When the vein walls deteriorate, varicose veins develop. The weaker vein walls allow it to expand as blood pressure increases. The valves in your vein that maintain the blood flow in one direction can't work correctly as your vein stretches. Slow-moving blood collects or pools in your vein, causing it to enlarge, bulge, discolor, and twist. Most of the time, it doesn't cause any significant issues, but if your varicose veins are causing you pain and discomfort, please consult a vein specialist as soon as possible.