If a 37-year-old RP patient has an initial total point score of 900 db, what is the probability that the patient will become legally blind (that is, have a total point score of < 250 db) by age 50?

Ophthalmology

Retinitis pigmentosa (RP) is a genetic ocular disease that results in substantial visual loss and in many cases leads to blindness. One measure commonly used to assess the visual function of these patients is the Humphrey 30−2 visual field total point score. The score is a measure of central vision and is computed as a sum of visual sensitivities over 76 locations, with a higher score indicating better central vision. Normals have an average total point score of 2500 db (decibels), and the average 37-year-old RP patient has a total point score of 900 db. A total point score of < 250 db is often associated with legal blindness. Longitudinal studies have indicated that the change in total point score over N years of the average RP patient is normally distributed with mean change = 45N and variance of change = 1225N. (Assume the total point score is measured without error; hence, no continuity correction is needed.)