Achromatopsia

Definition

Definition of Achromatopsia
Achromatopsia (ACHM), is a medical syndrome that exhibits symptoms relating to at least five separate individual diseases. Although the term may refer to acquired disorders such as color agnosia and cerebral achromatopsia, it typically refers to an autosomal recessive congenital color vision disorder, the inability to perceive color AND to achieve satisfactory visual acuity at high light levels (typically exterior daylight). The syndrome is also present in an incomplete form which is more properly defined as dyschromatopsia. The only estimate of its relative occurrence of 1:33,000 in the general population dates from the 1960s or earlier.

Symptoms

Symptoms of Achromatopsia
The syndrome is frequently noticed first in children around six months of age by their photophobic activity and/or their nystagmus. The nystagmus becomes less noticeable with age but the other symptoms of the syndrome become more relevant as school age approaches. Visual acuity and stability of the eye motions generally improve during the first 6–7 years of life (but remain near 20/200). The congenital forms of the disease are considered stationary and do not worsen with age.

The five symptoms associated with achromatopsia/dyschromatopsia are:

1. Achromatopia
2. Amblyopia (reduced visual acuity)
3. Hemeralopia (with the subject exhibiting photophobia)
4. Nystagmus
5. Iris operating abnormalities

Causes

Causes of Achromatopsia
Acquired achromatopsia/dyschromatopsia is a condition associated with damage to the diencephalon (primarily the thalamus of the mid brain) or the cerebral cortex (the new brain).

1. Thalamic achromatopsia/dyschromatopsia is caused by damage to the thalamus, a portion of the brain stem. It is most frequently caused by tumor growth since the thalamus is well protected from external damage.
2. Cerebral achromatopsia is a form of acquired color blindness that is caused by damage to the cerebral cortex of the brain, rather than abnormalities in the cells of the eye's retina. It is most frequently caused by physical trauma, hemorrhage or tumor tissue growth.

The known causes of the congenital forms of achromatopsia are all due to malfunction of the retinal phototransduction pathway. Specifically, this form of ACHM seems to result from the inability of cone cells to properly respond to light input by hyperpolarizing. Known genetic causes of this are mutations in the cone cell cyclic nucleotide-gated ion channels CNGA3 (ACHM2) and CNGB3 (ACHM3) as well as the cone cell transducin, GNAT2 (ACHM4).

Diagnosis

Diagnosis of Achromatopsia
In both forms of congenital achromatopsia, the signs of achromatopsia may not present until age three to six months of age. The first signs are usually nystagmus, a shaky pendular movement of the eyes and squinting or other signs of aversion to bright light.

The diagnosis of rod monochromatism and blue cone monochromatism is based on medical history, color vision testing, electroretinogram testing, and a lack of significant retinal signs in young patients. Mild pigmentary retinal changes or “bulls eye” pigmentation may occur in older patients.

In the past, early diagnosis was complicated since infants could not perform the color vision testing to confirm the diagnosis. Today, genetic testing may aid in diagnosis of some of the most common forms of rod monochromatism.

Clinical

Clinical of Achromatopsia
Achromatopsia is characterized by reduced visual acuity, pendular nystagmus, increased sensitivity to light (photophobia), a small central scotoma (which is often difficult to demonstrate), eccentric fixation, and reduced or complete loss of color discrimination. Hyperopia is common. Nystagmus develops during the first few weeks after birth and is followed by increased sensitivity to bright light.

Best visual acuity varies with severity of the disease; it is 20/200 or less in complete achromatopsia and may be as high as 20/80 in incomplete achromatopsia. Visual acuity is usually stable over time, but both nystagmus and sensitivity to bright light may improve slightly.

Treatment

Treatment of Achromatopsia
The best treatment for light sensitivity is to address the underlying cause. Once the triggering factor is treated, photophobia disappears in many but not all cases. Patients with photophobia will avert their eyes from direct light (sunlight and room lights), or may seek the shelter of a dark room or wear sunglasses. A study by Stringham and Hammond, published in the Jan-Feb issue of Journal of Food Science, discusses the improvement in visual performance and decrease in light senitivity (glare) in subjects taking 10mg Lutein and 2mg Zeaxanthin per day.

Prevention

Prevention of Achromatopsia
There are currently no known ways to prevent congenital achromatopsia. However, acquired achromatopsia may be prevented by following routine safety measures to protect against brain damage, such as wearing a bicycle helmet or a seatbelt.