When the eye pressure is greater than the arterial blood pressure, arterial blood cannot enter the eye to nourish the retina. A sudden build up of pressure leads to acute blindness and damage to the optic nerve. A slower build up causes few symptoms, yet leads to the same results. It occurs in a dog because it possesses physical or physiologic traits that predispose it to glaucoma. These are usually predetermined by genetics.
Secondary glaucoma means that the disease is secondary to, or caused by, another condition.
A common example is a penetrating wound to the eye. Other causes would include bleeding in the eye, inflammation within the eye, attachments or scarring between the iris and lens, and luxation or displacement of the lens. Lens luxation can cause glaucoma and likewise chronic glaucoma can cause lens luxation. The pain may present itself with the dog rubbing his eye with his paw, against the furniture or carpet, or your leg. This is a common, and often unnoticed, early sign.