Suhail Wahab KhalilIsra School of Optometry

The normal human retina contains two kinds of light sensitive cells: the rod cells (active only in low light) and the cone cells (active in normal daylight and responsible for color perception).

The different kinds of inherited color blindness result from partial or complete loss of function of one or more of the different cone systems.

Color VisionIs the ability of the eye to

discriminate between colours excited by light of different wave lengths.

Color BlindnessIs the inability to perceive difference between some of the colours that other people can distinguish.

Normally, there are three kinds of

cones (each one sensitive to a

specific range of wavelengths):

L(Long) ---"red" cones (64%)

(725 – 647 nm, peak : 565 nm)

M(Middle) ---"green" cones

(32%)

(575 – 492nm, peak : 535 nm)

S(Short) ---"blue" cones (2%)

(492 – 450 nm, peak : 420 nm)

Trichromatic theory: Also called as young helmholtz theory: It postulates the existence of three kinds

of cones Each cone containing a different

photopigment and maximally sensitive to one of three primary colours i.e. Red, Green and Blue.

Monochromacy: occurs when two or all three of the cone pigments are missing and color and lightness vision is reduced to one dimension.

Total color blindness

Dichromacy: occurs when only one of the cone pigments is missing and color is reduced to two dimensions.

Partial color blindness

red-green

blue-yellow

Also known as rod monochromacy, complete achromatopsia, and

typical monochromacy.

A rare, non-progressive inability to distinguish any colors as a result of absent or nonfunctioning retinal cones.

See everything as white, black, or some shade of gray

Typically caused by a missense mutation (a switched amino acid)

in the CNGB3 gene.

CONGENITAL DEFECTS ACQUIRED DEFECTS

1 Affects both eyes equally 1 May affect one eye only or asymmetric

2 Usually a R-G defect is found 2 B-Y or R-G defect

3 Other visual functions normal 3 Other visual functions also abnormal

4 Stable through out life time 4 Variable, depend on test and diseases conditions

5 Learned to adapt- can label objects

5 Cannot name color correctly

6 More prevalent in male 6 Equally Prevalent in male and female

7 One or more Cones are absent 7 Cones are present but work at decreased/altered spectral sensitivity

8 Causes:  X-linked recessive gene

8 Causes: diabetes, glaucoma, medication, aging, and Chemical Exposure

The objectives of assesment of color vision is to: Screen for color deficiency. grade the severity of color vision

defect. Classify the type of defect.

PSEUDOISOCHROMATIC COLOUR TEST:

most commonly employed tests- eg.-

ISHIHARA PLATES and

HRR(HARDY,RAND,RITTLER)

plates Ideal for paediatric

testing of congenital color blindness.

designed in four ways

1st plate- for demonstration and malingerers.

(2-9) plate- Transformation plates: normal person sees one figure and a CVD sees another.

(10-17)plate-Vanishing plates: normal person see the figure while a CVD person will not.

Pseudoisochromatic colour plates (18-21)plate-Hidden-

digit plates: normal person does not see a figure while a CVD will see the figure.

(22-25)plate-Diagnostic plates: seen by normal subjects, CVD one number more easily than another. Protans only see the no. on the right side and deutans only see the no. on the left.

Should be used at 75 cm, day light, right angle to visual axis, 3 sec time.

It is a hue arrangement type test.Patients are asked to arrange 15 coloured caps in sequential order based on similarity from the pilot colour cap at a distance of 50 cm.

Currently No treatment. Some filters (goggles/tinted contact

lenses) may help to distinguish the colours but not in the identification of colours.

The purpose of this is to eliminate certain lights and modify the light reaching the eyes so that the receptors receive correct information.