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In the paper by White et al (this issue, pp322–328)1 non-invasive tests for the detection of intracranial aneurysms are evaluated. Combining these tests is shown to improve the diagnostic performance, the combination of power Doppler and CT angiography having the greatest sensitivity (0.83) for aneurysm detection with a strong agreement with the gold standard of digital subtraction angiography (κ 0.84).
This paper shows that the performance of these non-invasive imaging techniques is improving. One of the concerns raised is that small aneurysms, of 5 mm or less, cannot be reliably identified. This concern has been diluted by findings of the International Study on Unruptured Intracranial Aneurysms (ISUIA)2 which indicated that aneurysms under the size of 10 mm have a very low rate of spontaneous rupture (about 0.05%/year) and do not justify the risks of treatment in most cases.3 While the technology progresses, what remains to be established are the indications for their application, particularly in the context of the very low complication rate (0.07% risk of permanent neurological deficit) for conventional digital subtraction angiography (DSA),4 which does require a short inpatient stay.
What are the indications for imaging for suspected intracranial aneurysms? These can be divided into two groups—those patients who have had a subarachnoid haemorrhage (SAH) and those who have not. For patients with an SAH, the indications are firstly to investigate the cause of an acute bleed, secondly to follow the progress of other unruptured aneurysms in the patient, and thirdly, surveillance for those who have received endovascular treatment. Currently, investigation of acute SAH requires conventional DSA. The consequences of inadequate information in the investigation of a ruptured aneurysm is considerable, and a 0.83 sensitivity for non-invasive imaging is not sufficient. The surgical technique is also directed by the anatomy of the aneurysm as shown on DSA. The decision to treat unruptured aneurysms detected at the time of DSA after SAH can usually be taken at the time after recovery from treatment of the ruptured aneurysm. There is rarely an indication to follow such aneurysms. The risk of rupture is 0.5%/aneurysm/year and the decision to treat therefore depends on the age of the patient and the risk of the treatment, taking into account the size and location of the aneurysm. In general, small aneurysms in elderly people do not require treatment (and there is no indication for repeat imaging) whereas younger patients with aneurysms above 10 mm should be counselled and offered treatment. In terms of endovascular surveillance DSA is the standard. Non-invasive imaging that can detect a residual aneurysm neck and refilling would realise considerable cost savings.
For patients without SAH, recent evidence has indicated that there is no benefit in screening asymptomatic relatives.5 The criteria for a satisfactory screening test are not fulfilled, with potential morbidity of treatment exceeding any potential benefit. Asymptomatic lesions found incidentally on imaging which approach 10 mm in size may lend themselves to non-invasive surveillance.
Finally, does non-invasive imaging offer an advantage over conventional DSA in addition to the negligible complication rate? In certain circumstances, especially giant aneurysms, CT angiography can offer benefits in defining the relation between the aneurysm, respective cerebral arteries, and skull base, assisting in planning and risk assessment.
In summary, the paper by White et al 1 illustrates the considerable progress that has been made in refining non-invasive imaging of cerebral aneurysms. In addition to refining the sensitivity of these techniques, future developments need to parallel modern ideas on clinical indications for aneurysm detection and surveillance.