Findings of the International Subarachnoid Aneurysm Trial and the National Study of Subarachnoid Haemorrhage in context

Concern has been expressed about the applicability of the findings of the International Subarachnoid Aneurysm Trial (ISAT) with respect to the relative effects on outcome of coiling and clipping. It has been suggested that the findings of the National Study of Subarachnoid Haemorrhage may have greater relevance for neurosurgical practice. The objective of this paper was to interpret the findings of these two studies in the context of differences in their study populations, design, execution and analysis. Because of differences in design and analysis, the findings of the two studies are not directly comparable. The ISAT analysed all randomized patients by intention-to-treat, including some who did not undergo a repair, and obtained the primary outcome for 99% of participants. The National Study only analysed participants who underwent clipping or coiling, according to the method of repair, and obtained the primary outcome for 91% of participants. Time to repair was also considered differently in the two studies. The comparison between coiling and clipping was susceptible to confounding in the National Study, but not in the ISAT. The two study populations differed to some extent, but inspection of these differences does not support the view that coiling was applied inappropriately in the National Study. Therefore, there are many reasons why the two studies estimated different sizes of effect. The possibility that there were real, systematic differences in practice between the ISAT and the National Study cannot be ruled out, but such explanations must be seen in the context of other explanations relating to chance, differences in design or analysis, or confounding.