They result from an increase of pressure within the veins, which cause them to swell.

Reasons for pressure increase;

Hereditary, especially if father is affected
Prolonged standing in one spot (decreases muscle pumps which pump blood back to your heart)
Height (Taller people are more likely to be affected)
Extreme muscle effort, ie. lifting heavy weights
Pregnancy (foetal development causes the uterus to push on veins)
Obesity (have an increased amount of blood within the veins)
Valve incompetence ie. the valves don’t work properly

Treatment;

Conservative – Do nothing

Non-operative – Compressive stockings, increase activity

Sclerotherapy – Injections

Operative – Remove the varicose veins