Glaucoma is a group of conditions of the eye where the fluid pressure inside the eye is increased. Anything that interferes with the drainage of fluid (aqueous humour) from the eye can cause glaucoma, not just inherited diseases. Glaucoma is painful, and if untreated leads to degeneration of the optic nerve and blindness. Glaucoma is one of the leading causes of blindness in dogs. Primary glaucoma refers to glaucoma that occurs in both eyes, is strongly breed related and has been documented or is strongly suspected to have a genetic basis, is associated with increasing age and results in characteristic changes to the optic nerve. Increased intraocular pressure (IOP) within the eye is a common result of the changes that occur within the eye with glaucoma, although damage to the optic nerve and other nerve tissue has been shown to occur prior to any increase in IOP in dogs and humans in recent studies. Hence increased IOP is not the sole characteristic that defines the presence of glaucoma.There are two main categories of primary (inherited) glaucoma:Primary Open Angle (POAG)Primary Narrow Angle/Primary Closed AngleThe term “open angle” glaucoma refers to the drainage angle in the eye, which is where fluid drains from the inside of the eye. The drainage angle is located between the cornea and the iris, and open angle glaucoma means that structurally this angle is not abnormally narrow (at least in the early stages of the disease), although something in the drainage angle is not working properly to allow for normal fluid drainage.This type of glaucoma is relatively uncommon, and is seen most commonly in the Beagle, in which it is inherited as an autosomal recessive disorder. A gene mutation has been identified in affected Beagles which encodes a protein involved in the structural matrix that supports cells (including those around the eye). This gene is called ADAMTS10. A DNA test is available for POAG in the Beagle and several other breeds, for example, the Norwegian elkhound has been shown to have a mutation at a different location to the beagle on the ADAMTS10 gene.With glaucoma eyes may appear red and more prominent than usual, and the cornea may have a slight blue-green haze/tinge. The pupil is dilated, and affected eyes are painful. Early signs are more difficult to detect, but the eye may be teary, slightly red and sensitive to light. Glaucoma is diagnosed with a tonometer, which measures the pressure inside the eye. Gonioscopy may be used by a veterinary ophthalmologist to look at the drainage angle inside the eye, in order to determine the form of the glaucoma. The disease is generally managed with medication (regular eye drops) although in severe cases the eye may be removed to relieve pain. If the condition is diagnosed before vision is lost, there are procedures to attempt to save vision, such as surgical destruction of several areas of the ciliary body, which produces the aqueous humour, in order to reduce fluid production and therefore pressure in the eye permanently. This is a specialist procedure, and offers the best hope for salvaging vision.