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Colour vision deficiency part 2--assessment in clinical practice.

The second part in a series about colour vision, this article describes the methods available for identifying defects in practice, with an overview of the clinical considerations and the relative merits of each technique.

Course code: C-34466|Deadline: February 14, 2014

Learning objectives

To understand the various methods for identifying colour vision defects (Group 3.1.4)

Learning objectives

To understand the various methods for identifying colour vision defects (Group 3.1.4)

Introduction

Identification of red-green colour deficiency is an integral part of a complete eye examination approved by the GOC and endorsed by the College of Optometrists. An efficient screening test should be given to all new patients and the result recorded. Further assessment at subsequent visits is unnecessary unless occupational advice is needed or an acquired abnormality is suspected from reported symptoms or clinical findings.

Screening

The efficiency of a screening test is described in terms of specificity and sensitivity compared with a gold standard reference test. The spectral anomaloscope, such as the Nagel anomaloscope, is the accepted reference test for congenital red-green colour deficiency. A large number of normal trichromats and colour deficient people must be examined to obtain accurate figures. If an anomaloscope is not available, the efficiency of a new test can be compared with that of a validated test.

The specificity of a test is the percentage of normal trichromats correctly identified as normal (true negatives). Low specificity shows that some normal trichromats are incorrectly identified as colour deficient (false positives). Specificity of 95% indicates that 5% of the normal population is incorrectly identified as colour deficient (a very large number of people). Sensitivity is the percentage of colour deficient people correctly identified (true positives). Low sensitivity shows that some colour deficient people are not identified (false negatives). Sensitivity of 95% indicates that 5% of colour deficient people are not identified (a very small number of people). This is acceptable if those not identified have minimal/slight deficiency but not if people with severe deficiency are missed.

The Nagel Anomaloscope

The Nagel anomaloscope is a direct view spectroscope that presents a Rayleigh match (red + green = yellow). The bipartite field subtends about 2.5 degrees to ensure that only cones are stimulated. One half of field contains monochromatic yellow (589nm) and the other, a mixture of monochromatic red (670nm) and green (546nm). Normal colour vision is dichromatic in this spectral range. The characteristics of the red-green mixture ratios selected to match yellow identify normal or defective red-green colour vision and diagnose the type and severity of deficiency. The matching ranges of protanomalous and deuteranomalous trichromats form two separate distributions outside the normal matching range. Protanomalous trichromats require more red to match yellow and deuteranomalous trichromats require more green (see Figure 1). The matching range on the red-green mixture scale is highly correlated with the peak wavelength separation of the two expressed photopigments determined by genetic analysis. (1) Protanopes and deuteranopes are able to match all red-green mixture ratios. Protanopes are distinguished by the low yellow luminance needed to match pure red.

An anomalo-quotient calculated from a single match or from the midpoint of a matching range does not show the severity of deficiency. A redgreen discrimination index (RGI) can be used if a single measurement is needed. RGI is calculated as (1-(R-MR)/73) where R is the matching range obtained, MR is the mean normal matching range for the instrument and 73 is the number of units in the red-green matching range. RGI is zero for dichromats and close to unity for normal trichromats.

Clinical tests for defective colour vision

Clinical tests consist of pigment colours and comprise a visual task that is easy to understand. Two tests are needed; one test identifies redgreen deficiency and classifies protan and deutan deficiency, while the second test is used to estimate the severity and identify significant tritan deficiency. Dichromats are not identified with pigment tests. Appropriate illumination is an important consideration when undertaking these tests as colour appearance changes with the spectral content of the light source. White light illumination, with balanced spectral emission, is needed to preserve the intended appearance of pigment tests. Natural "north sky" illumination, equivalent to Standard Source C, is ideal; tungsten illumination should not be used.

Identifying red-green deficiency

Inheritance

The most usual transmission of congenital red-green deficiency is from maternal grandfather to grandson. Boys with a colour deficient brother have a 50% risk. Girls with a colour deficient father and affected maternal relatives may also have colour deficiency.

The Ishihara plates

Pseudoisochromatic screening tests aim to identify red-green deficiency and classify protans I and deutans. Confusion colours must be carefully selected and luminance contrast clues eliminated. A number of tests have been produced but clinical audits show that the Ishihara test is the most efficient. Background knowledge of the different design parameters is helpful (see Table 1). Each plate should be viewed for about four seconds. Neither the hidden digit plates nor the pathway designs are required. Sensitivity of the 16 transformation and vanishing designs of the 38-plate test is 97% if three errors are made and 94% on four errors. (2,3) Specificity is 100%, compared with the Nagel anomaloscope, if 'misreadings' are distinguished from errors on vanishing designs. Fewer than 2% of normal trichromats make more than three 'misreadings' (two 'misreadings' on the 12 Transformation and Vanishing designs of the 24-plate test). Misreadings arise from completed loops in the serif numeral design such as '6' or '3' being interpreted as '8' (see Figure 2).

