Based on a presentation about Policy Advocacy on Color Vision Deficiency (CVD) by Dr. Warapat Wongsawad M.D., ophthalmologist and head of Vitreo-Retina Division at the Mettapracharak (Wat Rai Khing) Hospital, Thailand, at the recently held Thailand Ophthalmology Forum
Color vision deficiency or CVD affects approximately 1 in 12 men (8%) and 1 in 200 women across the globe. Patients who are not suffering from severe types of color vision deficiency may not even be aware of differences among colors that may be obvious to the rest of us and may only find about their condition after being tested.
The retina comprises three types of photoreceptor cells: rods, cones and the intrinsically photosensitive retinal ganglion cells (ipRGCs). While rod cells are responsible for scotopic vision (vision under low-light conditions), cones are responsible for color vision and function the best in relatively bright light. These cone cells are densely packed in the fovea centralis and reduce in number towards the periphery of the retina. There are about six to seven million cones in a human eye and they are most concentrated towards the macula. Three different types of cones (type S, M and L) are responsible for receiving short, medium and long wavelengths from the visible spectrum. There are a variety of factors that may lead to color vision deficiency.
The cause of deficient color vision in the majority of people is due to genetic factors which are inherited. Other patients may become color vision deficient as a result of other diseases, such as diabetes and multiple sclerosis. They may acquire the condition over time due to the aging process, medication or other factors. Chromosomes contain genes with instructions for the development of cells, tissues and organs. The 23rd chromosome is made up of two parts – either XX chromosomes if you are female or XY chromosome if you are male. If you are color vision deficient, it means that the instructions which are contained in the 23rd chromosome for the development of your cone cells are faulty and the cone cells might be missing or less sensitive to light. There might be a development error in the pathway to the brain as well.
The “faulty” gene for color vision deficiency is found only on the X chromosome and so for a male to be color vision deficient the faulty color vision deficiency gene only has to appear on his X chromosome. For a female to be color vision deficient it must be present on both of her X chromosomes.
If a woman has only one color vision deficiency gene she is known as a carrier but would not be color vision deficient. When she has a child, one of her X chromosomes will be passed down to the child. If she gives the X chromosome with the faulty gene to her son he will be color vision deficient, but if he receives the “good” chromosome he would not be color vision deficient.
This inherited color vision deficiency is caused by abnormal photopigments. These color-detecting molecules are located in the cone cells and, in humans, several genes are required for the body to synthesize photopigments. Defects in these genes may lead to color vision deficiency.
There are three major types of color vision deficiencies, which depend upon photopigment defects in the three different kinds of cones that respond to blue, green, and red light. Red-green color vision deficiency is the most common (affecting 99% of color vision deficiency patients), followed by blue-yellow color vision deficiency. A complete absence of color vision or total color vision deficiency – is rare.
Testing color vision
Clinically, Ishihara color test plates are the most commonly used tool to test for color vision deficiency worldwide due to their easy usability and high accuracy. However, in order to test for the severity of color vision deficiency, the gold standard is the Nagel’s anomaloscope.
Early symptoms in children
The main symptom of color vision deficiency is a difficulty in distinguishing colors or in making mistakes when identifying colors.
» Using the wrong colors for an object –e.g. purple leaves on trees, particularly using dark colors inappropriately
» Low attention span when coloring in worksheets. Color vision deficient children may not like to color in pictures or want to play counting or sorting games with colored blocks or beads.
» Denial of color issues
» Problems in identifying red or green color pencils or any color pencil with red or green in its composition. (e.g. purple, brown)
» Excellent night vision
» Identification of color may be made worse by low level light, small areas of color and colors of the same hue
» Smelling food before eating
» Excellent sense of smell
» Sensitivity to bright lights
» Reading issues with colored pages or worksheets produced with color-on-color
» Children may complain that their eyes or head hurt, if looking at something red on a green background, or vice versa.
Treatment and management
There is currently, no treatment for inherited color vision deficiency although research in gene therapy is ongoing. Patients may use special color friendly contact lenses and glasses for the color vision deficiency which are able to aid in the perception of colors.
Editor’s Note: The Thailand Ophthalmology Forum was held at the Sukosol Hotel in Bangkok on 2-4 August 2018. The 3-day forum was held in collaboration with the ASEAN Ophthalmology Society (AOS) and the 3rd Taiwan-Thailand Retina Society Joint Meeting.
