desaturated and similar colors occurs first. Then, defects in blue color discrimination are common in RP. The color matches of RP patients, are on average more protanomalous (i.e., requiring a greater than normal red/green mixture ratio during color matching) than those in normally sighted individuals [60]. As the retinal degeneration progresses, bright red and orange colors are the last hues that are typically lost.

Most RP patients become increasingly sensitive to light, which can include bright sunlight or diffuse glare, as in white cloudy weather. Many RP patients complain of visual impairment or of discomfort in bright light, independent of cataract. The amount of intraocular light scatter in RP has been correlated to visual field area [2]. It is possible that when vision is reduced due to the retinal degeneration, even a minimal further reduction due to bright light may move the patient into a range of functional disability [18]. Also, RP patients require a longer time to recover visual acuity following transitions between areas with different levels of light. Therefore, they experience difficulty when transitioning from a bright sunny day outdoors to dimmer indoor lighting, or vice versa [26]. To help with light sensitivity and glare, the majority of RP patients always wear sunglasses or tinted lenses when outdoors on sunny days. Most wear them only sometimes on cloudy days, and rarely if ever indoors [58].

5.2.4  Vision Fluctuations in RP

There are visual phenomena present in RP patients that are unexplained at present. The most striking is the presence of “good” and “bad” days. Many patients report having good and bad days, without any clear correlation with ambient lighting or weather. Visual acuity and contrast sensitivity measures are two to three times more variable in legally blind RP patients when compared to normally-sighted individuals [31]. Variability in visual acuity or visual field appears to increase as visual acuity or visual field is reduced in RP; however, contrast sensitivity does not appear to vary according to the level of remaining contrast sensitivity.

Periodic shifts or changes in the way the retinal degeneration affects patients’ ability to function and accomplish important tasks leads to experiences of increased disability at potentially critical times. Some RP patients indicate that stress or fatigue decreases vision temporarily, and that their vision improves when these factors are alleviated. Day-to-day decreases in visual field test results appear to be related to corresponding periodic increases in perceived stress or decreases in general health. Research in this area, based on patient feedback and focused on investigating the concerns pertinent to patients, is currently being conducted to understand and mitigate RP patients’ visual fluctuations that can result in significant distress, morbidity, and reduced quality of life [32]. Attempts to understand and manage these aspects of retinal disease processes may also help identify therapies and improve the reliability of outcome measures in clinical trials. Day-to-day fluctuations in retinal sensitivity in response to electrical stimulation with prostheses and resulting shortterm variations in visual function are also likely to occur with retinal prostheses.