Color vision deficiency, also known as color blindness, is a condition that affects an individual’s ability to differentiate between colors, specifically those of similar hues. The inability to distinguish between colors results from either a partial or total loss of color vision, depending on the type of color blindness present (National Institutes of Health [NIH], n.d.).
Symptoms
The most common indicator that someone has color blindness is the lack of ability to decipher between the three primary colors, red, yellow, and blue, or the lack of ability to decipher between colors of similar shades, such as dark blue and black (NIH, n.d.). In more severe cases of color blindness, such as when achromatopsia occurs, vision problems arise as additional symptoms (NIH, n.d.). Some vision problems that are apparent with severe color blindness are: photophobia (light sensitivity), side-to-side eye movements that appear to be involuntary, and a decrease in vision sharpness (NIH, n.d.). Farsightedness and nearsightedness have also been observed in people with more severe cases of color blindness, but when these occur, the person only has one or the other, never both (NIH, n.d.).
Types
Color blindness can be classified into two main types: incomplete or complete. Incomplete color blindness is the milder form where an individual can still see and differentiate between some colors (NIH, n.d.). Complete color blindness is the more severe form where an individual cannot see any colors within the visible spectrum of light, therefore that person only sees black, white, and shades of gray (NIH, n.d.). Incomplete color blindness can be further divided into two types: red-green color blindness, which is by far the most common, and blue-yellow color blindness, which is less common (NIH, n.d.). Individuals with red-green color blindness have trouble deciphering between red, yellow, and green (NIH, n.d.). Red-green color blindness affects males more often than females, affecting nearly one in twelve males and one in two-hundred females (NIH, n.d.). Red-green color blindness tends to affect people with a Northern European heritage more often than people with other ethnical backgrounds, for reasons still quite unclear (NIH, n.d.). Individuals with blue-yellow color blindness have trouble deciphering between varying shades of blue and green and between dark blue and black (NIH, n.d.). Blue-yellow color blindness affects both males and females equally, affecting nearly one in ten thousand individuals (NIH, n.d.). Blue-yellow color blindness does not appear to affect any specific ethnicity more than the other (NIH, n.d.).
Complete color blindness, also known as monochromacy or achromatopsia, can also be further divided into two categories: incomplete and complete (Neitz & Neitz, 2000). In incomplete achromatopsia, such as blue cone monochromacy, an individual only has one type of functioning cone out of three, so that individual’s ability to decipher between colors is extremely impaired (Neitz & Neitz, 2000). Blue cone monochromacy, specifically, is a rare type of color vision deficiency that affects males more often than females (NIH, n.d.). This type of color blindness only occurs in one out of every one hundred thousand individuals (NIH, n.d.). Blue cone monochromacy, even though characterized under complete color blindness, allows an individual to still see a single color, which is why it is classified as incomplete achromatopsia (Neitz & Neitz, 2000). In complete achromatopsia, such as rod monochromacy, an individual does not have any of the three types of cones functional (Neitz & Neitz, 2000). Without any of the three types of cones functional, an individual cannot see any color, only a grayscale. While it is possible that individuals with achromatopsia can see a single color, as mentioned previously, due to the possibility of the single functioning cone, it is not very common and most individuals’ color vision is completely absent (Neitz & Neitz, 2000).
Causes/ pathophysiology
When light enters the eye, in the form of a wave, it enters through the cornea, passes through the lens, where it is focused, and then travels to the retina (Mayo Clinic, 2018). The retina, a light-sensitive layer of tissue in the back of the eye, is connected to the optic nerve which transmits the visual signals processed within the retina to the brain (Neitz & Neitz, 2000). The retina includes two types of photoreceptor cells: rods and cones (Neitz & Neitz, 2000). The rods are responsible for vision in low-light environments, such as those that occur at night, whereas the cones are responsible for vision in bright-light environments, such as those that occur during the day (NIH, n.d.). Both the rods and the cones, through the utilization of pigments, allow the eyes to recognize and respond to varying wavelengths of light and transmit these visual signals to the brain (NIH, n.d.; Mayo Clinic, 2018). The light-sensitive pigment in the rods is rhodopsin, which is encoded by a gene on chromosome three (Neitz & Neitz, 2000). The light-sensitive pigment in the cones is opsin, which comes in three different types depending on the type of cone (NIH, n.d.). The pigments of both the rods and the cones contain chromophore 11-cis-retinal, which is the light-sensitive portion of the pigment (Neitz & Neitz, 2000). When light is absorbed by the eye, the 11-cis-retinal undergoes a conformational shape change which specifically stimulates the opsin of the cones, triggering biochemical reactions to occur and a neural signal to be sent to the brain about the light absorbed (Neitz & Neitz, 2000). The rods and cones are able to transmit the signals from visual stimuli to the brain because they contain chemicals that decompose through the biochemical reactions triggered by the activated opsin, which promotes the sending of the neural signal (Chaudhari & Shah, 2012).
