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Qualitative versus quantitative assessment of cerebrovascular reserves
H Yonas1, R R Pindzola, C C Meltzer
1Department of Neurological Surgery, University of Pittsburgh Medical Center, Pennsylvania 15213, USA.
Insights
Qualitative cerebral blood flow (CBF) measures are insufficient for identifying patients at high risk for stroke due to carotid occlusion. Quantitative CBF methods are more reliable for patient management and clinical trials.
Area of Science:
- Neurology
- Radiology
- Vascular Medicine
Background:
- Symptomatic carotid occlusion poses a significant stroke risk.
- Quantitative cerebral blood flow (CBF) studies identify high-risk patients by detecting paradoxical CBF decrease during vasodilatory challenge.
- Qualitative CBF techniques are proposed as an alternative for risk stratification.
Purpose of the Study:
- To evaluate the sufficiency of qualitative CBF measures in predicting stroke risk in patients with symptomatic carotid occlusion.
- To compare the accuracy of qualitative CBF assessment with quantitative xenon-enhanced computed tomography (Xe/CT) CBF.
Main Methods:
- Quantitative CBF data from 94 patients with symptomatic carotid occlusion using Xe/CT before and after acetazolamide challenge were analyzed.
- Qualitative ratios were derived from quantitative CBF data to mimic single photon emission tomography (SPECT) analysis.
- Patients were classified based on the change in asymmetry index, with a threshold of < -10% considered abnormal qualitative reactivity.
Main Results:
- Qualitative assessment showed a 61% sensitivity and 75% specificity for detecting compromised cerebrovascular reserves.
- Fifty percent of patients deemed compromised by qualitative criteria did not exhibit a quantitative 'steal' response (false positives).
- Eighteen percent of patients not deemed compromised by qualitative criteria had a quantitative 'steal' response (false negatives), indicating significant discrepancies.
Conclusions:
- Qualitative CBF methodology is not sufficiently accurate for identifying high-risk patients with compromised cerebrovascular reserves.
- Quantitative CBF methods, specifically Xe/CT, provide a more reliable assessment for individual patient management.
- Accurate risk stratification is crucial for effective clinical trial design and patient care in carotid occlusion.
Objective:
Quantitative studies of cerebral blood flow (CBF) combined with a vasodilatory challenge have defined a subgroup of patients with symptomatic carotid occlusion who have an increased risk for stroke. These are patients whose CBF paradoxically decreases in response to a vasodilatory challenge. Recent reports suggest that qualitative CBF techniques, such as single photon emission tomography with 99m-hexamethylpropyleneamine oxime, can also define the same high-risk subgroup. To determine whether qualitative measures of CBF are sufficient for predicting the risk of stroke, we converted our quantitative CBF data, obtained with xenon-enhanced computed tomography (Xe/CT), to qualitative ratios in a manner similar to that used with single photon emission tomography data.
Methods:
We analyzed CBF values within the territory of the middle cerebral artery for 94 patients with symptomatic carotid occlusion. Values obtained using Xe/CT before and after the intravenous administration of 1 g of acetazolamide were used to derive an asymmetry index: (Coccl - Cnon)/Cavg x 100. The difference between the postacetazolamide asymmetry index and the baseline asymmetry index was used to classify the patients into groups according to CBF values. The threshold for abnormal qualitative CBF reactivity was defined as a percent change in the asymmetry index of less than -10%. Quantitative (Xe/CT) CBF was considered abnormal ("steal" response) when the response to acetazolamide (percent change) on the occluded side was a decrease of 5% or greater.
Results:
Of 34 patients whose cerebrovascular reserves were considered compromised based on qualitative criteria, 17 (50%) did not have a steal response as defined by quantitative Xe/CT CBF (i.e., false positive). Eleven of 62 (18%) who were not considered compromised by qualitative criteria had a steal response (i.e., false negative). Our data indicate that a qualitative approach has a 61% sensitivity and a 75% specificity for detecting patients with compromised reserves. Further, the positive predictive value of this method is only 50%. Therefore, the two methodologies do not predict the same patients as having compromised reserves.
Conclusion:
Previous studies have shown that patients at high risk for stroke can be identified with quantitative CBF methods. This study shows that the important subgroup cannot be accurately defined with qualitative methodology. The implications of using the more reliable methodology are important for individual patient management and for designing clinical trials.