Achromatopsia

Some have greater sensitivity to light than others, some are totally without color vision while others have a limited amount of color vision, and some have better visual acuity than others. Achromatopsia affects one birth out of every 40,000, and is more prevalent in societies where the marriage of blood relatives is common: In this instance, frequent intermarriage increases the risk of two sets of defective chromosomes being combined, leading to achromatopsia. | Excerpts from the New York Times Article on Achromatopsia, Nov. 7, 1992Blind, lost in an explosion of whiteness, like a winter traveler staggering through a blizzard,

Ms. Futterman and others with rod monochromacy lack cones, the photoreceptor cells in the retina that respond to color and are responsible for day vision. They can see only through the grace of their rods, photoreceptors that control night vision and are extremely sensitive to dim light. Rods are terrific at picking up the weakest of light signals and can detect even a single photon soaring through space, but as the light brightens toward daylight, or the indoor equivalent, the rods quickly saturate into uselessness.

For the normal-sighted, the cones pick up where the rods wash out. For those without cones, they simply cannot see above certain brightness; the screen turns to snowy blankness. And, because only the cones possess the red, blue, and green pigments that are sensitive to colors, people with rod monochromacy can, in the best of circumstances, see only varying types of brightness, a palette of silvers and diamonds. “Cones swamp out the rods in normal vision, so this is an excellent way of studying rod function without worrying about cones.

This pattern of cone distribution has led her and others to propose that the genetic defect behind the disorder disrupts, not the body’s ability to generate cone cells, but its power to usher those cells to their proper position on the retina, once the cones have been created. Rod photoreceptors are poor at detecting light falling into the red range of the spectrum; rod monochromats not only cannot see the color red, they are relatively insensitive to red light. Thus, glasses are designed to allow in only light that is in the red range of the electromagnetic spectrum, a small fraction of the sun’s overpowering glow.

Many with the disorder are proud night owls, who love going out after dark. Even children with the disorder keep vampire hours. Debra O’Bayley of Santa Rosa, California, whose daughter Elise has rod monochromacy, said the child will do anything in order to stay up late. She’ll be up until eleven at night, playing outside, if she can con us into it,” said Ms. O’Bayley. “She’ll play in her sandbox, on her bouncer, anything to be outdoors when she feels free. ” For older monochromats, though, life was trial by improvisation.

Ms. Futterman, who grew up in a relentlessly sunny and hade-free town in Texas, spent the first 17 years of her life without any help at all, not even a pair of sunglasses. She was terrified of going out onto the school playground during recess. She learned to make do, perpetually squinting to shield out light or blinking her eyes frequently and relying on the afterimage that remains on the retina to help her get around. There is a wide range of visual functioning among those of us with achromatopsia. The visual acuity we have for near or far varies greatly among individual achromats, hypersensitivity to light varies greatly also, and colorblindness may be total or partial.

Excerpts from the Island of the ColorblindTotal, congenital colorblindness, or achromatopsia, is surpassingly rare, affecting perhaps only 1 person in thirty or forty thousand. Never having seen color, had no sense of loss. ” But congenital achromatopsia, she pointed out, involves more than colorblindness. What was far more disabling was the hypersensitivity to light and poor visual acuity which also affect congenital achromatopes. More adept at night vision to begin with, was perfectly at ease in the dim lighting and led us to a table.

Those born with ‘The Maskun’ (the Pingelapese word for achromatopsia, pronounced Mah-skoon) have less chance of marrying, partly because it is recognized that their children are likely to be affected, partly because they cannot work outdoors in the bright sunlight, as most of the islanders do. ”One old woman indignantly refused to try any sunglasses. She had lived eighty years as she was, she said, and was not about to start wearing sunglasses now. No one born here with the maskun finds himself isolated or misunderstood, which is almost the universal lot of people born with congenital achromatopsia elsewhere in the world. She had to avoid bright light and to contend with a great deal of misunderstanding.

