Hi
If there is no bluish ulceration (infection) gunk on the surface, then you are dealing with a fast onset cataract (if it looks a little like a cluster of stars inside the eyes - lots of tiny white dots) or nuclear sclerosis (if it is a more even fog developing in the lens). Either can come up strongly within a matter of days and develop fully within a week, as I know from my own experiences with a number of piggies. Old age cataracts/sclerosis are not uncommon.
We do not usually recommend to use flash on cameras in guinea pigs but if the eye shows up in the photos from a dark indoors space shining purple or blue instead of red, then you are dealing with a cataract or sclerosis. If the eye is dark from ulceration gunk sitting on the eye surface, then that is either a scratch of the cornea (potentially from something stuck under and eyelid) or from a hay poke.
Cataracts and nuclear sclerosis are not operable in guinea pigs. They are not painful. It is like a thickening piece of white/grey paper is developing in the eye for the affected piggy but the transition with a fast onset can be a bit difficult as it takes time for the other senses to adjust and to take over some of the functions. Unfortunately, in many cases the other eye will follow within a matter or weeks or months, leading to severely impaired sight or blindness.
Eyesight is thankfully the weakest sense in guinea pigs whereas it is our strongest in humans. Piggies will compensate especially with smell, touch (whiskers) and hearing. During the transition period when the second eye goes, I would recommend to not change the layout and to especially not wash any furniture and cosies at higher temperatures to allow the piggy to adjust and rebuild their mental map. Once that has happened, you won't notice much difference and you can make changes as long as you don't remove the scent of any furniture. Ramps can be safely used if they are made shallower if needed and are covered with a strong scent retaining material, like a carpet off-cut. Blind piggies follow scent spoors and they can happily zoom around following their own scent spoor in a loop or another piggy's scent spoor during roaming time. Or even jump and climb on things they can strongly smell and have memorised the height of.
I hope that this helps you? If you have concerns re. ulceration, we recommend to have some plain eye gel (preferable to drops) at home for first aid. They help to wash the ulceration gunk out, rehydrate the eye and create an ideal healing environment in deeper levels and help to speed up the healing process in case of an injury.
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Caring for Older Piggies and Facing the End - A practical and supportive information collection (contains a list of age related issues with further links)
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First Aid: Immediate Care Measures and Non-medication Products
I hope that this helps you?