Jiaying (Jayne) Zhang, MD 

It is widely accepted that Diffusion-weighted Imaging (DWI) is the most sensitive and reliable at detecting early ischemic tissue. However, there exists a lesser known phenomenon called Reversed Discrepancy (RD) between Computed Tomography (CT) and DWI that seems to contradict this general concept. Reversed Discrepancy is the failure of DWI to detect early CT ischemic lesion showing parenchymal hypoattenuation. Though the significant of early ischemic changes on CT within 3 hours of symptom onset can be controversial, it can be indicative of critical hypoperfusion and irreversible ischemia with high risk of symptomatic hemorrhage after tissue plasminogen activator (tPA) treatment.

In this study by Kawano et al, the authors aimed to describe the clinical significance of RD including the factors associated with the development of RD in stroke patients within 3 hours of onset. A total of 164 patients with ischemic stroke in the anterior circulation who underwent both CT and MRI within 3 hours of onset were retrospectively analyzed. RD was found in 24% of the study population. Interestingly, RD group patients tended to be older, had a higher admission NIHSS, higher rate of atrial fibrillation and higher rate of proximal ICA/MCA occlusion. Atrial fibrillation was also independently associated with the presence of RD.

What is striking about this study is the frequency of RD identified this group of patients presenting within the 3 hour window. This means, as the authors pointed out, that RD is more commonly encountered in acute stroke imaging than previously thought. But what is the pathophysiological significance of this phenomenon? Although the exact cause is not known, it is theorized that RD may be related to early spontaneous reperfusion resulting in pseudonormalization of Apparent-diffusion coefficient (ADC) and hence by this mechanism explain the potential reversibility of RD. Another mechanistic consideration to explore in the future is the state and extent of the collateral circulation on the symptomatic side. Perhaps poor collateral flow is a factor associated with the development of RD and that is why patients with RD tend to have a higher NIHSS and rate of proximal occlusions.