Similar small colour differences are employed and only minimal/slight deuteranomalous trichromats that have a discrimination threshold close to this value make fewer than eight errors. About 40% of colour deficient people see both protan/deutan classification figures but patients can be correctly classified by assuming that the less clear figure is not seen. Typical Monochromats are able to interpret some designs correctly using perceived luminance contrast. Blue Cone monochromats can only read Plate 1. It has been suggested that heterozygous women might be identified by a small number of errors on the Ishihara plates or by a large number of 'misreadings': this is not the case. Although the mean number of misreadings made by heterozygotes is slightly greater than that of normal trichromatic men the difference is not clinically significant. (4) Compound mixed heterozygotes are known to have normal colour vision. A small number of the most accurate screening plates can be used for rapid screening. A different selection of plates is recommended for young children containing low numbers that can be identified verbally by most three-year-olds (see Table 2). A card is placed over one side of the plate if there are two numbers. The Ishihara Test for Unlettered Persons is an alternative and has transformation designs with circles and squares (see Figure 3). (5) The Colour Vision Testing Made Easy Test (Home Vision Care, USA) includes designs with circles, squares and stars that identify moderate/severe red-green deficiency. (6) The Matsubara test has poor colour reproduction and should not be used. (7)

Grading severity

Three clinical grading tests that aim to identify people with significant (moderate or severe) deficiency are in general use (see Table 3).

The AO HRR test (discontinued) and the Richmond HRR test 4th Edition (2002)

The HRR test is intended to identify and grade the severity of congenital protan, deutan and tritan deficiency. (8) Tetartan designs were included to identify abnormality of a supposed fourth yellow sensitive photopigment but are not needed. All the plates have vanishing shape designs composed from average neutral colours that provide negative evidence of colour deficiency. The test is not accurate for screening; specificity may be as low as 72% if two errors (not seen or missed figures) on the six low threshold red-green designs is the fail criterion. Sensitivity is about 83% if three missed figures is the fail criterion. Only two arbitrary grades of severity can be distinguished with confidence (see Table 3). (9-12) Protan/deutan classification is good and significant tritan deficiency can be identified with the large colour difference grading plates. False-positive tritan results are obtained with screening designs after 55 years of age. Women have poorer discrimination of neutral hues than normal trichromatic men and miss a larger number of small colour difference 'vanishing' designs intended to identify red-green deficiency. (13)

The Farnsworth D15 test

The D15 contains hues from an incomplete hue circle with Munsell value 5 and chroma 4. These are contained in caps that subtend 1.5 degrees at a test distance of 50cm. The aim is to identify significant protan, deutan and tritan colour confusions. A small number of hues are available to identify tritan deficiency. The task is to arrange the hues in a natural order beginning with a reference cap. Significant confusions are demonstrated when hues from opposite sides of the circle are mingled in the arrangement. Classification of the type of deficiency is obtained from the direction of the crossing lines (see Figure 4). A circular results diagram with no crossing lines is a pass. An interlacing pattern is a fail. About 50% of colour deficient men fail the D15, including 98.5% of dichromats. (14) Inspection of the results diagram is preferred to numerical methods of analysis or scores that are difficult to relate to test performance. (15-17) Some typical monochromats are able to arrange the hues according to perceived contrast. Blue cone monochromats make a large number of nonspecific red-green confusions.

The City University Test 2nd Edition (CU2) (discontinued)

The CU2 test is based on the Farnsworth D15 and has 10 charts that display a central colour and four peripheral hues. (18) The task is to select the peripheral hue that looks most like the central colour. Three of these are typical protan, deutan and tritan confusions and the fourth hue is adjacent in the D15 sequence. An error on one plate is a fail. Ambiguous protan/deutan classifications are found in 60% of results and are an artefact of the test design. (18) The CU2 is slightly different from the first edition but has the same number of plates. The third edition of City University Test was introduced in 1998 containing screening designs in a unique format and a small number of new grading plates; the test has not been validated.

Protans make fewer errors than deutans on the D15 test and the CU2 because perceived luminous contrast is available as an aid to obtaining good results (see Table 3).

