What is the bump in the arms of dialysis patient?

Fistula +/- aneurysm: Dialysis patients need a mature vein that can withstand repeated puncture which is created by diverting arterial flow into a nearby vein. Needling the same site causes a loss of pain nerve fibers, and thus patients prefer that site for repeated use. This can cause fibrosis and thinning of the wall at that site, resulting in aneurysmal dilatation. This is not in danger of rupturing.

Dialysis access: I think you are referring to the permanent dialysis access, which is created by joining the patients own veins to the arteries so that they have a high blood flow through which two needles can be placed to get blood out of the body to the machine and back after dialysis. These fistula may become large uniformly or only at specific places. Many times they do not cause problems but your dialysis n.