ACHROMATOPSIA

Achromatopsia is an inherited visual disorder in which affected individuals are colorblind, have a loss of visual acuity, and are sensitive to bright light.

WHAT IS ACHROMATOPSIA?

This visual disorder is due to a lack of cone photoreceptor cells in the retina. Normal eyes have about 6 million cones that predominantly are located in the central area of the retina. There are three types of cone photoreceptor cells that are responsible for normal color vision: red (also known as L cones), green (M cones), and blue (S cones). A balanced distribution of these photoreceptor cells is necessary for normal color vision.

Achromatopsia is an inherited non-progressive visual disorder. Inherited means that people are born with this condition; non-progressive means that it does not become worse with time. Affected individuals are colorblind and have visual acuity loss, extreme light sensitivity and nystagmus (rapid, involuntary, oscillatory motions of the eyeball). The condition also is known by the name rod monochromacy.

Rods are not used for color vision (except for black, white, and shades of gray) or for central vision. For this reason, persons with rod monochromacy are partially or completely colorblind and have very poor visual acuity.

Three primary forms of rod monochromacy have been identified.

• Complete Achromatopsia is characterized by the inability to perceive colors other than black, white and shades of gray. Those with this visual disorder have no real concept of color. They experience the world much as if it were a black and white photograph with varying shades of gray. This might not be too much of an impediment if patients had good central vision and could perceive images in good detail. Unfortunately, the lack of functioning cone photoreceptor cells means that they cannot. Visual acuity commonly is 20/200 or less.
• Incomplete Achromatopsia also is characterized by profound color impairment. However, patients have a small residual amount of color vision due to the presence of some functioning cone photoreceptor cells in the retina. Visual acuity also is slightly better and typically ranges between 20/80 and 20/120.

  A subtype of incomplete achromatopsia is blue cone monochromacy. In this condition, both red and green cone photoreceptor cell functions are absent, but blue cone functions are present. Color vision and visual acuity remain impaired.

• Cerebral Achromatopsia is a form of complete colorblindness that is caused by damage to the cerebral cortex of the brain, as opposed to abnormalities in retinal photoreceptor cells. It nearly always is the result of trauma or brain damage from illness. Thus, persons with this visual disorder most often are diagnosed by neurologists, rather than ophthalmologists.

  Loss of color perception in those with cerebral achromatopsia neither co-occurs with severe visual acuity impairments nor with photophobia. However, these individuals do have poor spatial acuity and prosopagnosia (difficulty recognizing faces). People acquiring this visual disorder report that their surroundings are all in shades of gray. They are capable of perceiving gray because of prior experience with color vision. This makes it possible for them able to perceive the absence of color as gray. Conversely, those with inherited complete rod monochromacy find the concept of gray just as confusing as the concept of any other color.

WHAT CAUSES ACHROMATOPSIA?

Rod monochromacy is an inherited genetic disorder. Thus far, four genes are known to be associated with this condition: CNGA3 located on chromosome 2 (2q11.2), CNGB3 located on chromosome 8 (8q21.3), GNAT2 located on chromosome 1 (1p13.1), and PDE6C located on chromosome 10 (10q24 ).

Blue cone monochromacy has a different genetic underpinning. It is an X-linked disorder that is associated with lesions in the opsin genes OPN1LW (opsin 1 long wave), which encodes pigment for red cone photoreceptor cells of the retina, and OPN1MW (opsin 1 middle wave), which encodes pigment for green cone photoreceptor cells.

WHAT ARE THE CHANCES OF HAVING CHILDREN WITH ROD MONOCHROMACY?

It is important to understand that if both parents are carriers, the probability is 50% (1 in 2) that their non-affected children also will be carriers. For this reason, children should be informed that they might be carriers as they approach adulthood.

For those living with achromatopsia, 100% of their children will be affected if their mate also has this visual disorder; the risk is 50% (1 in 2) that their children will be affected if their mate is a carrier; and the risk is 100% that their children will be carriers if their mate is not a carrier.

HOW COMMON IS ACHROMATOPSIA?

This visual disorder affects approximately one in 40,000 live births. Its prevalence varies in different parts of the world. Because there is a genetic link, it is more common in regions where there is a high rate of consanguineous marriages (marriages between relatives).

WHAT ARE THE MAJOR SYMPTOMS OF THIS VISUAL DISORDER?

Rod monochromacy is present from birth. It usually is first noticed by parents when their child is about six months of age, when she or he begins to show signs of photophobia (sensitivity to light) and/or nystagmus (irregular eye movements). As the child ages, color vision and visual acuity problems will become evident. Other symptoms that have been reported in some individuals include:

• amblyopia (eye misalignment or differences in image quality between the two eyes).
• hermeralopia (inability to see clearly in bright light).
• Iris operating abnormalities
• prosopagnosia (difficulty recognizing faces), predominantly in those with cerebral achromatopsia.

