Screening for UIAs has major implications for both the individual at risk and the population as a whole. The financial cost of treatment on a per-case basis of aneurysmal SAH has been shown to be significantly greater than that of elective treatment of an UIA. Studies suggest that elective patients have a shorter length of stay and less associated morbidity and mortality, except Hunt and Hess Group V, where early mortality limits medical expenditure. This suggests that early detection and prophylactic occlusion of cerebral aneurysms may significantly reduce treatment costs. Any saving made, however, has to be balanced against the cost of treating lesions that, if left untreated, would never rupture and the cost of implementing such a screening programme.

In any screening programme, it is detection rather than characterization that is the goal. Until recently, DSA has been the only investigation with a satisfactory sensitivity and specificity for detecting aneurysms but it is costly, invasive and has associated risks and is therefore not suitable as a widely used mass screening tool. With neuroradiological advances, the use of MRA and CTA as screening investigations has been increasing. MRA involves no radiation or contrast and has no known risk to the patient, other than those associated with ferromagnetic implants. MRA is therefore preferred, particularly for patients with a history of adverse contrast reactions or at an increased risk of contrast toxicity. CTA, on the other hand, may prove to be better at screening for de-novo aneurysms in the presence of aneurysm clips. Any aneurysm identified by non-invasive methods can be further characterized by DSA.

The effectiveness of screening also depends on the temporal profile of aneurysm formation. If indeed aneurysms do arise over a relatively short time period, during which they are most likely to rupture, a single aneurysm screen may fail to identify the most hazardous lesions, resulting in a wholly ineffective screening program. Neurosurgeons and radiologists may then be left to treat a large number of stable aneurysms, many of which would never have ruptured.

Craniotomy for UIA has until recently been uniformly reported to yield excellent outcomes. These figures have recently been disputed by the ISUIA study results suggesting that mortality and morbidity may be as high as 14.4% [4]. This means that the impact of screening on aneurysm-related death and disability may be disappointing, particularly if the ISUIA results are shown to be nearer the truth.

It is currently less expensive to screen and treat any population of patients that has a minimum aneurysm prevalence of 5% and life expectancy of at least 20 years, assuming a 2% annual rupture rate for UIA, a screening cost of $500 for MRA and a combined mortality and morbidity of treatment of 5% or less. Endovascu-lar treatment may reduce costs further if it is able to shorten hospital stay and therefore reduce costs. However, with its associated longer-term follow-up and higher risk of recurrence, the cost of repeat DSA and endovascular procedures means that this is not currently the case.

Schievink [23] currently recommends screening first-degree relatives aged 18-65 years once a familial aggregation has been established, as 10% of these individuals will have IAs and one-third of these lesions will be greater than 5 mm. In those who have a negative MRA, screening is undertaken 5-yearly to detect de-novo aneurysm formation. Those with affected siblings are screened biannually during the corresponding decade of life at which their sibling was diagnosed. Screening is only considered for children if family members aged less than 18 years have been affected and all monozygotic twins are screened. Screening for IAs in families with only one affected member is not recommended. These individuals have only a modest SAH risk and screening identifies UIAs in only 2-4%.

We currently need a better understanding of the natural history of aneurysm formation and

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