She had no contact with anyone who could understand her experience of the world. 2- What Is It Like to Have Achromatopsia? | Being colorblind is, by far, the least troublesome manifestation of this vision disorder. Far more serious to cope with are the poor visual acuity (especially for distance) and hypersensitivity to light, sometimes called photophobia. | Their vision decreases as lighting increases and increases as lighting decreases. Only tasks that require color vision or good detail vision cause them to be mindful of their vision disorder in such situations. | How well achromats are able to see can change continually (and sometimes significantly) with every change in illumination. Depending on the quantity of light, the quality of light, the direction of the light, and other factors, achromats can experience reasonably good vision, then seriously limited vision – and then can be very nearly blinded by light, as they move from place to place. | | Achromats are acutely aware of the wide range of illumination levels that are encountered in this world.

At the “friendliest” end of the spectrum are dimly lit restaurants, theaters, bars, and basements. At the other end of the spectrum are playgrounds, beaches, and large expanses of pavement, snow, or water during the daytime. | Individual Differences| No two people with this vision disorder would give the same answer to the question, “What is it like to have achromatopsia? ” This is because there are so many variables in people’s lives. Some networkers are complete achromats, having total colorblindness, severe light sensitivity, and low visual acuity even under the most favorable lighting.

Others are incomplete achromats, with some color vision, less photophobia, and somewhat better acuity. | | Our vision problems constitute only one aspect of who we are as individuals. All of the other aspects of who we are represent other sets of variables. How we experience the impact of achromatopsia in our lives is influenced by our aptitudes, dispositions, strengths, and weaknesses. We have different types and degrees of sensitivity. We are as diverse as any other set of people on this planet. | | Also, the circumstances of people’s lives have considerable influence in determining how difficult or limiting having achromatopsia can be.

For example, someone growing up under adverse conditions, with little or no access to adaptive aids or assistance in coping with vision problems will experience achromatopsia very differently from someone growing up with a caring family and in visually comfortable surroundings, provided with special services and adaptive aids. | | Persons whose only vision disorder is achromatopsia do not need to adjust to progressive vision loss or to prepare for further vision loss. The Problems of Achromatopsia 1- Day Blindness (Hemeralopia): The first and most significant problem is Hemeralopia or day blindness.

This is a severe intolerance to light that can severely impair the patient’s vision. Rather than improve vision, as it does for most of us, light profoundly obscures their vision. We will discuss hemeralopia in detail on this website. If we control the light, we greatly improve the functional vision of achromatopsia patients. 2- Profound Colorblindness: The second problem is a lack of color vision. This is often a total loss of color vision in Rod Monochromats. Some Rod Monochromats, however, may have incomplete forms and may retain traces of some color vision.

In Blue Cone Monochromats, only visual input from the rod cells and blue cones still function. Thus patients retain the blue channel of color vision. They lack the red and green channels. This does not mean that they see the color blue as most do, but that objects that are blue may be more easily seen. 3- Reduced Visual Acuity: The third problem is reduced visual acuity related to the loss of cone cells, which normally reside in the center of the retina providing our sharpest vision. The vision of the patient is also constantly affected by the amount of light present.

In our examination room with the lights lowered, the patient may read the acuity chart, but be severely hampered when the lights are turned up. If we control the light, the patient’s visual acuity, though still impaired will be functionally improved. 4- Nystagmus: The fourth element is nystagmus, the rhythmic shaking or movement of the eyes which is often one of the first things that alerts parents that their child‘s vision may be affected. Nystagmus may add to the variation of the vision. Stress is just one of many factors that may affect the speed of the nystagmus and thus clarity. – Emotional and Social Impact: The mechanics of dealing with all of these problems can create a whole set of social problems. The day blindness may cause constant squinting, looking down to avoid light and wearing dark sun lenses inside.

