Understanding Achromatopsia
Laura K, Windsor, O.D., F.A.A.O.
Richard L. Windsor, O.D., F.A.A.O., D.P.N.A.P.
Achromatopsia is an inherited condition that affects approximately 1 in every 33,000 Americans. It is also known as rod monochromatism. This condition is associated with color blindness, visual acuity loss, extreme light sensitivity and nystagmus. It is a condition found throughout the world with varying incidence.
There are two primary forms, the complete achromatopsia and incomplete achromatopsia.
Complete Achromatopsia
Achromatopsia means “without color” and is defined as little or no function of the cone cells. Persons with achromatopsia are only able to perceive black, white and shades of gray. Patients with complete achromatopsia have no real understanding of the concept of color. A color like red may be perceived as dark gray while yellow may be perceived as a light gray. The vision is much like that of a black and white photograph with varying shades of gray.
Incomplete Achromatopsia
Patients with incomplete achromatopsia have profound color impairment, but do have a small residual amount of color vision and slightly better visual acuity due to the presence of some functioning cone cells in the retina.
Inheritence
Achromatopsia is a recessive inherited condition. It requires both parents to contribute a gene in order for the condition to occur. All the offspring of an achromat may carry one gene for achromatopsia. In order to pass the condition onto their children, it would require having children with someone else carrying the same gene. This would be extremely unlikely in today’s very mobile populations.
In the western Pacific, however, there is a small island of Pingelap that has a high incidence of achromatopsia. This population has been studied significantly because of this condition. Through genealogy, it was traced back to 1775 to one man who survived a typhoon which killed most of the islands residents. This man had a mutation of the CNGB3 gene which is essential in the eye’s photoreceptors and ultimately vision. He passed this gene on to his family members who have subsequently been affected by achromatopsia. Today, 6% of the population of this small island has the condition.
The other group of highly studied achromats is Western European families. The genetic studies there have found another gene that is responsible for their vision problems. The CNGA3 gene was found to cause the condition in this population.
Both of these genes affect the cone cells of the eye. Cone cells are responsible for our central, detail vision. Additionally, the three different types of cone cells are responsible for our color vision. They are packed by the millions into the macula of the retina. The peripheral retina is made of rod cells. These cells are more sensitive to light, but cannot differentiate color.
Early Detection in Children
Achromatopsia is present from birth. The first signs may be the presence of nystagmus, a pendular quivering of the eyes and light sensitivity with squinting in bright light. An electroretinogram may show an abnormal photopic or daylight signal while maintaining a normal scotopic or night vision signal. When the child is old enough, color vision testing like the Sloan Achromatopsia Test can further confirm the diagnosis.
Hemeralopia (Day Blindness) -- Extreme Light and Glare Sensitivity
In order to function across a wide range of lighting conditions, we must switch between two different visual systems, the photopic and scotopic systems. The photopic system uses cone cells that see color, function best in bright light and provide our best vision (20/20) due to the concentration of cones in the macula of our retinas. The scotopic system functions only in very low levels of light. It uses rod cells that are able to function only at very low levels of light. This provides our night vision.
In patients with achromatopsia, the cone cells are not functioning. Thus the only cells left are the rod cells. Thus when the achromat goes into bright light, the scotopic visual system, meant to be used only at night is suddenly overwhelmed. Achromats describe being “dazzled” and “blinded” by bright light. In full daylight, vision can easily become hazy and then completely washed out. This extreme light sensitivity is called photophobia. It may also be referred to as hemeralopia or day blindness. Achromats develop an aversion to bright daylight. To avoid problems, they often learn to work or play at night. Sometimes parents and friends misunderstand their aversion to light and try to force them to use more light and to travel outdoors in bright sunlight.
Areas where bright lighting and shade come together can also pose many problems for these patients. Their vision may be significantly impaired and they may not see what is in their path in the shaded area.
Vision Loss
Achromatopsia is also associated with stable central vision loss. The visual acuity of a person with complete achromatopsia is typically 20/200. In the incomplete form of achromatopsia the visual acuity may range from 20/60 to 20/100. This vision loss is stable throughout life, but does vary with exposure to bright lights as stated above. The stable vision throughout life differentiates achromatopsia from progressive cone dystrophies.
Nystagmus
Nystagmus is an involuntary rhythmic shaking or wobbling of the eyes that occurs when children are born with vision loss. Pendular nystagmus is where the eyes rotate back and forth evenly, much like a pendulum, and this is the type of movement noticeable in achromatopsia. Patients with early onset nystagmus do not notice the movement in their vision when their eyes shake. However, increases in the nystagmus can cause temporary decreases in vision. Stress can cause an increase in the movement and a further decrease in acuity.
There may be a certain eye position or head tilt that slows the movement and allows better visual acuity. This unusual position should be encouraged and is called a “null point”.
Red Central Contact Lenses
An important new treatment for those with achromatopsia is the use of red central soft contact lenses. These contact lenses have a small red circle that when properly positioned looks like the pupil of the eye. On the eye they appear as dark circles looking just like the normal pupil of the eye. These red contact lenses not only reduce the light entering each eye, but allow primarily red light to enter the eye. Red light allows the remaining rods to function better, and in complete achromatopsia, where the patient sees no color, the patient is not aware of the red hue. In the incomplete form of achromatopsia, patients report it enhances their ability to detect red stoplights and red brake lights.
