Thursday, June 4, 2015

color vision deficiency

Ishihara cupcakes
What allows us to see color?
The retina (the tissue that lines the back of the eye) contains specialized cells called photoreceptors that respond to light. There are two types of photoreceptors: rods and cones.  Rods are responsible for vision in dim light, and cones are responsible for vision in bright light as well as color vision. There are three types of cones; each type is sensitive to a different wavelength, and thus a different color of light. An S cone is sensitive to short wavelengths (blues), an L cone is sensitive to long wavelengths (reds), and an M cone is sensitive to medium wavelengths (greens). The information gathered from these cones is used by the brain to create our perception of color.  Normal trichromatic color vision involves the presence and proper functioning of all three cones.  Color vision defects arise when one of these cones is missing or altered.

What is color blindness?
The term "color blind" is misleading, as most people use the term to describe someone with a color vision deficiency. They can see colors, but have trouble distinguishing between certain colors and shades.  Color vision defects can be inherited or acquired. 
  • Inherited: These color vision deficiencies are present at birth, affect both eyes equally, and are stable over a person's lifetime. They occur in about 8% of males (the highest prevalence is found in non-Hispanic whites 1 ) and 0.5% of females (2).
  • Acquired: Unlike inherited deficiencies, these occur during a person's lifetime, may affect one eye or both eyes unequally, and may progress. Eye diseases or injury, the use of certain medications, or exposure to certain chemicals can cause an acquired color vision issue.

What types of color vision deficiency exist? 3
1. Achromatopsia, also known as rod monochromatism, is true, total color blindness. There is a complete absence of functional cones, so everything is seen in shades of gray. This rare and severe condition is typically associated with other signs like light-sensitivity, poor vision, and nystagmus (eye jitter). 
    • Inheritance pattern: Autosomal recessive
    • Prevalence: 0.00003% of males 
  • Partial or incomplete achromatopsia refers to even rarer conditions in which there is only one of the three types of cones present/functioning.  Blue cone monochromacy (BCM) is the most common of these, but with a prevalence of one in 100,000, it is still very rare.  
2. Dichromacy is a category of color deficiency in which there are only two types of cones present instead of three. 
  • Protanopia- missing L (red) cone. 
    • Inheritance pattern: X linked recessive
    • Prevalence: ~1% of males, ~0.1% of females
  • Deuteranopia- missing M (green) cone. 
    • Inheritance pattern: X linked recessive
    • Prevalence: ~1.5% of males, ~0.01% of females
  • Tritanopiamissing S (blue) cone. 
    • Inheritance pattern: Autosomal recessive
    • Prevalence: 0.008%, males=females
3. Anomalous Trichromacy is a category of color deficiency in which all three types of cones are present, but one type of cone's sensitivity is off.  Specifically, one cone's spectral sensitivity is shifted, resulting in reduced sensitivity to the wavelength of light (color) that the cone is intended to absorb. These are the most common and most mild color deficiencies.
  • Protanomalymalfunctioning L (red) cone.
    • Inheritance pattern: X linked recessive
    • Prevalence: ~1% of males, 0.01% females
  • Deuteranomalymalfunctioning M (green) cone, or "green weak." This is the most common color vision defect.
    • Inheritance pattern: X linked recessive
    • Prevalence: ~5% of males, ~0.4% females
  • Tritanomaly- malfunctioning S (blue) cone. 
    • Inheritance pattern: Autosomal dominant
    • Prevalence: 0.0002%, males=females
"Protan" is used to refer to both protanopia and protanomaly, and "deutan" is used to refer to both deuteranopia and deuteranomaly. Protan and deutan defects are both considered red-green color deficiencies. "Tritan" defects (tritanopia and tritanomaly) are considered blue-yellow deficiencies.
For the nerds out there, a Punnett not-so-square depicting why males are far more likely to present with an X-linked recessive inherited trait:
The biology behind red-green color blindness
So if a boy is red-green color deficient, it is likely that the boy's maternal grandfather was also red-green color deficient, and the boy's mother/grandfather's daughter was a carrier.

How is color deficiency diagnosed?
Color deficiency is usually detected in childhood, either when a child has difficulty with color naming or when he/she is checked during a routine eye exam.  Some tests that may be used include:

  • Pseudochromatic plates: These tests involve distinguishing numbers or symbols amongst colored dots.  The Ishihara test is probably the most recognized of these tests (and the inspiration for the cupcakes seen at the top of the page).  The Ishihara test does not test for blue-yellow color deficiencies, whereas the HRR test does.   
  • Arrangement tests: These tests involve arranging discs in color order, beginning with a fixed pilot disc. Two such tests are the Farnsworth-Munsel 100 (interestingly has only 85 discs, not 100) and the Farnsworth  D15. There is an online version of the D15 test here. Because color perception varies depending on the type of display and lighting you are working with, this test is not definitive or diagnostic. Based on my sample size of 2 (thanks Matt and Lee), it seems to be fairly accurate.
  • Lantern: The Farnsworth Lantern (Falant) is a color-naming test typically used by US federal agencies to test color vision. 
  • Anomaloscope: This is a color matching test. Half of a circle is presented as pure yellow, and the other half can be adjusted by the observer to varying proportions of red and green to achieve a perceived match.  

Fun Fact: All adult male squirrel monkeys are red-green color deficient. 

How are color vision deficiencies treated?
For the most part, those with color vision deficiencies learn to recognize color by different means (ie: brightness, location) and develop their own system of coping, whether that involves a special way of organizing things, or asking others for help when matching things. There is no cure for color vision deficiencies at this time, but there are some products out there that can potentially enhance color discrimination.
  • Xchrom Lens: This is a red-tinted soft contact lens that is worn in the non-dominant eye.  The red tint is intended to enhance color perception by changing the lightness of colors (ie: makes greens look darker), increasing the number of shades a person can see. 
  • Chromagen:  Chromagen lenses are available in glasses or contact lens form.  A range of 8 colored filters are available, and a series of tests are done to determine which color(s) is/are appropriate.  The filters are intended to change the wavelength of each color going into the eye, potentially enhancing color perception and discrimination. Chromagen lenses are also marketed as part of dyslexia therapy.
  • EnChroma:  These lenses are designed for people with anomalous trichromacy (all 3 cones are present, but one cone's sensitivity is shifted).  EnChroma lenses use a notch filter to restore the spectral separation between cones, potentially reducing color confusion and enhancing color perception. 
  • Gene therapy? This may be in our future.  In 2009, researchers from the University of Washington and the University of Florida (Go Gators!) cured color blindness in two squirrel monkeys using gene therapy 4

CliffsNotes: Most color vision deficiencies are inherited, not acquired. The most common form is red-green deficiency in males.