'An absence of vision of the first sort'

MANILA, Philippines – In medical school, I used to know of someone who was color blind. When asked “what color is this?” pointing to a slide with pink and blue skin tissue, he would gleefully shout, “Magenta!” — his standard answer to all questions about color. I do not know how that unfortunate fellow fared in microbiology where we were asked to identify the type of bacteria based on whether we saw red or blue organisms.

But I know he would not have made a good officer on deck aboard a seafaring vessel. You see, boats have color signals for where they are going: red means they are heading portside or toward the left, and green means they are turning starboard or toward the right. That’s how ships conventionally signal each other as to where they were headed, in case they were on a collision course, so they could avoid running into each other. It was the deck mate’s job to determine where the ship ahead was turning, based on the color signal it flashed. So you would definitely not want to be on board a ship with a color-blind officer on deck! Neither would you want to be on board a plane with a color-blind pilot who has difficulty distinguishing the red or white lights on a runway at night, colors that indicate whether a plane is “on glide” for proper landing. Or worse, would you want to be in the vicinity of a bomb diffuser (think “Hurt Locker”) who could not tell the correct color of the wire he needs to cut??? There are many applications for color in the working world but for these jobs, you would understand why normal color vision is a MUST. Such is the reason why seamen and pilots have to undergo color vision tests before they are accepted for employment. And why they routinely get seen by eye doctors or ophthalmologists for screening. 

Color blindness is usually congenital. Congenital color blindness affects about eight in 100 males and four in 1,000 females of the human population. The more common occurrence among men is due to the nature of genetic transmission in the inheritance of this hereditary disorder. The eye’s retina has photoreceptors responsible for transforming light energy into an electrochemical nerve impulse that is transmitted to the brain for interpretation into the images that we see.

There are two types of photoreceptors: the rods and the cones. Rods are responsible for vision in dim conditions or during nighttime, while cones are responsible for vision in well illuminated conditions or during daytime. Color is seen during daytime or in good lighting because the cones contain any one of three pigments sensitive to the colors red, green or blue. Color deficiency, be it color blindness or color weakness, is due to the absence or defects of any or all of these color sensitive cones.

In its most serious form, where there is a congenital lack of functioning cones in the retina, absent color vision is associated with very poor visual acuity, aversion to bright lights and “roving eyes” or nystagmus.

Then there are persons who do have cones but all cones contain the same visual pigment. They have normal visual acuity but their world is suffused with only one color, blue, or green, etc. and the absence of any other color denies them the ability to correctly discriminate hue, since they have never had previous color experience. Imagine your whole life looking through a single colored filter and seeing everything in different shades of only that one color. How could you possibly identify that color?

Most congenital color vision defects, though, do not affect visual acuity and are “red-green” defective. There is a confusion of reds, browns, olives and golds. Pastel pinks, oranges, yellows and greens look similar to each other. Congenital blue-yellow defects are rare. In this, patients confuse pastel blues with pastel yellow or green in addition to calling some deep blues, gray. Blue-yellow defects are more common in acquired color blindness.    

When acquired, color blindness usually affects vision as well. An example is ethambutol optic neuropathy (ETON). Patients on high dose or prolonged treatment with the anti-tuberculosis medication, ethambutol, start to experience blurred vision, defects in color discrimination and constriction in their visual fields. When detected early through monitoring of visual acuity, color vision testing and visual field testing, stopping the medication can possibly result in improvement of symptoms.

For congenital color blindness, however, as far as present day medical treatment is concerned, THERE IS NO CURE. Retinal cones cannot regenerate nor can they acquire pigments that are not present since birth. Methods touted as “cures” for color blindness — wearing of filtered or red glasses, reading and re-reading of color vision plates under bright sunlight — are NOT cures.

This is the reason why people interested in jobs that require normal color vision (seamen, pilots, etc.) need to be tested PRIOR to embarking on arduous, time-consuming and costly training for their respective professions. Doing so will eliminate the futility of all the training and studying only to find out at the end of the training that one is not qualified for the job due to color blindness. Prospective maritime or aviation students would do well to consult an ophthalmologist for color vision testing with Ishihara plates and the Farnsworth-Munsell 100-hue test.

Pilots, when landing a plane at nighttime, need to know if they are on target for landing. Two red lights and two white lights flashing at the runway mean the plane is just at the right height and may safely land. The fate and lives of passengers on board largely depend on seeing those colored lights correctly. Next time you cut out a heart-shaped paper that your three-year-old toddler correctly identifies as “red,” remember that and know how lucky your child is to be gifted with a world of colors.

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For comments and inquiries, or to get in touch with an accredited member of the Philippine Academy of Ophthalmology, call the PAO secretariat at (02) 813-5318 or send an e-mail to [email protected]. Or send regular mail to Unit 815, Medical Plaza Makati Condominium, Amorsolo corner De la Rosa Sts., Legaspi Village 1229, Makati City.