Blood-brain barrier

The blood brain barrier (BBB) forms a physical resistance to the passage of lipophobic substances form cerebral capillaries into the brain and is a key reason why there is no CSF enhancement following IV contrast media in CT and MR imaging.

It is formed by a combination of endothelial cells, pericytes, and astroglial and perivascular macrophages along the cerebral capillary walls.

In general, capillary walls in the human body can comprise of three different types:

• continuous: present in areas which have a BBB
  • continuous interendothelial tight junctions
  • no pinocytosis
  • no fenestrations
• fenestrated: present in areas which lack the BBB
• sinusoidal: not found in the brain

In the brain, the majority of capillary walls are of the continuous type with tight junctions and a continuous basement membrane

Areas which have fenestrated capillaries and thus lack the blood brain barrier are termed :

• circumventricular organs (CVO) and include:
  • area postrema
  • median eminence
  • subforniceal region
  • posterior pituitary
  • pineal gland
  • organum vasoculosum
  • lamina terminalis
• choroid plexus
• dura mater

Generally lipophilic solutes can cross the BBB which include:

• caffeine
• ethanol
• radiopharmaceuticals used in PET and SPECT imaging
• CO₂
• O₂
• nicotine

Hydrophilic solutes in general on the other hand are unable to cross it

• e.g. water soluble CT/MR contrast media

There are a multitude of conditions that are associated with disruption of the BBB. They include:

• temporal lobe epilepsy (TLE) ¹
• posterior reversible encephalopathy syndrome (PRES)
• hypoxia, ischaemia and infarction
• tumours
• inflammatory conditions, e.g. meningitis
• trauma
• intracranial irradiation
• multiple sclerosis
• progressive multifocal leukoencephalopathy: the JC virus can cross the BBB