Introduction:
Atrial fibrillation (AF) is a major cause of ischemic stroke. Patients with undiagnosed AF lack the stroke prevention provided by oral anti­coagulants. The aim of this study was to compare the in-hospital mortality rate between ischemic stroke patients diagnosed with AF de novo and ischemic stroke patients diagnosed with AF prior to hospitalization for stroke.

Material and methods:
We identified patients admitted to the Neurology Center during the years 2013–2014 with acute ischemic stroke and AF. We analyzed in-hospital outcomes in patients with newly diagnosed AF and those with known AF. The study endpoint was death during hospitalization.

Results:
The study included 2,000 patients with acute ischemic stroke, out of whom 579 patients (29%) were diagnosed with AF. AF was newly diagnosed in 123 patients (21.2%) (new-AF group), while 456 patients (78.8%) had a history of AF (previous-AF group). The mean National Institutes of Health Stroke Scale (NIHSS) score at admission was 7.2 points in the new-AF group and 3.7 in the previous-AF group (p < 0.001). In-hospital death was more common in the new-AF group (13 patients, 10.6%) than in the previous-AF group (16 patients, 3.5%) (p = 0.003). In multivariate analysis, the NIHSS score at admission in the new-AF group was associated with higher mortality, while in the previous-AF group, the NIHSS score at admission and multiple ischemic foci were risk factors of in-hospital mortality.

Conclusions:
Newly diagnosed AF in ischemic stroke patients significantly worsens prognosis compared to patients previously diagnosed with AF. Early detection of latent AF and subsequent use of anticoagulation are important in preventing severe stroke.