PES or PEX (pseudoexfoliation glaucoma) is an age-related disease which features accumulations of microscopic granular amyloid-like protein fibers in various parts of the body, especially the eyes. It can lead to poor vision, glaucoma and various other problems. It was first described in 1917.
It's generally thought of as a systemic disorder, but its cause is unknown.
There are suggestions that it may have a genetic component, that it may be caused by a virus, or that it's the result of free-radical damage, or excess sunlight.
The number of people with the disease is unclear, but is probably in the region of 1 per 5000. Because populations are in general 'aging' it's thought that the prevalence of PES is likely to increase.
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