As humans we perceive the world around us in the visible spectrum spanning 480 to 740 nanometers. This narrow range nevertheless encompasses the infinite number of hues that we can see around us. The normal human eye can sense and distinguish these colors with the use of cone shaped cells in the retina, called ‘cones’.
Individuals with color blindness lack the ability to accurately distinguish between several colors to varying degrees due to defects in their cone cells.
There are three different types of cones
- Red Cone or Protan
- Green Cone or Deutran
- Blue Cone or Tritan
Causes of Color Blindness
- Genetic defects in the cones: These are inherited genetic abnormalities in the cones. This is the most common cause of Color Blindness. Men are more likely to be affected as the condition is most commonly inherited in an X-linked recessive pattern.
- Direct Injury to the cones, the optic nerve carrying signals to the brain or to the areas in the brain processing color vision.
- Cataracts: This is a clouding of the eye lens which commonly occurs in old age and causes objects to be dimmer and less sharp than normal.
- Neurological Disorders like Alzheimer disease and Parkinson Disease
Types of color blindness
There are three broad categories of color blindness determined by the type of cone affected.
- Red-Green Color Blindness: This is the most common, with a male predominance and affects roughly 8% of men and 0.5% of women with Northern European ancestry. Affected individuals have defects in either their red cone (protan) or green cone (deuteron) resulting in difficulty distinguishing between the colors red, orange, yellow and green.
- Protanomaly: The red cones are abnormal, leading to dimming of reds, yellow and oranges, making them look more like green. This affects about 1% of all males. The defect does not pose major problems and individuals live normally.
- Protanopia: The red cones are non-functional, making red appear black. The orange and green appear more yellow. This also affects 1% of males.
- Deuteranomaly: The green cones are abnormal where, the colors yellow and green appear closer to red. This condition is the most common, affecting 5% of males. It is mild and doesn’t affect daily life to a large extent.
- Deuteranopia: The green cones are non-functional, with the color green appearing beige and reds appearing brownish-yellow. This affects about 1% of males.
- Blue-Yellow Color Blindness: This is a rarer form of color blindness where the blue cones (tritan) are abnormal or missing. This affects males and females equally as the genetic defect is not on the X chromosome.
- Complete Color Blindness: This is very rare, and results in difficulty in distinguishing any of the colors. Affected individuals would likely see everything in only one colour or in greyscale. This form is frequently associated with poor vision and/or sensitivity to bright light. This condition affects both males and females equally.
Inheritance Patterns of Color Blindness
The location of a genetic defect on the chromosome determines the pattern in which a condition is inherited. Color Blindness most commonly occurs due to a genetic defect in the cone cells. Most of the genes responsible for the cones are located on the X chromosome.
The X and Y chromosomes determine the genetic sex of an individual. Men have one X chromosome (from their mother) and a Y chromosome (from their father), while women carry two X chromosomes (one from each parent).
Red- Green color blindness is an X-linked recessive disease. This means that if a male child has the abnormal gene on his X chromosome, he is almost sure to manifest the condition. A female on the other hand would need to have both X chromosomes affected to manifest symptoms.
The other types of color blindness are determined by genes present on the autosomal chormosomes. These are the non-sex chromosomes which are present in 2 sets in every individual. There is no preference for either males or females when inherited.
Symptoms of Color Blindness
The symptoms most commonly presents in childhood, but is often not noticed until much later.
- Difficulty distinguishing between colors while choosing clothes, painting etc.
- Confusion between the reg-green-amber of traffic signals for those with red-green blindness.
- Difficulty determining colors of fruits when ripe and unripe
Diagnosis of Color Blindness
Screening of all individuals for color blindness and especially male children can identify this condition early. The following tests are most commonly used:
- Ishihara Color Test: This is most commonly used test for Red-Green blindness screening. It consists of a series of cards where a number or a pattern is to be identified. Individuals with color blindness have difficulty or are unable to visualise the patterns.
- The Cambridge Color Test: This is a computer based test, very similar to the Ishihara charts. The individual is required to identify the letter C present against a background color by pressing a corresponding computer key.
- The Anomaloscope: In this test, the individual looks into an eyepiece showing a semicircle of yellow light which can be adjusted for brightness and a lower semicircle of re-green light which can be mixed to varying degrees. The goal is to make the color match. The degree of different sources used indicate the type of color blindness.
- The Fansworth Lantern Test: this test is used by army recruitment to determine is a prospective recruit has severe color blindness. The US military uses this in recruitment.
Treatment of Color Blindness
- There is unfortunately no available method to treat color blindness. Some mild forms of red-green color blindness can be corrected using specialised lenses to enhance the colors. Mobile applications are helpful in identifying different colors in photos to aid shoppers buy clothes and fruits.
- Traffic signals are often designed in patterns or in a defined order of colors so that color blind individuals can depend on that rather than the color. This assists but doesn’t fully remove the risk of misinterpreting signals.
- Color vision deficiency. American Optometric Association. http://www.aoa.org/patients-and-public/eye-and-vision-problems/glossary-of-eye-and-vision-conditions/color-deficiency. Accessed Mar. 19, 2019.
- Facts About Color Blindness. National Eye Institute. https://nei.nih.gov/health/color_blindness/facts_about. Accessed Mar 20,2019