A 58-year-old man with hypertension presents to the emergency department 4 hours after acute onset of right-sided weakness and facial droop. Non-contrast CT (NCCT) brain is performed. The structure marked **A** in the diagram is noted to show loss of normal gray-white differentiation, appearing hypoattenuating relative to the underlying white matter, while the remainder of the brain appears relatively normal on initial imaging. Which of the following best describes the significance of this finding in the context of acute stroke management?

Explanation

## Why "It is the insular ribbon sign, an early indicator of MCA territory infarction visible within 3-6 hours, suggesting the patient is within the window for thrombolysis and mechanical thrombectomy" is right The insular ribbon sign is a subtle but clinically important early sign of MCA territory infarction on NCCT brain, characterized by loss of normal gray-white differentiation along the lateral margin of the insular cortex (structure **A**), appearing hypoattenuating relative to underlying white matter. This sign is visible within 3–6 hours of stroke onset, when CT may otherwise appear normal or near-normal. Recognition of this sign is crucial because it confirms acute ischemia in the MCA territory and places the patient within the therapeutic window for both intravenous thrombolysis and mechanical thrombectomy. The clinical scenario (4 hours post-onset) aligns perfectly with the temporal window for this sign's appearance and intervention eligibility (Harrison 21e Ch 426; ASPECTS scoring and early MCA stroke signs). ## Why each distractor is wrong - **"It represents a hyperdense MCA sign indicating acute thrombosis, which mandates immediate anticoagulation rather than thrombolysis"**: The hyperdense MCA sign is a different early finding—a bright (hyperattenuating) thrombus visible within the MCA vessel itself. The insular ribbon sign is hypoattenuating (dark), not hyperattenuating, and indicates parenchymal ischemia, not just vessel occlusion. Both signs support thrombolysis/thrombectomy, not anticoagulation alone. - **"It is a normal variant of insular cortex anatomy and does not indicate acute ischemia; the patient should be observed without intervention"**: The loss of gray-white differentiation in the insular cortex is pathologic in the acute stroke setting, not a normal variant. Misinterpreting this as normal would delay critical intervention and worsen outcome. - **"It indicates involvement of the lentiform nucleus, which carries a better prognosis than insular involvement in acute MCA stroke"**: The finding is specifically in the insular cortex (**A**), not the lentiform nucleus (putamen, structure **C**). While both may be involved in MCA territory infarction, insular involvement is a recognized early sign on NCCT and is incorporated into ASPECTS scoring; misidentifying the location would lead to incorrect prognostication. **High-Yield:** The insular ribbon sign—loss of gray-white differentiation in the insular cortex on early NCCT—is a subtle but critical early sign of MCA stroke (visible 3–6 hours post-onset) that confirms acute ischemia and guides urgent reperfusion therapy decisions. [cite: Harrison 21e Ch 426; ASPECTS scoring; early MCA stroke imaging signs]