Appropriate use of CT perfusion following aneurysmal subarachnoid hemorrhage: a Bayesian analysis approach

Abstract

Background and purpose: In recent years CTP has been used as a complementary diagnostic tool in the evaluation of delayed cerebral ischemia and vasospasm. Our aim was to determine the test characteristics of CTP for detecting delayed cerebral ischemia and vasospasm in SAH, and then to apply Bayesian analysis to identify subgroups for its appropriate use.

Materials and methods: Our retrospective cohort comprised consecutive patients with SAH and CTP performed between days 6 and 8 following aneurysm rupture. Delayed cerebral ischemia was determined according to primary outcome measures of infarction and/or permanent neurologic deficits. Vasospasm was determined by using DSA. The test characteristics of CTP and its 95% CIs were calculated. Graphs of conditional probabilities were constructed by using Bayesian techniques. Local treatment thresholds (posttest probability of delayed cerebral ischemia needed to initiate induced hypertension, hypervolemia, and hemodilution or intra-arterial therapy) were determined via a survey of 6 independent neurologists.

Results: Ninety-seven patients with SAH were included in the study; 39% (38/97) developed delayed cerebral ischemia. Qualitative CTP deficits were seen in 49% (48/97), occurring in 84% (32/38) with delayed cerebral ischemia and 27% (16/59) without. The sensitivity, specificity, and positive and negative predictive values (95% CI) for CTP were 0.84 (0.73-0.96), 0.73 (0.62-0.84), 0.67 (0.51-0.79), and 0.88 (0.74-0.94), respectively. A subgroup of 57 patients underwent DSA; 63% (36/57) developed vasospasm. Qualitative CTP deficits were seen in 70% (40/57), occurring in 97% (35/36) with vasospasm and 23% (5/21) without. The sensitivity, specificity, and positive and negative predictive values (95% CI) for CTP were 0.97 (0.92-1.0), 0.76 (0.58-0.94), 0.88 (0.72-0.95), and 0.94 (0.69-0.99), respectively. Treatment thresholds were determined as 30% for induced hypertension, hypervolemia, and hemodilution and 70% for intra-arterial therapy.

Conclusions: Positive CTP findings identify patients who should be carefully considered for induced hypertension, hypervolemia, and hemodilution and/or intra-arterial therapy while negative CTP findings are useful in guiding a no-treatment decision.

Fig 1.

A, Graph of conditional probabilities for CTP determining DCI in aSAH. The blue curve represents the spectrum of posttest probabilities for a positive CTP finding. The red curve represents the spectrum of posttest probabilities for a negative CTP finding. The horizontal black lines represent the treatment threshold posttest probabilities of 30% and 70% for HHH and intra-arterial therapy, respectively. B, Graph of conditional probabilities for CTP determining vasospasm in aSAH. The blue curve represents the spectrum of posttest probabilities for a positive CTP finding. The red curve represents the spectrum of posttest probabilities for a negative CTP finding. The horizontal black lines represent the posttest probability treatment thresholds of 30% and 70%, for HHH and intra-arterial therapy, respectively.

Fig 2.

A 65-year-old woman with a CTP examination 6 days following aSAH. A, The MTT map demonstrates diffuse prolongation of MTT in the vascular territory of the right middle cerebral artery (arrows). B, The CBF map from the same level demonstrates a reduction in CBF in a similar distribution. C, The GCP for DCI is customized for this patient on the basis of the Hunt and Hess scale scores, as a sample clinical predictor. The vertical dashed lines represent the pretest probabilities of 35% and 61% for Hunt and Hess scale grades 1–2 and 3–5, respectively. The horizontal dashed lines indicate the posttest probabilities for these 2 Hunt and Hess scale grade classifications. The posttest probabilities for a positive CTP finding remain above the HHH treatment threshold and do not contribute to treatment decisions. However, the posttest probabilities of a negative CTP finding are below the HHH treatment threshold and do alter treatment decisions. Thereby, performing CTP to assist in HHH treatment decisions is considered appropriate in both low and high Hunt and Hess scale grades.