Action-like wall

The veins dilate gradually, reaching the last valve at the distal end and the vena cava at the proximal end. If the fistula is large and the pressure in the vein increases sharply, a pulsating mass may be formed locally due to venous swelling a few weeks after the trauma, much like a pseudoaneurysm. When the fistula is small, the vein gradually dilates at the fistula, the lining of the vein thickens, and fibrous tissue grows. As the vein wall gradually thickens, an "action-like wall" is formed.