Symptoms vary among individuals in terms of their presence and severity.

HOW IS THE DIAGNOSIS MADE?

Diagnosis of rod monochromacy is made by an ophthalmologist. Depending on the child's age, they may not be capable of completing the color vision screening tests. However, the presence of nystagmus, photophobia, amblyopia or other visual acuity problems will provide essential information to inform the diagnosis. The following clinical tests may be used.

• Standard tests for colorblindness, such as Ishihara Pseudoisochromatic Test (PIP), which contains 7 plates designed for pediatric diagnosis, Hardy Rand Ritlers Pseudoisochromatic Test (HRR), which comes in a pediatric form called HRRP, and City University Test, which tests both for red-green and blue-yellow colorblindness.
• Tests for severe colorblindness, such as the D-15 and Munsell 100 Hue, that require patients to align colored disks into a continuous pattern.
• Visual field assessment through a Perimetry Test may reveal small central scotomas, areas of vision loss corresponding to the central area of the retina (fovea).
• Optical Coherence Tomography can be used to assess losses of cone signals from the retinal photoreceptor integrity line.
• Electroretinography (ERG) is a test to measure the electrical response of the photoreceptor cells of the eye (rods and cones).
• Genetic Testing can confirm variants in the CNGA3, CNGB3, GNAT2, and PDE6C genes. This testing is provided by the non-profit John and Marcia Carver Laboratory at the University of Iowa in the USA, the United Kingdom Genetic Testing Network in the UK, and the DNA Testing Centres of Canada in Canada. In addition to confirming affected individuals, genetic testing can confirm if you are a carrier.

Patients sometimes are misdiagnosed as having a different condition called cone dystrophy. However, cone dystrophy is a progressive disease that can lead to blindness. It also has a genetic underpinning, but it has been associated with dominant mutations in the GUCA1A gene located on chromosome 6 (6p21.1). It is believed that other genes in the same region of chromosome 6 also may be involved, but this has not yet been confirmed. In contrast, rod monochromacy neither is progressive nor does it lead to blindness and has different genetic underpinnings.

WHAT TREATMENTS ARE AVAILABLE?

There is no cure for achromatopsia. Treatment focuses on helping the affected individual to live successfully with this visual disorder.

Parents should have their children checked for visual acuity problems (refractive errors). Prescribing corrective lenses for conditions such as farsightedness (hypermetropia), nearsightedness (myopia) and astigmatism can improve vision to some extent, but will not restore normal levels of vision.

Red central soft contact lenses are an important new treatment for those with rod monochromacy. These contact lenses have a small red circle at the center of the lens that when properly positioned looks like the pupil of the eye. Red central soft contact lenses reduce the light entering each eye and also restrict the type of light entering the eye to the red spectrum. Red light has been found to help rods in the retina to function better. The red hue does not interfere with vision in patients with complete achromatopsia because these individuals do not perceive color. For those with the incomplete form of the visual disorder, the lenses appear to enhance their ability to detect emergency signals, such as flashing red lights, red stoplights and red brake lights on cars and trucks.

For those preferring to use eyeglasses, tinted glasses or red glasses will help reduce the sensitivity to light and enhance visual functioning in moderate light situation. In full sunlight outdoors or in bright indoor spaces, very dark tinted lenses are necessary. These typically are of the wrap-around variety or lenses mounted in standard frames with opaque side shields attached.

Low vision aids also have been found helpful in persons with rod monochromacy. In addition to tints, hats and visors to control light sensitivity, use of magnifiers and large-print books can help with reading and small telescopes can be used for viewing signs and seeing faces at a distance.

HOW WILL THIS CONDITION AFFECT MY CHILD'S EDUCATION?

With adequate help from teachers, children with rod monochromacy usually are able to attend regular classes. However, teachers must be sensitive to the fact that many school activities involve color coding, such as standing on the red line in the gymnasium and use of color in learning materials, including quizzes and exams. Front seat placement, large-print books, and magnifying devices also can enhance a child's educational experience. More severely affected children may benefit from services designated for the visually impaired available in schools.

REFERENCES:

Boger WP, Petersen RA. Achromatopsia. In: Pavan-Langston D, ed. Manual of Ocular Diagnosis and Therapy, 4th ed. Boston: Little, Brown and Company, 1995:283

Heywood CA and Kentridge RW. Achromatopsia, Color Vision, and Cortex. Neurologic Clinics of North America 2003; 21(2):483-500.

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