Achromatopsia - Colour Blindness

By Dr. Veena G.D., MBBS, DOMS

Colour blindness is a hereditary condition wherein the individual becomes deficient to one of the primary colours but other faculty of vision like visual acuity, optic nerve etc. are normal.

Hereditary Facts

The hereditary aspects of colour blindness commonly seen are “Portan and Deutan” blindness.

Portan is mild deficiency in red sensitivity and is called portanomaly.

Deutan is mild deficiency in green sensitivity and is known as Deuteronomaly.

Sometimes complete lack of sensitivity of red is “Portonopia” while green is “Deuteranopia”.

Achromatopsia is also referred as ‘Day Blindness’.

How Colour Blindness happens
In the retina, there are two kinds of neuro-sensory cells – Rods and Cones. Rods are responsible for night vision whereas cones are responsible for day and colour vision. Cones are further classified into three types. They are the red cones, the green cones, and the blue cones. A balanced distribution of these cells is necessary for normal colour vision. If a child is born with non-functioning cones, they will have colour blindness - Achromatopsia. Sometimes children have a reduced complement of the cones, in which case they will have partial or incomplete achromatopsia.

Prevalence

Portan and Deutan colour blindness conditions both are X-linked recessive hereditary anomaly. Approximately 8–10% male population is colour blind with one of these forms of colour blindness while affliction in female population is rare. They just carry the genes but do not suffer from colour blindness. 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 / cousins).

Rare Forms

Still rare forms are :

Tritans-anomaly (mild deficiency for blue colour sensitivity)
Tetaratans-anomaly (mild deficiency for yellow colour sensitivity).

Other Causes for Colour Blindness

The eyes of these persons otherwise appear normal with normal visual acuity. Sometimes drug induced colour blindness might be encountered with prolonged usage of certain drugs e.g. Chloroquine and Avitaminosis. These patients should be counselled to avoid occupation which require good colour vision.

Colour Vision Testing

It is mandatory to demonstrate the adequate colour vision for certain jobs like driver’s license, Army, Transportation, Pilot or Traffic Police etc.

1. Screening defective colour vision from normal.

2. Qualitative classification –

Portans and Deutans – 4% in all male

0.4% in all female

Tritans (blue) and Tetaratans (yellow) are extremely rare

3. Qualitative Analysis – Here the degree of colour deficiency is measured as in mild, medium or marked.

Technique

The standard colour blindness testing is conducted with primary dots printed on a background of similar dots in a confusion of colours or grays. These dots are set in patterns, shapes, numbers or letters. That would be recognized by normal individual but not perceived by those with colour perception defects. Some of the standard colour blindness vision test charts are –

Ishihara’s colour vision chart
Stilling colour vision chart
Hardy-Rand-Ritter [H-R-R] colour vision chart.

Ishihara’s colour vision chart

Diagnosis

On a routine test, ophthalmologist makes the diagnosis. The essential clues are the person may have altered light sensitivity and reduced vision with a very rare horizontal repetitive movement of the eye (Nystagmus). Further diagnosis can be confirmed by a specialized and sensitive test called ElectroRetinoGraphy (ERG).

Treatment

Children should be checked for refractive errors (need for glasses). Prescribing glasses to correct refractive conditions such as far-sightedness (hyperopia), near-sightedness (myopia) and astigmatism can improve the vision somewhat but will not restore normal levels of vision. Dark tinted glasses or red glasses will help reduce the sensitivity to light and enhance visual functioning.

Prevention

Avoid consanguineous marriages. Good genetic counselling to be done if there is a past history of colour blindness in the family. Parents and teachers are to be involved in counselling and treating the child with colour blindness. If complete lack of colour is encountered then special glasses with the prescribed tint can be used. Enrolling in specially designated schools where visually impaired are cared for is advised.

Kid wearing red tinted

glass for colour blindness