CT perfusion metrics and transit time

James Anstey provides his thoughts on the recent developments in delayed cerebral ischaemia following a subarachnoid haemorrhage (SAH). Unlike TBI, where outcomes have plateaued after 20 years, outcomes have steadily improved for aneurysmal SAH. Early intervention, with an increasing amount of coiling as opposed to clipping as well as ICU all likely playing a part.

However, there is still a subsection of patients who deteriorate three days or more post their event. This is likely due to delayed cerebral ischaemia (as opposed to pure vasospasm). This is a diagnosis of exclusion in a patient who deteriorates after three days post bleed and without hydrocephalus, seizures, infection or another identifiable causal pathology.

There are several pathophysiological factors at play. Firstly, microcirculatory problems, including vasoconstriction in capillary beds and clumping with endothelial damage. This is perhaps why treatments to improve perfusion have had little success. Next, a combination of cortical spreading ischaemia and angiographic vasospasm.

Gold standard diagnosis of vasospasm remains the catheters angiography. Transcranial Doppler and CT angiography are both being used more and more and certainly have a role to play. CT angiography in particular stacks up reasonably well to catheter angiography and has a negative predictive value approaching 100%. One potential problem is overcalling the narrowing at times and has occasional artefacts. Transcranial Doppler is used occasionally however has challenges with reliable operators, is user dependent and only visualises a part of the cerebral circulation.

Patients deteriorate, and we of course want to make sense of it. But what do we do thereafter? Hypertensive therapy with the aim to improve cerebral perfusion is often the go to method. James shares his thoughts on this technique, with reference to the current literature. Similarly, we diagnose vessel narrowing as the problem, however therapies that reverses this does not seem to confer good clinical outcomes. There is a large list of failed therapies because of this fact.

This raises lots of questions about this patient group. Jame's main messages are to not become obsessive with vasospasm, use CT angiography as a good substitute for catheter angiography and be cautious of vasodilator therapies as they generally do not seem to affect long term prognosis.

This #CodaPodcast was recorded in November 2018 as part of Brain, a CICM Neuro Special Interest Group meeting.

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