Onyx is a new agent for endovascular embolisation i.e. blocking off of abnormal vessels by injecting a blocking agent through a tube (catheter) in the blood vessel. The catheter is introduced through the groin and guided into the abnormal area of blood vessels. Calf AVMs respond well to embolisation, and Onyx, which is a new agent, is superior to cyanoacrylate, the older agent. Recent published series in peer reviewed standard medical journals have shown the superiority of Onyx in efficacy and with reference to side effects and complications. Although the agent is far from perfect it is currently the finest one available. Its full composition is 6%, 8% or 20% ethylene vinyl alcohol copol dissolved in dimethyl sulfoxide.