BackgroundThe optimal time to commence anticoagulation in patients with atrial fibrillation (AF) after ischaemic stroke or transient ischaemic attack (TIA) is unclear, with guidelines differing in recommendations. A limitation of previous studies is the focus on clinically overt stroke, rather than radiologically obvious diffusion-weighted imaging ischaemic lesions. We aimed to quantify silent ischaemic lesions and haemorrhages on MRI at 1 month in patients commenced on early (<4 days) vs late (≥4 days) anticoagulation. We hypothesised that there would be fewer ischaemic lesions and more haemorrhages in the early anticoagulant group at 1-month MRI.MethodsA prospective multicentre, observational cohort study was performed at 11 Australian stroke centres. Clinical and MRI data were collected at baseline and follow-up, with blinded imaging assessment performed by two authors. Timing of commencement of anticoagulation was at the discretion of the treating stroke physician.ResultsWe recruited 276 patients of whom 208 met the eligibility criteria. The average age was 74.2 years (SD±10.63), and 79 (38%) patients were female. Median National Institute of Health Stroke Scale score was 5 (IQR 1–12). Median baseline ischaemic lesion volume was 5 mL (IQR 2–17). There were a greater number of new ischaemic lesions on follow-up MRI in patients commenced on anticoagulation ≥4 days after index event (17% vs 8%, p=0.04), but no difference in haemorrhage rates (22% vs 32%, p=0.10). Baseline ischaemic lesion volume of ≤5 mL was less likely to have a new haemorrhage at 1 month (p=0.02). There was no difference in haemorrhage rates in patients with an initial ischaemic lesion volume of >5 mL, regardless of anticoagulation timing.ConclusionCommencing anticoagulation <4 days after stroke or TIA is associated with fewer ischaemic lesions at 1 month in AF patients. There is no increased rate of haemorrhage with early anticoagulation. These results suggest that early anticoagulation after mild-to-moderate acute ischaemic stroke associated with AF might be safe, but randomised controlled studies are needed to inform clinical practice.