All of these impact a person’s interaction with other individuals. It can be especially difficult for a young person. | 3- Inheritance Factors Associated with Rod MonochromacyWith rod monochromacy – the form which affects most members of our network – there is no family history of the defect, with the exception of certain unusual circumstances, such as are mentioned in the irst paragraph on the following page. Males and females are equally affected. Persons with this vision disorder have inherited 2 faulty genes, 1 from each parent. This is known as autosomal recessive inheritance. When both parents carry the faulty gene but do not they have rod monochromacy, the odds are 1 in 4 that a child born to them will have the vision disorder. The birth order of affected and nonaffected siblings in network families illustrate the range of possibilities in keeping with these odds.

In some families 3 normally sighted children have been born before a child with achromatopsia was born. In others the second-born child or the third-born has been the one to have achromatopsia. | | A rod monochromat does not risk passing on this condition to offspring unless he or she happens to mate with another carrier. If a rod monochromat were to mate with a carrier of the same faulty gene who does not have the condition, the odds would be 50-50 that offspring would be affected if two rod monochromats were to mate, all of their children would be expected to have the condition.

No rod monochromat in our network is known to have children or grandchildren with this condition, and none is known to have parents, grandparents, or ancestors affected by the condition. All of this is in keeping with the known inheritance factors. In fact, one of the ways diagnosticians can determine that a patient has rod monochromacy rather than blue cone monochromacy or certain other retinal conditions is by noting that the condition has not “run in the family. ” It is rare for someone to carry the gene and even rarer for two persons carrying the gene to mate.

The likelihood that two persons with the same gene will mate is higher if they are related or if they live in a tightly knit or geographically isolated community. Thus, there is a higher incidence of rod monochromacy in certain parts of the world, such as the tiny atoll of Pingelap and an enclave inhabited by Pingelapese on Pohnpei Island. Based on what is known about autosomal recessive inheritance,* the mating choices for rod monochromats which would bring risk of passing on this disorder would include the following: 1.

Mating with another rod monochromat: all children would be expected to have rod monochromacy if the same abnormal gene is involved. 2. Mating with a known carrier of the same faulty gene (50% chance of passing on the defect). For example, the parents and children of rod monochromats are carriers. 3. Mating with a suspected carrier: suspected carriers include anyone closely related to someone who has rod monochromacy (e. g. , a sibling or a grandchild). If the suspected carrier has the gene, chances are 50-50 that offspring will inherit the defect if he or she mates with a rod monochromat.

Apart from the circumstances listed above, rod monochromats in the U. S. and in most other parts of the world are not likely to have descendants with rod monochromacy, because (1) it is highly unlikely that they will happen to mate with a carrier of the same flawed gene, and (2) it is just as unlikely that their children will mate with a carrier. The children of rod monochromats, although they are carriers, are most likely to mate with non-carriers, in which case their children (i. e. , the grandchildren of rod monochromats) have a 50% chance of being carriers and a 50% chance of not inheriting the gene.

My son is a carrier because he has inherited from me one abnormal gene in this set of genes. He does not have rod monochromacy, because he inherited the normal gene (the dominant one) in this set from his father, who has normal vision. The only way my son would have any chance of passing on this defect to his children would be by mating with a carrier – a very unlikely possibility. If he should happen to mate with a carrier, the chances of their having a child with rod monochromacy would be 1 in 4 – i. e. , the same odds that existed for the parents of all rod monochromats in this network.

In mating with a non-carrier, the odds are 2 in 4 that my son’s children would not even inherit the faulty gene. The Genetics of Rod Monochromacy The most common form of achromatopsia, rod monochromacy, is considered an autosomal recessive disorder. Genes are the pieces of hereditary material which exist in every cell of our bodies, giving directions as to what specific tasks each cell must do to make our bodies function. Many of our genes are involved with cells that have specific functions in the eye. Some of the eye genes instruct the body on how to make cone cells in the retina and how to make them function correctly.

For every gene, we have two copies: one inherited from our mother and one from our father. In autosomal recessive diseases a disorder only occurs if there is a defect in both of our copies of the gene. One abnormal copy of the gene is inherited from one parent and the other abnormal copy is inherited from our other parent. The parents are considered carriers, since they each have one abnormal copy of the gene but also a “protective” normal copy. If there is an error in one copy of a gene, the matching normal gene can usually instruct the cell properly. It acts as a backup system to cover up the error.