The cones within the retina, which provide color vision, can be classified into three basic groups: “L” cones, “M” cones, and “S” cones (NIH, n.d.). The “L” cones are cones sensitive to long wavelengths of light, such as that from yellow-orange light, and they contain opsin encoded by the OPN1LW gene (NIH, n.d.). The “M” cones are cones sensitive to middle wavelengths of light, such as that from yellow-green light, and they contain opsin encoded by the OPN1MW gene (NIH, n.d.). The “S” cones are cones sensitive to short wavelengths of light, such as that from blue-violet light, and they contain opsin encoded by the OPN1SW gene (NIH, n.d.). The OPN1LW and OPN1MW genes are located on the X chromosome, whereas the OPN1SW gene is located on chromosome seven (NIH, n.d.). When individuals have mutations in any of their opsin genes, they have one of the inherited forms of color blindness. Abnormal opsin pigments within the “L” or “M” cones, or absence of either of these types of cones, results in red-green color blindness (NIH, n.d.). If both of these types of cones are absent but the “S” cone is still present, blue cone monochromacy results (NIH, n.d.). Since the genes for the opsin pigments in the “L” and “M” cones are encoded on the X chromosome, red-green color blindness and blue cone monochromacy are considered X-linked disorders (NIH, n.d.). Abnormal or absent opsin pigment within the “S” cone results in blue-yellow color blindness (NIH, n.d.). Since the gene for the opsin pigment in the “S” cone is encoded on chromosome seven, blue-yellow color blindness is considered an autosomal disorder (NIH, n.d.).
Acquired color vision deficiencies are not linked to chromosomal gene mutations and can occur throughout life from several factors, including: eye diseases and disorders, antibiotics, prescription medications, toxic chemicals, eye trauma and injury, age, and other physiological diseases and disorders (Chaudhari & Shah, 2012; NIH, n.d.). Macular degeneration, which is a degeneration of part of the retina, or other eye disorders which affect the retina, optic nerve, or visual cortex, can all cause color vision deficits in individuals when they arise (Chaudhari & Shah, 2012; NIH, n.d.). The side effects of specific antibiotics or prescription medications, such as high blood pressure medications, can also cause individuals’ perceptions of color to change when under use (Chaudhari & Shah, 2012). When individuals are exposed to certain toxic chemicals, such as carbon monoxide, fertilizers, styrene, or organic solvents, in a laboratory or other work space, they too put themselves at risk of color vision deficiencies (Chaudhari & Shah, 2012; NIH, n.d.). Eye trauma or injury only affects color vision when the damage is in the retina or portions of the brain that transmit, integrate, and process visual stimuli produced by colors (Chaudhari & Shah, 2012). A stroke is a common example of an injury that can cause damage to the parts of the brain involved in relaying and interpreting color stimuli (Chaudhari & Shah, 2012).
As individuals get older, as with any other portion of the body, the retina and all its components involved in processing color stimuli become degraded and altered, which can cause partial loss of, or alterations to the color vision of the older individuals (Chaudhari & Shah, 2012). The nerves of the visual cortex, where color stimuli are processed, become neuropathic in physiological diseases such as Alzheimer’s disease, which can also result in color vision deficits (Chaudhari & Shah, 2012). In disorders such as diabetes, the pigments within the cones of the retina can get disturbed, the lens of the eye can get discolored, and there can be pathological changes in the neurons that transmit the visual stimuli (Chaudhari & Shah, 2012). Each of these three factors can play a role in causing color vision defects in the affected individuals. When an individual is a chronic alcoholic, his/her optic nerve can eventually atrophy, which also results in color vision changes as seen with the other physiological disorders, due to a loss of transmission of visual stimuli to the visual cortex in the brain (Chaudhari & Shah, 2012). In general, acquired color blindness occurs from an alteration in the retina or portions of the brain involved in deciphering colors.
