Prinzmetals variant angina is caused by coronary artery spasms. A coronary artery spasm is a temporary, abrupt, and focal (restricted to one location) contraction of the muscles in the wall of an artery in the heart. This spasm constricts the artery, slowing or stoping blood flow. A prolonged spasm can cause chest pain, or even a heart attack (myocardial infarction). 
What causes Prinzmetals variant angina ?