When the affected area is pressed with a clear, clean carrier paddle or a clear, special tongue depressor, pulsation is visible and the capillaries around the nevus are dilated and arranged in a radial pattern to the naked eye. When the central point is pressed, the entire nevus disappears, and when the pressure is removed, the blood is seen to fill up from the centre outwards.
Spider nevi occur above the trunk, especially on the face, neck and hands, and can also occur in traumatic areas.