The ASPECTS template is weighted in favor of the striatocapsular region
Introduction
Stroke is the second commonest cause of disability in the world and a major cause of mortality (Bonita, 1992). Recent meta-analyses show that thrombolysis with recombinant tissue plasminogen activator (rt-PA) reduces morbidity in selected patients with acute ischemic stroke (Hacke et al., 2004, Wardlaw et al., 2003). One concern with the use of this therapy is the risk of precipitating brain parenchymal hemorrhage, hence the interest in using imaging techniques to predict the likelihood of this complication. Thrombolysis is then withheld from patients at high risk of hemorrhage if the imaging model can accurately predict this risk.
Previous studies have proposed that the risk of intracerebral hemorrhage (ICH) following thrombolysis is increased in patients with middle cerebral artery (MCA) territory infarcts in whom visual inspection of acute CT scans indicates involvement of >1/3 of the arterial territory by infarction (Barber et al., 2000, Fiorelli et al., 1999, Larrue et al., 1997, Tanne et al., 2002). This prediction rule is currently used in MRI-based thrombolytic trials such as Desmoteplase In Acute ischaemic Stroke [DIAS] (Hacke et al., 2005). In this paper, we refer to this guideline as ‘the rule of 1/3’.
Another bedside tool in prediction of hemorrhage, the Alberta Stroke Program Early CT Score (ASPECTS) system, was devised to help quantify early ischemic changes in the MCA territory on CT scans (Barber et al., 2000). The ASPECTS template comprises two slices, one at the level of the thalamus and striatum and one 2 cm superior to this level. One point is deducted for partial or total involvement by infarct in any of the 10 regions. According to its developers, an ASPECTS rating of 10 represents no visible infarction and a score of 0 represents diffuse ischemia throughout the MCA territory. ICH complicating thrombolysis is less likely to occur in patients with an ASPECTS rating between 8 and 10 in this patient group (Barber et al., 2000). However, the predictive value of a lower ASPECTS score for symptomatic thrombolysis-related hemorrhage did not remain in a recent re-analysis of the pivotal rt-PA trial. This conflicting finding leads to questions regarding the usage of this scale within 3 h of ischemic stroke onset as a predictive tool.
The regions in the ASPECTS template are not of uniform size suggesting that different importance (or weight) is implicitly assigned to each of the 10 regions. One consequence is that smaller regions may contribute disproportionately to the estimated risk of intracerebral hemorrhage following thrombolysis predicted by the ASPECTS score. We assessed this explicitly by determining the contribution to the MCA territory infarct volume corresponding to each ASPECTS region. This allowed us to test the hypothesis that the ASPECTS template uses an unequally weighted scoring system with greater weighting of the striatocapsular region (internal capsule and caudate and lentiform nuclei). The prime clinical motivation behind this study was to better understand the internal structure of the ASPECTS method as the use of different imaging criteria is likely to affect the proportion of patients considered eligible for thrombolysis. To examine this specifically, we tested whether an ASPECTS score of 7 corresponds to one-third involvement of the MCA territory by infarction.
Section snippets
Patient selection
The CT images were chosen from patients presenting with an acute stroke syndrome between January 2001 and February 2003 with scans showing infarct localization appropriate to the clinical presentation. The inclusion criteria were the presence of MCA territory infarction and availability of electronic and hard copies of subacute CT scans [performed 5–10 days after stroke onset] (Phan et al., 2005). In previous studies, the issue of infarct visibility has confounded the estimation of infarct
Results
Nineteen patients (12 males and 7 females) with infarcts in the MCA territory were studied with the mean age of 74 years (range: 65–88 years). The median infarct volume was 46 ml and interquartile ranges were 16 ml–80 ml (see Table 1).
Using the criteria of Landis and Koch (1977) for interpreting the kappa score, the interrater agreements among the 4 raters were substantial for the ASPECTS method (κ = 0.67).
Weights for the regions varied from 0.2% to 10.6% of the maximal DA-MCA volume. The sum
Discussion
This study of the structure of the ASPECTS scores, using MCA infarct territory volume as a comparator, showed that the ASPECTS method implicitly assigns different weights to each region. The striatocapsular region is given the greatest weight of all the regions with the clinical implication being that involvement of this region is likely to exclude patients from thrombolysis if the ASPECTS criterion is used.
We used the infarct volume of the MCA territory as a way of examining the internal
Acknowledgment
Funding: Dr. Thanh G Phan is supported by a Postgraduate Medical Research Scholarship awarded by the National Health and Medical Research Council, Australia.
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