Glaucoma is a disease that progressively damages the optic nerve at the back of the eye.  There are a number of forms of glaucoma, but generally it gradually and painlessly causes loss of parts of the field of vision.  Sometimes but not always glaucoma is associated with high pressures inside the eye; other risk factors include a family history (first degree relative) with glaucoma, age, diabetes and high blood pressure.  Because it is symptomless until well advanced, glaucoma can be difficult to diagnose, but newer technologies such as optical coherence tomography are making earlier diagnosis possible, along with long-established tests such as measuring the pressure inside the eye and the fields of vision with perimetry.  Glaucoma is most usually treated with drops to reduce the pressure inside the eye, but sometimes surgery is required.