If collaterals compensate for arterial occlusion or stenosis, symptoms may be negligible or absent. Poor collaterals may predispose to impaired reperfusion no reflow reperfusion injury or haemorrhagic transformation
- Outcome defends very much on what the fallback position is following vascular narrowing. If you have a spare parachute i.e. enough blood vessels providing cerebral blood flow above the levels that lead to infarction then the result of vessel occlusion from thrombosis or embolism is much diminished. The collateral ability of a vessel is ultimately determined by luminal calibre.
- CBF thresholds of 17 and 10 mL/100 g per minute are universally reported by reference textbooks and routinely used as those values discriminating between normal tissue (including benign oligemia; CBF >17 mL/100 g per minute), penumbra (17 to 10 mL/100 g per minute) and infarct core (<10 mL/100 g per minute)15
- Circle of Willis: anterior communicating connects right and left side circulations via the ACA and the p comm connects anterior and posterior circulations by joining PCA and MCAs.
- Extracranial and intracranial vessels e.g. via the ophthalmic artery where reversal of blood flow may provide secondary collateral support. There are anastomoses from the facial, maxillary , and middle meningeal arteries to the ophthalmic artery and dural arteriolar anastomoses from the middle meningeal artery and occipital artery through the mastoid foramen and parietal foramen
- Between Major arteries of the COW: The number and size of these anastomotic vessels are greatest between anterior and middle cerebral arteries, with smaller and fewer connections between middle and posterior cerebral arteries and even less prominent terminal anastomoses between posterior and anterior cerebral arteries.
- Leptomeningeal vessels with cortex
- The main stimulus to collateralisation is ongoing ischaemia and so those with a chronic ischaemic process may well develop sufficient backup that occlusion is silent. An extreme variant is Moyamoya disease where the collateral from and these can actually bleed.
- Those with progressive carotid stenosis can occlude with no symptoms at all. However when ICA occlusion is sudden and unplanned e.g. dissection the outcome can often be worse. There has been research to show that there are many with occluded carotids with no or minimal symptoms.