Outline the advantages and disadvantages of the various techniques used in the diagnosis and monitoring of vasospasm secondary to aneurysmal subarachnoid haemorrhage.
Answer and interpretation
Techniques that have proven or demonstrated potential in the diagnosis and monitoring of vasospasm include:
- In the conscious patient, may be detected clinically by new focal neurology or a drop in GCS.
- Advantages: No additional costs and readily available, can be repeated easily, non-invasive (usually), has to be performed at the bedside.
- Major disadvantage is lack of specificity often necessitating CT/angiography. Also lacks sensitivity, vasospasm can occur without a clinical correlate, early in the disease. Operator dependent.
Conventional 4 vessel DSA angiography:
- Remains the gold standard for diagnosis of vasospasm.
- 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. Risk of stroke (quoted about 1%, but probably a little lower) just from the angio, plus the dissections etc. that occur as well. Detects vessel narrowing, not necessarily poor flow to distal tissue in all cases (either increased flow rate through narrow vessel or collateral supply. May lead to over treatment.
Transcranial Doppler (TCD):
- It is low risk, performed at the bedside, non-invasive and able to be repeated daily enabling trend analysis.
- Disadvantages: The technique is however operator dependent and there is high interobserver 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.
- May be combined with perfusion allowing characterisation of both vascular anatomy and associated perfusion abnormalities.
- 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 delayed ischaemic neurological deficit (DIND).
- Disadvantages: 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. Similar disadvantages as per angiography with respect to transport, radiation (for CT), contrast exposure, interpretation by experts.
- 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.
- 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.
- The use of measures of tissue oxygenation using parenchymal sensors and microdialysis for monitoring biochemical indices of ischaemia are largely research tools.