The term “varicose vein” refers to any abnormally dilated vein (>3mm in diameter) that is visible through the skin surface.
Veins are present in a wide variety of sizes (ranging from microscopic, up to 2-3cm diameter in the torso). Although the diameter and precise anatomic location of veins has some degree of variability, veins in the legs can generally be divided into two categories (“superficial” veins and “deep” veins). The “superficial” veins are located relatively close to the skin (or within a few centimeters of the skin surface), whereas the “deep” veins are more centrally located in the deep muscular compartments of the legs.
To help prevent blood from flowing in the wrong direction many veins in the legs contain small flaps of tissue on their internal surface that function as one-way valves. These valves allow blood to flow towards the heart, but prevent blood from flowing in the wrong direction. When these valves are not functioning correctly, blood flow can occur in the wrong direction which increases the back-pressure within the veins in the lower parts of the leg.
Although less common than valve dysfunction, other potential causes of increased pressure within the veins in the lower leg can can occur when upstream veins become clogged with clot (thrombus), or narrowed due to scar tissue. The relationship between a venous blood clot and the current (or ultimate) effect of venous blood-flow is complex. Sometimes blood clots can completely obstruct venous blood flow, and on other occasions blood clots are only “partially occlusive”, in which case blood can continue to flow around the clot.
The catch-all medical name for condition where the veins in the lower part of the leg are under increased pressure is “chronic venous insufficiency” (CVI).