT1 (De Vivo syndrome) causes infantile epilepsy treated with the ketogenic diet (which provides ketone bodies that use MCT1).
• LAT1 transports L-DOPA, methyldopa, and large neutral amino acids; competition between these explains some classic Parkinson's drug interactions.
• P-glycoprotein pumps many drugs back out of brain capillaries; the reason loperamide does not make you sleepy.

10. Imaging the BBB#

• Contrast-enhanced MRI: gadolinium leakage indicates BBB disruption.
• MR DCE perfusion: permeability mapping.
• PET / CT-perfusion: indirect via tracer kinetics.

11. Oedema types#

12. Evidence summary#

13. Self-check#