Ocular melanosis (OM), also known as ocular melanocytosis or melanosis oculi, is a congenital disease of the eye, which affects approximately 1 in every 5,000 people, and is a risk factor for uveal melanoma. In dogs, this disease most commonly affects Cairn Terriers. It also is called Pigmentary Glaucoma because this condition often results in glaucoma (which is a high pressure in the eye that can cause discomfort and blindness). 

Ocular Melanosis is inherited, although the specific genes and mode of inheritance is not completely understood. The condition causes pigmented (i.e. brown) cells to accumulate in the eye and eventually block the drains that are responsible for removing fluid from the eye. These “clogged” drains will cause the pressure inside the eye to increase. The high pressure in the eye will damage the retina (the layer in the back of the eye that senses light) and the optic nerve (the nerve that sends the signal from the retina to the brain). Dark pigment also may be deposited on the sclera (white part of the eye).