My eye doctor's office specializes in treating patients with certain eye disorders such as strabismus, nystagmus, and related conditions--hence tertiary care. All his patients are referred to him by other eye doctors (secondary care). As a result, they have very specialized equipment to test for those disorders and related ones (they were the only ones to discover some of my eye problems because of their specialized equipment) because they assume other conditions have already been tested for at the secondary care step. Since my secondary care physician missed most of my eye problems, and my tertiary care physician doesn't even check for certain eye problems, my glaucoma went unnoticed until this summer. In fact my eye doctor has told me straight up that I am one of the very few patients they treat that has problems outside of the field that they specialize in.