Q: My colleague at work has a son who was born without color receptors in his eyes. How common is this? A: The normal eye has both cones and rods. The over 6 million cones in the eye function best in bright light and are the color-sensitive receptors -- there are red, green and blue cones.
Q: My colleague at work has a son who was born without color receptors in his eyes. How common is this?
A: The normal eye has both cones and rods. The over 6 million cones in the eye function best in bright light and are the color-sensitive receptors -- there are red, green and blue cones.
The part of the eye that gives us our sharpest vision is the fovea, which has a high density of cones, each connected to a separate nerve.
The more than 120 million rods in the normal eye, most in the periphery of the retina (the "picture screen" in the back of the eye), are not color sensitive and are much less densely packed than the cones in the fovea.
Up to 100 rods may be connected to the same nerve fiber. However, the rods are up to 100 times more sensitive to light than the cones (they can respond to as little as a single photon), so they function well in dim light. This explains why it is hard to make out colors in dim light and why vision is not as sharp in a dimly lit environment.
Complete achromatopsia is a congenital condition where a baby is born with few or no functioning cones. There have been several genes identified with this condition, and it is inherited in a recessive fashion, meaning the child must get one copy of the defective gene from each parent.
Achromatopsia affects about 1 in every 30,000 people in the U.S. However, on the island of Pingelap in Micronesia, up to 1 in 17 people are affected because of a very high incidence of the defective gene in this population.
This increased likelihood of the defective gene is thought to be due to inheritance from one affected man who survived a typhoon in 1775 that wiped out a large percentage of the island's population (a book by Dr. Oliver Sacks called "The Island of the Colorblind" tells this story).
There is also a condition called incomplete achromatopsia where the patient will have decreased functional cones (sometimes profoundly decreased), but will have some of their color vision and some measure of their high acuity vision preserved.
People with achromatopsia will have an aversion to bright light since they lack cones, which function well in bright light, and only have the very light-sensitive rods. They will blink frequently and turn away from a bright light source.
Of course, these patients will not be able to see in color. Without cones, the high acuity vision spot (the fovea) does not function. Hence, the patient must try to utilize their less acute peripheral vision, which is typically less than 20/200 vision.
They may also have nystagmus, which is involuntary jittery eye movement, due in part to the reflex of trying to utilize the parts of the eye in the periphery that are functioning normally. When some or all of these symptoms are present shortly after birth, suspicion of some eye problem will be raised and a diagnosis of achromatopsia may be considered.
A specialized test called electroretinography measures the electrical response of the eye to light and can be used to verify the diagnosis of achromatopsia, even in infants. Children and older patients may have other testing done to see if they have color vision and, of course, their visual acuity can also be tested.
Patients with achromatopsia will need to limit eye exposure to bright light. They may benefit from contact lenses that have a red center that covers the central part of the eye to minimize light to the fovea and allow any cones that are there to function better. They may also benefit from darkly shaded glasses.
The student with achromatopsia will need to be seated at a classroom desk away from bright lights and direct sunlight. Because of their decreased visual acuity, they will also benefit from other techniques designed to help those with limited vision, such as large print books and magnification devices.
It is also important that the school is aware and takes measures to accommodate the student's vision limitations, since many things in school are designed for children with color and high acuity vision.
Achromatopsia is not a progressive condition and, despite how it limits the patient's vision, it does not lead to blindness. However, these patients should be followed by an ophthalmologist to ensure that the other aspects of their vision are maximized.
Since this is an inherited condition, family counseling should also be offered.
Jeff Hersh, Ph.D., M.D., F.A.A.P., F.A.C.P., F.A.A.E.P., can be reached at DrHersh@juno.com.