Patients with lone atrial fibrillation uniformly exhibit abnormal atrial histology, most commonly compatible with myocarditis, suggesting an underlying structural or inflammatory substrate rather than a purely electrical phenomenon.
Abnormal atrial histology was uniformly found in multiple biopsy specimens in all patients with LAF. It was compatible with a diagnosis of myocarditis in 66% of patients (active in 25%) and of noninflammatory localized cardiomyopathy in 17% and was represented by patchy fibrosis in 17%. The cause of the pathological changes, which were found only in atrial septal biopsies but not in biventricular biopsies, in 75% of patients remains unknown.
Frustaci et al. (1997) studied this question.