I have seen several such cases . when I was doing my static pressures with Kevin Sullivan in the late 80's early 90 we did sequential angiograms. I too noted that accessory veins present after initial maturation of BC AVF would over time have the accessory veins pruned off even though the static pressures remained unchanged. Even when central stenosis at the arch occurred in some and intra access pressure increased, the accessory veins were still not visible. I always wondered whether the change in shear forces did something at the orifice of the accessory veins. On the other hand I remember a patient with complete central occlusion and evidence of "collateral" flow with veins over the shoulder, the intercostals etc who was carrying 3 L of fluid in his arm but who maintained a flow through the system if you can believe it of 1.7 L (measured by US at the brachial artery) which basically did not change after his obstructed portion was "recanulized". so is there a difference among patients or how much pressure does it take to make the accessories open up and carry flow.
I really do not know and there are no studies to study the natural Hx since we can't do flow, pressure and angiographic studies under current re-imbursement rules.