Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned about cataract formation in a dog with a previous diagnosis of "macular degeneration" and whether these diseases are painful.
Dogs don't get true macular degeneration, but they do get degenerative diseases of the retina that are somewhat similar. These are not painful, but they do lead to vision loss.
In young dogs we see an inherited disease called Canine Multifocal Retinopathy. This usually starts when the pup is around 4 months of age, is due to deterioration of the pigment cells in the retina, affects both eyes and leads to eventual blindness.
In older dogs we see SARDS (sudden, acquired retinal degeneration) or PRA (progressive retinal atrophy.
We know PRA is inherited, and is a slow insidious disease that often starts with loss of vision at night and progresses to total blindness. It is due to deterioration of the retina.
With SARDS we are unsure whether it is inherited or not, but the blindness comes on very suddenly.
Cataracts can form in any eye. They can be congenital, meaning the dog can be born with the predisposition to form early on or them later in life. They can also be secondary to trauma, or secondary to metabolic changes (diabetes), and they can form secondary to retinal degeneration.
Cataracts themselves are not painful, but they often lead to inflammation and that can cause glaucoma. Both inflammation and glaucoma can be quite uncomfortable. With acute glaucoma the dog can be very, very painful.
Please let me know if you have any further questions.