color vision

Document Sample
color vision Powered By Docstoc
					Examining for Colour
   In the normal eye, the retinal cones contain three
    classes of photopigments which have maximum
    sensitivity in the long-wave (red), medium-wave (green)
    and short-wave (blue) parts of the spectrum.
   Colour deficiency arises when one of these
    photopigments is missing (dichromatism), or when an
    abnormal photopigment is present which differs in
    sensitivity from the corresponding normal pigment
    (anomalous trichromatism).
   Photopigment abnormalities vary from slight to severe .
    There are three types of colour deficiency and
     differences in severity within each type: protan and
     deutan defects are known collectively as red—green
     colour deficiency.
    1.   Protan defects: The long-wave photopigment is either
         missing or abnormal, causing loss of sensitivity to red light.
    2.   Deutan defects: The medium-wave photopigment is either
         missing or abnormal, causing loss of sensitivity to green
    3.   Tritan defects: The short-wave photopigment is either
         missing or abnormal, causing loss of sensitivity to blue light.
   The number of cone photopigments and their spectral sensitivity is genetically
   Genes which specify the long-wave and medium-wave sensitive
    photopigments are located on the X chromosome and the inheritance of
    protan and deutan defects is X-linked.
   About 8% of males (one in 12) and 0.4% of females (1 in 200) are affected.
   The different types of colour deficiency do not occur with the same
    frequency and deuteranomalous trichromatism predominates in both males
    and females (see Table).
   The gene specifying the short-wave photopigment is located on chromosome
    7 and tritan colour deficiency is inherited as an autosomal dominant trait.
   An equal number of males and females are affected.
   The prevalence of tritanopia is probably not greater than 1 in 10000 and
    tritanomalous trichromatism not greater than 1 in 500. Many clinical colour
    vision tests are designed to identify red—green colour deficiency only.
Routine colour vision screening
   Colour deficiency occurs in families, and parents may know that a child is
    likely to be colour deficient if another family member is affected.
   In an X-linked inheritance the most common transmission is from maternal
    grandfather to grandson. Boys with a colour-deficient brother have a 50%
    chance of being similarly affected.
   Colour-deficient girls must have a colour deficient father as well as colour-
    deficient maternal relatives.
   Children with severe colour deficiency often make colour naming mistakes or
    choose incorrect colouring materials at an early age.
   Many occupations require good colour vision and careers advisors need to be
    aware of colour deficiency.
   However, if screening is delayed until adolescence the child may already have
    been placed at an educational disadvantage.
   Colour vision assessment can easily be undertaken in stages:
         Ideally the first examination should be made when the child begins school at
         four or five years of age
         second examination after seven years of age when the child is capable of
         responding to adult screening methods. This second examination confirms the
         initial screening result, and aims to estimate the type and severity of colour
        A final more comprehensive examination, which includes tests of hue
         discrimination ability, can be given when career options need to be considered.
   Clinical colour vision tests are designed to perform different functions:
         Screening tests identify individuals with normal or abnormal colour vision.
        Grading tests estimate the severity of colour deficiency. Both screening and grading
         tests classify colour deficiency into protan, deutan or tritan but tests composed of
         pigment colours are not able to distinguish dichromats and anomalous
   There are four test design categories each involving a different visual task
    (Table 2).
      Pseudoisochromatic plates:
   Pseudoisochromatic plate tests are commonly
    used in the clinic to screen for colour vision
    Colours are carefully chosen based on the
    confusion lines.
   The most commonly used pseudoisochromatic
    plate in the clinic would be the Ishihara
    Isochromatic plates (for screening red-green
    colour vision deficiency).
   Pseudoisochromatic plates are designed in four ways:
       Transformation plates: where a person with normal colour vision sees
        one figure and a CVD person sees another.
       Vanishing plates: where a person with normal colour vision see the
        figure while a CVD person will not.
       Hidden-digit plates: where a person with normal colour vision does not
        see a figure while a CVD will see the figure.
       Diagnostic plates: designed to be seen by normal subjects with colour
        vision defectives seeing one number more easily than another.
   Most pseudoisochromatic plates are intended to be viewed for
    three or four seconds only and the verbal response should be
   Prolonging the viewing time reduces the efficiency of the test as
    it assists colour deficient people to obtain the correct result.
   Several special pseudoisochromatic tests for children
    have been produced with simple figures, such as
    familiar geometric shapes or pictures, which are easier
    for a child to name quickly.
   Other visual tasks such as colour matching games,
    selecting similar colours or finding the odd one out
    have also been tried.
   The Ishihara test for unlettered persons has the most
    easily understood format, and is the preferred test for
    children from four to seven years of age.
    Farnsworth-Munsell 100 hue test
   The F-M 100 hue has been designed to detect all types of colour vision
    abnormality from the mildest red-green defect to total achromatopsia.
   It separates persons with normal colour vision into classes of superior, average
    and low colour discrimination and measures the axes or zones of colour
    confusion in those with defective colour vision2.
   The F-M 10 hue test consists of 85 caps which form a perfect hue circle of the
    visual spectrum. The hue circle is divided into four parts for the testing. Each
    has an additional fixed or pilot cap at either end of the box and 22 or 21 loose
    caps. The four boxes render it impossible to confuse reds with greens, or blues
    with yellows.
   The F-M 100 hue test is the most comprehensive of the Farnsworth Munsell
    type tests, giving both differential diagnosis and progression of the disease.
   The test can be used to screen for any type of colour vision loss. However, the
    test takes a long time to complete, especially when a patient has an acquired
    loss and therefore must be tested monocularly2.
               Nagel anomaloscope
   Diagnosis of the exact type of red-green colour deficiency can only
    be made using spectral stimuli.
   The Nagel anomaloscope is the instrument of choice:
        The instrument consists of a Maxwellian-view spectroscope in which two
        halves of 2° bipartite field are illuminated respectively by monochromatic
        yellow (589 nm), and a mixture of monochromatic red and green
        wavelengths (670 nm and test 546 nm).
       Exact colour matches must be made by adjusting both the luminance of the
        yellow test field and the red—green matching ratio.
   This test examines practical hue discrimination ability and is more
    appropriate for young people over 13 years of age when career
    options are being considered.

Shared By: