They are unusual to the extent that, if especially if the person who had those symptoms had a mechanical heart valve, every effort should be made to rule out any other possible cause of the symptoms, because the alternatives to
warfarin are not good for someone who has a mechanical
heart valve. I think artificial heart valves are coming, probably within the next human generation, that will last much longer than current tissue valves and will not require warfarin like current mech valves do.