Carriers of achromatopsia have no visual symptoms and no abnormalities on eye examination. Therefore, they have no way of knowing that they carry the gene. Every individual in the world carries approximately 6 to 8 recessive gene abnormalities, of which only some involve the eye and vision. People only know which abnormal genes they carry if they have children with someone who happens to carry an abnormality in the same gene. Both individuals may pass their abnormal gene on to a child, who then gets a “double dose” (one from each parent), and that child shows the effect of the autosomal recessive disorder.

The likelihood that spouses carry abnormalities in the same gene is higher if the spouses are related to each other or if they come from a tightly knit or geographically isolated culture. This is why there is a higher incidence of achromatopsia in some populations. If both parents are carrying one abnormal gene and one normal gene (they are carriers), the chance with each pregnancy that they would both pass on their abnormal copy (rather than their normal copy) at the same time is 25%. This would produce a child with rod monochromacy.

There is a 50% chance that one parent would pass an abnormal gene and the other would pass his or her normal copy of the gene. In this case, the offspring would be a carrier just like the parents, but would not have rod monochromacy. There is a 25% chance that both parents would pass on their normal gene, producing a child who is not a carrier, who is unaffected, and who will not have offspring in subsequent generations who would be carriers or who would be affected. The offspring who are carriers would still have to have children with another carrier for there to be the possibility of having a child affected by rod monochromacy.

As for rod monochromats themselves, they have inherited two abnormal copies of the responsible gene, but they too would have to have a carrier partner (or an affected partner) in order for there to be any possibility of their having affected offspring. This is because any given individual usually passes only one copy of his/her genes to each offspring. For a rod monochromat to have affected offspring, the offspring would have to also inherit a copy of the abnormal gene from the other parent. Since the rod monochromat only has abnormal copies of the gene to pass on, all of that person’s offspring become carriers.

Since it is necessary for both parents to carry the same abnormal gene in order to produce affected offspring, families may have one or more affected siblings and yet have no other family history of the disease. Autosomal recessive disorders are usually found in one generation only (i. e. among the children of two carriers). Sometimes all children in a family will have rod monochromacy. This is possible because the chance for the parents to pass on their abnormal copies of the gene is the same for each pregnancy.

In families from geographically isolated cultures or cultures in which intermarriage between relatives is common, the disorder will often be found in more than one generation. This pattern is sometimes called “pseudo dominant inheritance,” a term that refers to a different type of inheritance, autosomal dominant inheritance, which is being mimicked, due to the high frequency of abnormal genes being passed around within this tightly knit group. It has already been discovered; as a result of studies conducted at several research centers, that more than one gene can cause the disorder.

Presumably, the genes that are involved determine the way cones are made and/or the way in which cones function. Each of us has thousands of genes distributed over 46 chromosomes. The Genetics of Blue Cone Monochromacy There are 3 kinds of cone cells in the retina, each kind defined by the color of pigment it contains: blue, green, or red. This relates to the color of light detected. It is the mixing and matching of input from the combinations of these cells that allows normal color vision. In complete achromatopsia (rod monochromacy), there is a generalized defect in cone cell formation/ function, so that all 3 types of cones are affected.

In blue cone monochromacy (BCM), cone cells develop normally, but the retina is unable to fill them with red or green pigment, thus leaving only blue cones. Patients affected with BCM usually show similarities to complete achromatopsia, including reduced central vision (often better than in complete achromatopsia), abnormal color vision, nystagmus (shaky eyes), and photophobia (aversion to bright light). Unlike rod monochromats, however, some patients with BCM can also show a progressive deterioration of the part of the retina which is populated by cones (the macula).

Almost always, only males are affected by BCM. The red and green cone cell pigments are produced by genes on the X chromosome, which is a chromosome involved with determining a child’s gender. Normal males have one X chromosome and one Y chromosome. Normal females have two X chromosomes and no Y chromosome. A patient who has BCM has a defect which prevents the formation of red and green pigment from a single X chromosome. A woman who carries this defect on one of her X chromosomes usually does not have BCM, because she also has a normal X chromosome that “protects” her from the defect.

