Air rarely enters the vascular system naturally, so it usually takes some medical invasive procedure to achieve it, the infamous exception being diving accidents (decompression). You can basically distinguish microscopic bubbles from macroscopic air embolism.
Microscopic bubbles can occur
- during the PFO test
- angiography (heart, brain)
- ECMO and heart-lung-machines
They lead to microscopic arterial embolisms in the brain that shine brightly on T2, less so in other sequences of MRI, CT cannot show them. Depending on the emboli zed artery focal symptoms can occur, but usually only multiple small emboli lead to problems, such as encephalopathy/delirium. Note that microemboli introduced on the venous side usually don’t pass the lung, unless there is right-left shunt (PFO, Morbus Osler, …).
Macroscopic air embolism happens in typical circumstances
- Central venous access (push and pull!)
- Biopsies and other pulmonary procedures
- Endoscopy (mostly EGD)
- brain surgery (posterior fossa mainly due to the sitting position)
Now in my experience there are two possible pathophysiologic mechanisms
- Fulminant cerebral venous embolism – this can happen (very rarely) in sitting patients; the air ascends into the jugular vein and then leads to partial occlusion of the sagittal and other sinuses with consecutive brain edema.
- Arterial embolism: It turns out that the lung only has so much capacity for air. If enough is introduced on the venous side, air migrates to the arterial side and then embolizes to the brain where it leads to visible air in the vessels on CT.
Interestingly, these kinds of strokes seem to have better collateralization (perhaps some blood can get through the bubbles), because there are plenty of case reports with hyperbaric treatment that turned out successful. Apart from that some experts use Trendelenburg’s to try to get the bubbles out.
Literature: there are some case reports, but no proper reviews that cover the above. Even “Uncommon causes of stroke” doesn’t mention cerebral air embolism. I’d love to hear about your experiences.