Colour blindness is the inability to distinguish certain colours. It occurs when one or more of the cone types is missing or defective to any extent. They may be absent entirely, or may be present, but unable to manufacture the necessary signals to the brain. It is characterized by abnormal colour matching and colour confusions. Colours which look different to people with normal colour vision can look the same to people with defective colour vision. For example, grass may appear green to colour normals, but be the same colour as orange for people with certain colour defective vision. There is also a marked reduction in the number of separate colours that can be distinguished in the spectrum and the relative luminous efficiency of the eye is altered1.
| Trichromatic | Possess all three cone pigments and has normal colour vision. |
|---|---|
| Dichromatic | Has complete deficiency in one cone pigment but preserves the remaining two cone pigments. |
| Monochromatic | Has only one cone pigment. |
| Achromatopic | Possesses no functioning cones. |
Dichromats have two cone receptors rather than three, and match all the spectral hues using two colour matching variables. There are three types of dichromatism depending on which of the three normal pigments is missing. Protanopes are the most common, lacking the long-wave ‘red’ sensitive receptors. Deuteranopes lack the middle-wave ‘green’ receptors, and tritanopes lack the short-wave ‘blue’ sensitive receptors. Protanopes and deuteranopes cannot distinguish red light from green light, while the rare tritanope has blue-yellow confusion2.
Partial deficiencies in one of the three cone pigments is called anomalous trichromatic. This means that to match a reference colour, three test colours must still be used, but the match is different from normal. e.g. a protanomalous trichromat requires more red than normal to match the reference2.
Protanomaly is deficiency in sensitivity to red, as well as poor red-green discrimination. A deficiency of green sensitivity, as well as blue-green and yellow-green insensitivity is called deuteranomaly. Tritanomaly is the deficiency of the cone pigment for blue as well as blue-green and yellow-green insensitivity2.
Monochromacy occurs when two of the types of cones malfunction resulting in single cone vision. This is relatively rare, with an incidence of about 1 in 30 000. Characteristics include reduced visual acuity, usually around 6/60 (20/200) but occasionally significantly better, scotopic spectral sensitivity with no Purkinje shift (shift from vision with rods, to vision with cones), photophobia (sensitivity to light), nystagmus ("wobbly eyes", where eyes are constantly trembles, and does not fix on a target normally) which usually disappears by age 30 years, and sluggish pupil reflex to light1.
Achromatopsia is the complete absence of any cones, or a very low normal cone density. The achromat can only see in black, white and shades of grey. Typical complete achromatopsia is characterized by reduced visual acuity from infancy, photophobia, pendular nystagmus that may diminish during adolescence, absence of foveal reflex, and irregular distribution of macular pigment1.
Defects arising during life can be due to general or ocular disease, a side-effect of medication, or as a consequence of toxic poisoning or head trauma. Changes in colour vision can be one of the first indications of disease, and are generally progressive. It is important to identify the cause early to avoid permanent damage1.
Most acquired colour vision defects come on gradually or suddenly and are easily identifiable, since they usually involve other abnormal visual symptoms such as reduced vision, a constriction of the field of vision in general or in a specific region, poor adaptation to the dark, etc. Acquired defects are often confined to one eye or one part of the visual field.
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