Smog Sign: Hazy Diffusion-weighted Imaging Restriction in Dense Axonal Tracts in the Pons on Hyperacute MRI with Remarkable Clinical Improvement After Intra-arterial Thrombectomy

Patient selection is of prime significance when considering intra-arterial thrombectomy (IAT) for patients with basilar artery occlusion (BAO). We present a case of BAO with the clinical locked-in syndrome and “smoggy” appearance diffusion restriction on diffusion-weighted imaging (DWI) in the pons on a hyper-acute magnetic resonance imaging (MRI) scan. A 62-year-old man was admitted with a National Institutes of Health Stroke Scale (NIHSS) of 21. An admission computed tomography angiogram (CTA) showed a mid-distal BAO. As the alteplase was being infused, the patient started declining with an examination suggesting locked-in syndrome. An emergent hyper-acute MRI showed true restricted diffusion on DWI at the entire right anterior inferior cerebellar artery territory, multiple punctate areas in the bilateral posterior inferior cerebellar artery territory, right > left hippocampus, and thalamic region mesial aspect, compatible with an acute posterior circulation infarct. He also had patchy or hazy pontine involvement with slight DWI signal changes but an impressive apparent diffusion coefficient (ADC) darkening in the bulk of pons. The patient was taken to the operating room (OR) for IAT within five hours of the witnessed stroke onset, successfully revascularized, and then discharged on the eighth day with NIHSS of 3. MRI at discharge showed a pronounced DWI restriction in the same areas involved in the hyperacute MRI done at admission. However, now the ADC hyper-intensity was less noticeable, with continued hazy and smoggy pontine signal changes on DWI. On the two-month follow-up, the patient had zero NIHSS and modified Rankin scale (mRS) scores, respectively. DWI-MRI changes in acute ischemic stroke behave differently in tract areas with a density of nerve axons, manifesting as a hazy or smoggy appearance: the “smog sign” on DWI. In hyper-acute MRI, "hazy" or "smoggy" diffusion restriction on DWI in different axonal tract areas like the pons can correlate with good functional outcomes if successful reperfusion therapy is offered.