Outline the advantages and disadvantages of the various techniques used in the diagnosis and monitoring for vasospasm secondary to aneurysmal subarachnoid haemorrhage.
Techniques that have proven or demonstrated potential in the diagnosis and monitoring of vasospasm include:
Clinical; in the conscious patient, may be detected clinically by new focal neurology or a drop in GCS.
Major disadvantage is lack of specificity often necessitating CT/angiography. EEG; May provide prognostic information, focal areas of slowing correlate with
angiographic vasospasm and a decrease in alpha to delta ratio strongly correlates with ischaemia. Sensitivity and specificity for detecting vasospasm is high.
Disadvantage: Not readily available however and their may be issues with interpretation.Conventional 4 vessel DSA angiography-
- remains the gold standard for diagnosis of DIND
- may allow therapeutic intervention (angioplasty) at the time.
Disadvantages -invasive, risks of bleeding, embolism, radiation/contrast exposure and transport. Requires skilled interventional radiology and therefore resource heavy.
Transcranial Doppler (TCD):
- It is low risk, performed at the bedside, non invasive and able to be repeated daily enabling trend analysis.
- The technique is however operator dependent and there is a high inter observer variability.
- Debate exists regarding correlation of flow velocity and vasospasm and although high velocities (> 200cm/sec) are predictive, lower velocity may not be as good.
- The technique may be more accurate when MCA velocity is indexed to the ipsilateral extracranial carotid artery (Lindegaard index, >3 strongly predictive).
- Colour coded TCD may offer greater accuracy than plain TCD alone.
CTA/MRI: may be combined with perfusion allowing characterisation of both vascular anatomy and associated perfusion abnormalities.
Image clarity will be affected by clip/coil and contrast related issues need consideration. The overall diagnostic capability of this modality however remains unclear until further prospective studies are performed. MR diffusion weighted imaging accurately identifies brain tissue at high risk of infarction; perfusion weighted imaging reveals asymmetries in regional perfusion. Both methods show correlation with DIND
-can be used to obtain a picture of brain perfusion and metabolism and have shown variable correlation with vasospasm as assessed by more conventional methods.
Disadvantages: They are resource heavy not easily available, radiation exposure, patient transport are issues.
The use of measures of tissue oxygenation using parenchymal sensors and microdialysis for monitoring biochemical indices of ischaemia are largely research tools.
This question closely resembles Question 2 from the second paper of 2013.