These lenses work much the way submariners in World War II turned red lights inside the submarine on to allow their eyes to adapt for night or “rod” vision before surfacing at night. With the contact lenses in place, mild sunglasses may be worn outside and no filters are usually required inside. We find that patients are astounded by the dramatic improvement of their functioning in bright light.
The contact lenses may or may not contain the patient’s prescription. We often fit these contacts under the patient’s eyewear. Thus, they can be worn during the day and removed in the evenings without the need for extra eyeglasses.
Dark red or plum filters may also be used to control light sensitivity. We also recommend wearing fit-over or wrap-around sunglasses to block excessive light from getting around eyewear. Also the use of visors and hats to limit light may be needed.
Low Vision Rehabilitation
Patients with achromatopsia respond very well to low vision care. The first step to rehabilitation is the use of tints, filters, hats and visors to control light sensitivity. This may occur from filters, environmental changes and now with the use of red center contact lenses. There are many different low vision aids and devices that can help patients function better. Simple magnification with microscopic eyewear or magnifiers can allow reading of regular print, dials, price tags, menus and thermostats. Telescopes can be used for spotting signs and seeing faces in the distance. Closed circuit television systems (CCTV), which use video magnification, enlarge print onto a screen for ease when reading and writing
School Issues
The child with achromatopsia finds the world full of color coding. School work is frequently color coded. For example, a child may be asked to determine how many red circles are present among the 10 on the page. Obviously, the student would be unable to perform this task. Also, coloring pictures and painting may be very difficult tasks to perform. It is crucial that color coding be eliminated from the child’s education.
Next, we must control light and glare. In the past, children had to wear dark sunglasses inside often stigmatizing the child as being different. Today with red central lenses, children with achromatopsia can control light and glare inside without the stigma of wearing sunglasses inside. Supplemental sunfilters should be worn outdoors to decrease light sensitivity. Also, seating the student in the classroom should be addressed by seating the student away from the windows and closes blinds to decrease glare on the chalkboard and windows. The student should be allowed to sit in the front of the room and be encouraged to go to the chalkboard when needed.
Due to the vision loss from achromatopsia, the use of large print reading materials should be addressed with each student on a yearly basis. In early elementary years, large print may not be needed, because most books are already written in large print. However, worksheets and homework should be enlarged. As the child progresses through school the use of large print may be necessary. All copies should be of good quality and enlarged to fit the child’s needs.
Many students can use bifocals, microscopes and/or stronger reading glasses to read regular print. Also, students should be allowed to hold the reading material close to their eyes for more magnification and clarity. Prolonged reading however may necessitate the use of large print and/or the use of a CCTV to reduce fatigue and eyestrain.
These patients also have nystagmus which can greatly affect their vision. When under stress the nystagmus can increase causing a decrease in their vision. Students should have extra time when taking tests. Additionally, students with achromatopsia should be allowed to turn their head or eyes into their null position for better vision.
Driving Issues
Patients with achromatopsia have good peripheral fields, moderate and stable vision loss, and respond well to magnification with bioptic systems. It would initially seem that these patients would be good bioptic driving candidates. The problem, however, is the severity of color blindness. In complete achromatopsia, patients have problems with traffic lights; brake lights and reading certain colored signs. With the application of Red Central Contact Lenses and modifications to aid seeing traffic signals we can now help many of these patients become bioptic drivers.
In incomplete achromatopsia, however, where patients still have a small amount of color vision, we can sometimes enhance the color detection of traffic lights and signs with filters and red contact lenses. This may enhance detection of traffic lights and signs, allowing some incomplete achromats to become bioptic drivers.
However, there may be certain situations where a traffic light is backlit from the rising or setting sun that can cause the traffic light to be very difficult to discern.
Other Resources
Oliver Sacks has written a wonderful book about the island of Pinelgap. His book is called The Island of the Color Bind and details the disease and the strange incidence of achromatopsia in this population. It was also made into a PBS documentary by the same name and was shown a few years ago. Oliver Saks has written many books, but is well known for his work with comatose patients which were made into the movie Awakenings starring Robin Williams.
An Internet resource on achromatopsia is the Achromatopsia Network. There website address is www.achromat.org . This website has wonderful information about achromatopsia to educate both the patient and their family.
Comment
There are no medical or surgical treatments for achromatopsia. In the future, we hope that genetic research may provide new methods of treatment. Early low vision intervention is crucial. Even in an infant, environmental modifications and adaptations in light exposure can be performed to help the child.
About the Authors
Drs. Richard and Laura Windsor are a father and daughter team of low vision specialists at the Low Vision Centers of Indiana. Dr. Laura Windsor was recently honored with the 2001 National Essilor Award. Her father is past recipient of the American Optometric Association’s National Optometrist of the Year 1999. They have recently completed a new movie on bioptic driving. Their work and patients have been featured on Breakthroughs in Science and the CBS Evening News: Eye on America. You may reach them at:
Dr. Laura Windsor drlaura@eyeassociates.com
Dr. Richard Windsor richw@eyeassociates.com
Website: www.eyeassociates.com