Diagnosis
The most efficient, and widely-used way to diagnose color blindness is to get an eye exam done which includes the use of printed pseudoisochromatic plates to test for color vision deficiencies (Neitz & Neitz, 2000). These plates help to diagnose not only the severity of an individual’s color vision loss but also the type of color vision deficiency present (Fomins & Ozolinsh, 2011). Rabkin tables and Ishihara tests, forms of pseudoisochromatic plates, are used most often to diagnose red-green color deficiencies specifically (Fomins & Ozolinsh, 2011). All of the various pseudoisochromatic test plates, no matter what form, include an object of focus, often a number or shape, and a background (Fomins & Ozolinsh, 2011). Both the object and the background are comprised of patches of random sizes, colors, and hues which led to these test plates gaining the nickname “color camouflage” plates (Fomins & Ozolinsh, 2011). It is fairly common for difficulties to arise when children of younger ages get tested by these plates since they are more prone to misunderstanding directions or getting stressed by the testing environment and the pressure to get the “correct answer” (Fomins & Ozolinsh, 2011). Due to these difficulties, the pseudoisochromatic plate tests tend to be inconsistent and therefore less reliable, so new methods of diagnosis are being encouraged (Fomins & Ozolinsh, 2011).
Treatment
Inherited color blindness has no cure at this time, but scientists are working on ways to utilize gene therapy and other retinal therapeutics to reinstate color vision in those that have either partial or complete color blindness (Mayo Clinic, 2018). A relatively recent study at the University of Pennsylvania utilized gene therapy, mediated by a recombinant adeno-associated virus, in order to restore cone function and color vision in daylight in humans and animals, specifically canines, that had forms of achromatopsia (Komáromy et al., 2010). Various types of the promoter for the human form of red cone opsin, a protein involved in sensing light, was utilized in the gene therapy (Komáromy et al., 2010). This study helped provide insight into future ways gene therapy can be used to target malfunctioning rods and cones in the retina.
Since the use of gene therapy for inherited color vision deficiencies is still being developed and tested, individuals with this form are currently recommended to implement a colored filter over their glasses or place colored contacts into their eyes in order to get some color vision back (Mayo Clinic, 2018). If an individual has an acquired color blindness, rather than an inherited one, the treatment is much simpler. These individuals can be treated by taking measures to remove the source of infection or reduce the effects of the eye disorders that caused the acquired color vision loss. For example, one can discontinue a medication if it appears to cause color blindness as a side effect (Mayo Clinic, 2018). When acquired color blindness is treated correctly, individuals can often get some color vision back but typically not all the color-visualizing ability they had originally (Mayo Clinic, 2018).
Correlations with other diseases
Even though there is currently no known association between color vision deficiencies and disorders of the eye that cause blindness, there have been individuals who have experienced color blindness as a symptom of the blinding disorders (Neitz & Neitz, 2000). Some of the blinding disorders that have been known to cause color vision deficiencies, include: macular degeneration, as mentioned previously, glaucoma, and diabetic retinopathy (Neitz & Neitz, 2000). Color blindness often occurs in conjunction with other physiological diseases that do not directly cause blindness, however, to such a great extent that these diseases have been classified as causes of color vision deficiencies (Chaudhari & Shah, 2012). Individuals can get acquired color blindness as a symptom from Alzheimer’s, diabetes, and chronic alcoholism, as discussed previously, but also from Parkinson’s disease (Chaudhari & Shah, 2012).
As more studies are carried out, more information can be obtained on if there are other external factors, such as other physiological disorders, that correlate to color blindness. The information from these studies can allow insight into how to more-efficiently prevent the development of acquired color blindness. As novel information arises about both types of color blindness, acquired and inherited, diagnostic tests and treatment plans for both can also improve.
References
Chaudhari, D. K., & Shah, J. S. (2012). Pathophysiology of altered color perception. Research in Pharmacy.
Color vision deficiency - Genetics Home Reference - NIH. (n.d.). Retrieved from https://ghr.nlm.nih.gov/condition/color-vision-deficiency.
Fomins, S., & Ozolinsh, M. (2011). Multispectral analysis of color vision deficiency tests. Materials Science, 17(1), 104-108.
Komáromy, A. M., Alexander, J. J., Rowlan, J. S., Garcia, M. M., Chiodo, V. A., Kaya, A., … Aguirre, G. D. (2010). Gene therapy rescues cone function in congenital achromatopsia. Human Molecular Genetics, 19(13), 2581–2593. doi: 10.1093/hmg/ddq136
Neitz, M., & Neitz, J. (2000). Molecular genetics of color vision and color vision defects. Archives of ophthalmology, 118(5), 691-700.
Poor color vision. (2018, November 6). Retrieved from https://www.mayoclinic.org/diseases-conditions/poor-color-vision/symptoms-causes/syc-20354988.