Abstract
Purpose: :
To model the use of Ishihara pseudoisochromatic plates in optic nerve dysfunction to investigate their usefulness in red, blue or combined color desaturation using a computer model, to compare them with the Hardy–Rand–Rittler plates as previously studied.
Methods: :
The 17 Ishihara plates were scanned into a PC, and were manipulated using Corel Photo–Paint 8 software, to desaturate each image from 100% color to 0% in 10% steps, using our previously presented technique. This was done for the master, red and blue channels individually. The manipulated images represent what an observer would see if their color vision was desaturated by that amount.
In 10 normal volunteers, we assessed the level of desaturation at which each subject could correctly identify each plate, and the results were amalgamated to calculate the mode % desaturation at which each plate was recognised.
Graphs were produced of the Ishihara score which we predict for each 10% loss of color saturation in the red, blue and master channel
Results: :
For master color desaturation, the the Ishihara score remained normal until vision had been impaired by 40% at which point there is an abrupt deterioration in the test scores down to 60% desaturation, followed by a gradual linear decline to 100%. For red desaturation, there is a relatively linear decrease from normal vision to 50% desaturated, at which point it plateaus at 1/17.
For blue desaturation, there is a linear decline from 10% down to 50% desaturation, at which point it plateaus at 1/17, until 90% desaturation.
Conclusions: :
Ishihara plates are relatively sensitive in the early phase of color desaturation for red or blue, but lose their discriminatory value when red or blue desaturation passes 50%. General color desaturation may not be detected until it reaches 40%, but is then relatively linear. Ishihara scores show an opposite and complementary pattern of decline in response to color desaturation in different hues, relative to the HRR plates; though neither alone provide a reliable means of monitoring color vision loss , the 2 different sets of plates together cover the range of color desaturation in each hue. As there is no single pattern of color desaturation in optic nerve disease, it is recommended that if the clinician wishes to monitor changes in optic nerve function closely, both sets of plates are used, and interpreted together, keeping in mind the patterns of effect found in these models.
Keywords: neuro-ophthalmology: diagnosis • neuro-ophthalmology: optic nerve • color vision