This other X chromosome can produce enough red and green pigment to allow her to have a fairly normal collection of red and green cone cells in addition to her blue cone cells, so her color vision should be close to (if not completely) normal. Males have only one X chromosome, so any defect on the X chromosome has no “protection,” since there is not a normal second X, as there is in females. The Y chromosome does not offer matched protective functions. As a result, if a male has the BCM defect on his X chromosome, no red or green pigment is formed.

This male is affected with BCM. Rarely, a woman may show signs of having BCM, due to a failure of her normal X to adequately protect her from the deficiency of cone pigment caused by the abnormal gene on her other X. The likelihood of this happening would be increased if the mother and father happen to be related to each other (e. g. , cousins). There are also great variations in the visual experiences of these individuals. For example, people suffering from monochromacy might mix up the following colors: * green and blue * red and black * yellow and white

Cone monochromacy: complete achromatopsia with normal visual acuity Rod monochromacy is still by far the most common type of complete color blindness. 3- Parenting Children and Teens with Achromatopsia * They are capable of helping me read a recipe; match socks, help keep track of a younger sibling, or get a younger sibling dressed in clothes that match. ” * “Some of my worst mobility nightmares are picnics and playgrounds * I discovered that this makes it easier for me to keep track of her. It’s easier for me to see dark colors in daylight.

Another good idea for keeping track of toddlers – which I didn’t think of until my kids were older – is to hang a little bell around their wrist or on their shoelaces or pin it to the back of their shirt (but never tie anything around a toddler’s neck). It can be reassuring to hear the bell tinkle and know that your child is somewhere nearby, even if you can’t see exactly where * can’t see far, can’t see colors, sensitive to light * My mom didn’t like to talk about it. I was led to believe, by nearly everyone around me, that it was better to pretend that I was ‘normal. ’ * I’ve tried being ‘teacher’ to my children and found it frustrating.

Whenever they would have a question about their homework, I would have to get close to their work or their reading material, and they seemed to hate having my head practically in their faces. There was no way that both I and the child could look at the same book or piece of paper at the same time. And comfortable lighting for them is too bright for me. * But my kids have so often had to teach me about these things, because I couldn’t see the insects, worms, or little purple flowers they would point to toddler, I plan to buy inexpensive little bells that can be attached to shoelaces to help me keep track of her movements outdoors.

Of course, I’ll be in the yard too, but I think the bells will help. Little kids think bells are neat. * I had many fears about his safety, especially in connection with my visual limitations in bright outdoor spaces, such as playgrounds and parks * Ever since she was small, she has felt a responsibility to watch out for me. She is the only one of my four kids who has shown an inclination to be ‘nurturing. * From the father of a 3-year old girl: “We recently moved to a house with a backyard facing east and with lots of trees and a big patio cover. It gets shade most of the day because of the patio roof and the trees.

There is only one window on the south wall, and it has shutters that our daughter can easily close, if it is too bright for her. In the place where we lived last year, she never went out to play in the backyard, which faced west. She plays in the backyard a lot now. ” * As a child, he always rooted for teams wearing dark jerseys. * She still gets lost (temporarily) often. I worry about the possibility of her injuring herself seriously. She’s fast and not very careful. * We kept mostly indoors. Once he got some good sunglasses, he started exploring the outdoors * I felt that being overprotective would not serve any good purpose.

I knew I wouldn’t always be able to be there watching out for curbs, etc. So they got used to helping themselves. I’m sure they have had more scraped knees, bumps, and bruises than normally sighted kids. * When my daughter was 9 months old, we took a trip to the beach, and that day I discovered how little she could see outdoors. We were sitting near a bunch of rocks, and each time she would drop one, she would feel for the rocks around her and then pick up the same one she had dropped, without ever looking down. I tried this with different rocks, dropping them in different locations, and she never failed to find them, but strictly by touch.