“Self-Fulfilling Prophecy” or Recognition Requires a Concept of Perception

“Self-Fulfilling Prophecy” or Recognition Requires a Concept of Perception To the Editor: The NINDS rt-PA Stroke Trial did not prospectively assess quantity and quality of ischemic brain parenchyma as detected by baseline CT.1 In this trial, no CT reading panel graded CT scans on the basis of predefined definitions. Now the trialists hypothesized that there would be disagreement among them about the presence of early CT changes.2 They randomly selected 70 baseline scans from the trial, and 16 of them reread the scans in a 1-day session. Among the investigators was 1 neuroradiologist who served as the “gold standard.” He was able to predict the lesion location at 24 hours in 96% (95% CI 92% to 100%) of the scans based on the information provided by the baseline CT. In comparison, the 16 raters of the NINDS group were much less sensitive (78%) and specific (57%) in detecting any early CT change or in identifying changes involving .33% of the MCA territory. With this poor performance, it is not at all surprising that the agreement beyond chance among these raters was somewhat low and heterogeneous. Moreover, the authors did not design and power their study to detect an effect of the method of film viewing, the effect of image quality, or an effect of the raters’ different experiences and training on agreement. To our surprise, however, they conclude from their data that all these factors do not affect the raters’ agreement. Can we generalize this experience? Correct interpretation of CT is a problem not only for the experienced stroke specialists of the NINDS rt-PA Stroke Trial Group. Emergency physicians in another study had an error rate in stroke detection by CT twice that of neurologists and radiologists, and only 17% of emergency physicians and 40% of neurologists achieved a 100% sensitivity for identification of intracranial hemorrhage.3 Missing the ischemic edema in its early stage on CT could result from technical limitations or the lack of a perceptual concept.4 Moreover, we do not agree with Dr Grotta’s assertion that the technique is the main problem. At least the group’s neuroradiologist was able to predict ischemic tissue damage from the baseline CT with high specificity, confirming the experience of others.5–7 The problem is the feature of findings in acute stroke and a lack of training to interpret these findings. Obviously, even experts in neurology are puzzled when anatomic information gets lost on CT because of gray matter hypoattenuation. The old prejudice that CT remains negative within the first 24 to 48 hours after ischemic stroke is still prevalent in review articles,8 although many studies have reported positive findings even within the first 3 to 6 hours of stroke onset.9 The specificity of positive CT findings for irreversible lesions is still widely ignored. Moreover, the diagnostic significance of a normal CT in a patient with stroke is underestimated by many stroke physicians and explained by a low sensitivity of the method. A normal CT in acute stroke means, however, that the patient will soon recover or the ischemic edema is delayed and appears after 6 hours, which occurs in about one third of the patients.7 This observation is really important for treatment strategies and the discussion about the therapeutic time window.10 Key to recognizing early ischemic changes on CT is understanding the concept of how stroke pathophysiology may appear on CT. Dr Grotta and his 20 coauthors like to remain vague in this regard: “The pathological significance of such findings, in particular hypoattenuation of the x-ray signal, is uncertain, but it probably represents increased tissue water content caused by early cytotoxic edema, which may or may not signal irreversible injury” (page 1529).2 Their uncertainty is reflected by the categories applied in this study: The authors differentiated between “loss of gray-white distinction” and “hypodensity,” although both mean x-ray hypoattenuation due to tissue water uptake. Unfortunately, the authors do not explain how they interpret the decreasing x-ray attenuation after arterial occlusion and do not discuss why CT enabled Dr Patel to predict irreversible ischemic damage with high specificity within 3 hours of stroke onset.2 The authors discuss abnormalities on diffusionweighted MRI that may be used instead to predict the irreversible damage, ignoring the relatively small database and a recent report on spontaneous reversibility of disturbed diffusion in TIA patients.11 Maybe white spots on MRI can be more easily detected than subtle changes of the gray scale on CT. Agreement among physicians seems not a problem exclusively with regard to radiological findings. We would very much like to know to what extent there is agreement among the NINDS investigators about the clinical categories “largeand small-vessel occlusion” on which they based their important conclusion that rt-PA is beneficial even in patients without large vessel occlusion. We fully agree with the authors that improved methods of recognizing early CT changes are needed not only for the NINDS rt-PA Stroke Trial Group. These methods should include formal training in reading CT for all physicians dealing with acute stroke and who cannot rely on a 24-hour neuroradiological service. Moreover, our scientific journals should further foster the comprehension of imaging by taking care of optimal image quality and interpretation. It should not happen again that an article about the need for an improvement in CT interpretation is accompanied by a CT printed upside down12 or that this journal publishes a CT with an easily visible ischemic lesion as a normal CT.13