Raised intra-ocular pressure is a significant risk factor for developing Glaucoma. It is a caused by a block of the outflow channels for intra-ocular fluid. Angle closure (acute) glaucoma appears suddenly and often with painful side effects and so is usually diagnosed quickly. Open angle (chronic) glaucoma tends to progress more slowly and so the patient may not notice it the disease has progressed quite significantly. Worldwide, it is the second leading cause of blindness.