A1 (Grade A — Definite): Ipsilateral large artery stenosis > 50% on CTA/MRA/angiography in the symptomatic territoryA2 (Grade B — Uncertain): Ipsilateral ≤ 50% stenosis with plaque ulceration, or atherosclerotic plaque without stenosis in appropriate territoryA3 (Grade C — Unlikely): Atherosclerosis present but in non-causal territory or distal vessel without direct stroke territory supply

S1 (Grade A — Definite): Classic lacunar syndrome with subcortical infarct ≤ 15mm in deep perforator territory (corona radiata, internal capsule, thalamus, basal ganglia, pons) with no other identified causeS2 (Grade B — Uncertain): Moderate-severe white matter hyperintensities (Fazekas 2–3) or lacunes present but infarct pattern not classic for lacunar originS3 (Grade C — Unlikely): Mild white matter hyperintensities (Fazekas 1) present but clinically implausible as stroke cause given imaging pattern

C1 (Grade A — Definite): High-risk cardiac source identified: AF/flutter, intracardiac thrombus, mechanical prosthetic valve, bacterial endocarditis, dilated cardiomyopathy (EF < 30%), myxoma, recent (< 4 weeks) large anterior wall MIC2 (Grade B — Uncertain): Medium-risk cardiac source: PFO + DVT/shunt evidence, moderate LV dysfunction (EF 30–50%), hypokinetic segment, LA/LAA enlargement without AF, non-bacterial thrombotic endocarditisC3 (Grade C — Unlikely): PFO alone without DVT/pulmonary embolism context or other embolic risk; presence of minor cardiac anomalies

O1 (Grade A — Definite): Specific identified rare cause: hypercoagulable state (antiphospholipid syndrome, protein C/S/AT III deficiency, factor V Leiden), vasculitis (CNS or systemic), CADASIL, Fabry disease, moyamoya, irradiation vasculopathy, paradoxical embolism confirmed, endocarditis (non-bacterial thrombotic)O2 (Grade B — Uncertain): Hypercoagulable state present but causal link uncertain (elevated factor VIII, mild thrombophilia); ANCA vasculitis with uncertain causal relationship to strokeO9 (Insufficient workup): Hypercoagulable panel not yet obtained; specialized testing not complete (e.g., genetic testing, vasculitis workup pending)

D1 (Grade A — Definite): Confirmed cervical or intracranial artery dissection on CTA/MRA (intimal flap, false lumen, mural hematoma, 'string sign', or double lumen) in supplying vessel of stroke territoryD2 (Grade B — Uncertain): Irregular vessel wall or subtle narrowing in appropriate territory but dissection not definitively confirmed on available imaging — additional MRI vessel wall imaging or DSA plannedD0 (Grade 0 — Absent): Good quality CTA/MRA of cervical and intracranial vessels — no dissection identified

ASCOD Classification — Stroke Etiology Phenotyping System

ASCOD Classification — Stroke Etiology Phenotyping System