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a person with blind spots or scotomas is suffering from which disease of the eye
The most common causes of central scotomas are disorders of the optic nerve, choroid or retina, such as macular degeneration.
what causes macular degeneration
No one knows exactly what causes dry macular degeneration. But research indicates it may be related to a combination of heredity and environmental factors, including smoking and diet.

Question: what is the average age of people with the disease
Self-contained rewrite: what is the average age of people with macular degeneration