Pretreatment Advanced Imaging in Patients with Stroke Treated with IV Thrombolysis: Evaluation of a Multihospital Data Base

Discussion

Our analysis of a large, multihospital data base found wide variation in the use of imaging in patients with acute ischemic stroke. From 2008 to 2011, the percentage of patients treated with thrombolysis who received only a head CT decreased slightly from 64% to 55%. If this trend continues, more than half of the patients treated with thrombolysis will receive some form of advanced imaging during their hospitalization. This growth in the use of advanced imaging is primarily attributable to increased use of CT perfusion but also to increased use of CT angiography and MR imaging.

Imaging has been an essential part of stroke treatment planning since the introduction of CT.²⁷ Unenhanced CT has been useful for demonstrating acute hemorrhage or other mimicking lesions, as well as showing cytotoxic edema from evolving infarction. CT is standard for trauma patients in emergency departments and has been the historical standard for patients with acute neurologic conditions, so CT is already fully integrated into emergency department culture and is readily available to patients with acute ischemic stroke. In fact, readily available head CT is a defining characteristic of a primary stroke center in the United States.²⁸ If the patient is already going to the CT scanner for the standard unenhanced imaging, it is generally not very time-consuming to add additional CT angiography and/or CT perfusion studies.² However, advanced CT, including high-quality CT perfusion and CT angiography, requires hardware and software that are not available at every hospital treating patients with stroke; this circumstance might limit adoption of these imaging techniques. As CT scanners are replaced and upgraded with time, advanced CT techniques are becoming uniformly available; this availability might account for some of the increased use demonstrated in our study. The clinical utility of such advanced CT imaging, however, is not universally accepted.

Our study showed that from 2008 to 2011, screening with MR imaging had a negligible increase from 15% to 16% of patients undergoing IV thrombolysis. These results suggest that MR imaging has not been widely adopted. MR imaging has been proposed as an effective means of identifying candidates for intravenous thrombolysis in the 3- to 4.5-hour time window.^{16⇓⇓⇓⇓⇓–22} The exact time of onset relative to the time of imaging or the time of thrombolytic administration is not available in the Perspective data base, so we cannot determine how often MR imaging was used for screening patients in the 3- to 4.5-hour time window. In addition, MR imaging is not typically as readily available as CT in an emergency setting. Safety screening for implants can slow down the process of acquiring MR imaging, especially if it is not possible to immediately obtain an accurate and complete history, which occurs reasonably often in the setting of acute ischemic stroke. Another impediment is that rapid, emergent acquisition of MR imaging is not part of the culture at many hospital emergency departments.

We found that patients who underwent MR imaging had lower in-hospital mortality rates compared with patients who underwent a head CT only or other types of advanced imaging. However, patients who received MR imaging or CT perfusion had a higher rate of being discharged to long-term care compared with the head CT group. These findings suggest that the use of advanced imaging may affect these patient outcomes; however, a causal link cannot be confirmed from our observational study. It is likely that patients treated with IV thrombolysis who underwent imaging with only unenhanced CT were clinically different from those who underwent additional imaging. Patients who underwent only CT were more likely to be admitted from the emergency department compared with patients who also underwent advanced imaging. While we performed multivariate logistic regression to minimize these differences in clinical characteristics between imaging groups, differences in unmeasured clinical variables may affect our results. For example, patients treated with IV thrombolysis who underwent additional imaging were probably more likely to have severe stroke compared with patients who only underwent CT imaging, with the additional imaging used to help determine whether mechanical embolectomy was warranted. We cannot quantify such a trend because we cannot assess stroke severity at presentation.

Our study of a national cohort of inpatients who received IV thrombolysis did not show that the use of CT angiography or CT perfusion significantly improved patient outcomes. However, these findings do not suggest that these imaging examinations are not useful in specific clinical settings, such as studies comparing IV thrombolysis with combined IV and intra-arterial treatments. In such studies, CT angiography may be useful in both identifying and characterizing pretreatment arterial occlusive lesions and in identifying post treatment recanalization effects.

This study has several additional limitations. We acknowledge that coding inaccuracies occur, which can affect a retrospective study of any administrative data base; however, it is unlikely that such inaccuracies would be more prevalent in one imaging group over another. As noted above, we cannot assess stroke severity at the time of presentation. We are unable to identify patients with similar stroke severity who did not receive intravenous thrombolysis, so we do not have a control group to allow assessment of the benefit from different treatments. We cannot determine whether screening imaging was used uniformly at each center and thus was not subject to selection bias. We also are unable to assess the time of presentation, but the patients undergoing intravenous thrombolysis would typically be expected to have presented early enough for initiation of intravenous therapy within 3 hours of symptom onset. However, some patients may have been evaluated with imaging specifically for broadening of the time window to 4.5 hours, especially with MR imaging.^{16⇓⇓⇓⇓⇓–22} Our study focused on the use of advanced imaging before or on the day of thrombolysis; the use of advanced imaging after treatment was not examined. Additional studies are needed to examine the use of advanced imaging in patients treated with IV thrombolysis and to examine the effect of this imaging on patient outcomes.