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Background:Anderson–Fabry disease (AFD) patients with cardiac involvement frequently complain of exertional symptoms. Previous studies have described a low incidence of resting left ventricular outflow tract obstruction (LVOTO) in AFD. We hypothesized that some symptomatic AFD patients have latent LVOTO which develops during physical exertion.Methods: Fourteen AFD patients (6 male (43%), mean age 54.3±9.9 years) with moderate to severe cardiac symptoms but without resting LVOTO (<30mmHg) on routine echocardiography underwent upright exercise stress echocardiography on a bicycle ergometer with simultaneous echocardiography during exercise and recovery. Latent obstruction was defined as a peak LV outflow tract gradient (LVOTG) ≥50 mmHg during or after exercise.Results: Six patients (43%) developed a LVOTG ≥50 mmHg during exercise (figure 1). In five patients the mechanism of obstruction was complete SAM of the mitral valve leaflets (MVLs). In one patient the LVOTG was the result of a narrow LVOT, the presence of a tendon running between the septum and the papillary muscles and contact between MVLs and septum. Patients with latent LVOTO had significantly smaller LV cavities compared to those patients without latent LVOTO (mean LVedd/BSA 21.5 mm/m2 and 26.2 mm/m2 respectively, p=0.007). In two cases surgical treatment of the obstruction resulted in a significant improvement in symptoms.Conclusion: We document for the first time the presence of provocable LVOTO in symptomatic AFD patients with cardiac involvement and successful surgical septal myectomy procedures in two females affected with the disease. These findings suggest that all AFD patients with unexplained exertional symptoms should undergo exercise stress echocardiography in order to detect and then treat latent LVOTO.