Varicose veins are the outward manifestation of blood pooling in the legs secondary to reflux and valve incompetence. They are described as abnormal veins on the skin surface >3 mm in diameter.
As their presence is a harbinger of underlying disease, their treatment is considered medically necessary if they cause symptoms, and therefore it is generally covered by insurance.
Conversely, spider veins are tiny veins on the skin surface measuring <1 mm in diameter. Spider veins are not necessarily a sign of underlying superficial vein disease and therefore they are considered cosmetic in terms of treatment.