Other hue discrimination tests

Other hue discrimination tests have been used to investigate and monitor progression in acquired colour deficiency. The Adams D15 has the same hues as the D15 but with Munsell value 5 and chroma 2. The Lanthony desaturated D15 has the same hues with Munsell value 8 and chroma 2 but is not an efficient screening test. False-positive tritan results are obtained in all age groups with clinical levels of illumination and test/retest consistency is poor. (19) The Mollon-Reffin Minimist test is intended to monitor acquired deficiency or as a rapid alternative to the D15. Significant protan and deutan deficiency can be identified in young children or disadvantaged groups but false positive tritan results are obtained. (20) Significant protan and deutan deficiency can be identified in young children or disadvantaged groups but false positive tritan results are obtained. (20)

The Farnsworth-Munsell 100 Hue test

The Farnsworth-Munsell 100 hue test (F-M 100) measures practical hue discrimination ability and was intended to identify normal trichromats with superior hue discrimination ability. There are 85 Munsell hues selected from a complete hue circle divided between four boxes. The caps must be arranged between two reference hues in each box. The colour difference between adjacent hues is smaller in the blue-green quadrant (Box 3) leading to false positive tritan errors after 55 years of age. Results are plotted on a circular diagram and discrimination ability is estimated from a total error score. Characteristic error patterns are found in severe congenital colour deficiency (see Figure 4). (21) Age-related mean error scores have been calculated to aid identification of acquired deficiency but must be accepted with caution due to examiner variance and differences in illumination. (22)

New techniques for examining acquired colour deficiency

New objective examination procedures performed on a high-resolution colour calibrated display unit are replacing pigment tests in major examination centres. The Colour Discrimination and Diagnosis (CAD) test presents stimuli of precise chromaticity and saturation as moving targets within a dynamic background of neutral grey dots that mask luminance contrast. The target moves along one of four diagonal directions and the subject presses a button to indicate the direction of motion. Thresholds that define red-green (RG) and blue-yellow (BY) sensitivity, within the neutral isochromatic zone, are plotted as x, y chromaticity co-ordinates in the CIE chromaticity diagram (1931).

The Cambridge Colour Test has been used to monitor progression of acquired loss of hue discrimination. The background consists of a matrix of grey dots. The test figure is a Landolt C constructed with the average neutral chromaticities found in congenital protan, deutan and tritan deficiency. The observer presses one of four buttons to indicate the position of the gap in the C.

Conclusion

The overview of the various colour vision tests described in this article provides the eye care practitioner with the knowledge to understand the diagnostic value of the assessments used in routine practice.

MORE INFORMATION

References Visit www.optometry.co.uk/clinical, click on the article title and then on 'references' to download.

Exam questions Under the new enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk/cet/exams.

Please complete online by midnight on February 14, 2014. You will be unable to submit exams after this date. Answers will be published on www.optometry.co.uk/cet/exam-archive and CET points will be uploaded to the GOC every two weeks. You will then need to log into your CET portfolio by clicking on "MyGOC" on the GOC website (www.optical.org) to confirm your points.

Reflective learning Having completed this CET exam, consider whether you feel more confident in your clinical skills--how will you change the way you practice? How will you use this information to improve your work for patient benefit?

Jennifer Birch was formerly a senior lecturer in clinical optometry at City University London. She is now a senior research fellow in the Henry Wellcome Research Laboratory in the Department of Optometry. She was a founder member of the International Research Group on Colour Vision Deficiencies, has written extensively on clinical aspects of defective colour deficiency and on occupational colour vision requirements and she was appointed to an Honorary Life Fellowship of the College of Optometrists in 2012.

Table 1 Function of the numeral designs of the 38 plate Ishihara
test in the identification of congenital red-green deficiency

PLATES                 FUNCTION                REALISATION

1                    Introduction           Seen correctly by
                                         luminance contrast all
                                         observers. Demonstrates
                                           the visual task and
                                         identifies malingering

2-9                   Screening          Different numerals seen
Transformation                          by normal trichromats and
                                            colour deficient.
                                          POSITIVE EVIDENCE OF
                                               DEFICIENCY

10-17                 Screening         A numeral seen by normal
Vanishing                                trichromats but not by
                                            colour deficient.
                                          NEGATIVE EVIDENCE OF
                                               DEFICIENCY

18-21            (Intended Screening)    Intended that a numeral
Hidden Digit            OMITt               is seen by colour
Do NOT use                              deficient but not seen by
                                         normal trichromats POOR
                                             SENSTIVITY AND
                                               SPECIFICITY

22-25               Protan/deutan        Different numerals are
Classification      classification         seen by protans and
LIMITED USE      ONLY SHOWN IF COLOUR   deutans. If both figures
                    DEFICIENCY IS       are seen, classification
                      IDENTIFIED        is according to the less
                                          clear figure Neither
                                          figure may be seen in
                                            severe red-green
                                               deficiency

Table 2 Selection of plates from the 38-plate Ishihara
test for rapid screening of adults and young children

                          Adults

Plate   Design Function   Normal     Red-green
                          Response   deficiency

1       Introduction      12         12
2       Transformation    8          3
3       Transformation    6          5
6       Transformation    5          2
7       Transformation
8       Transformation    15         17
10      Vanishing         2          --
14      Vanishing
15      Vanishing         7          --
24      Protan/deutan     3 5        Protans see 5
                                     Deutans see 3

        Children

Plate   Normal     Red-green
        response   deficiency

1       12         1 and 2
2
3
6       5          2
7       3          5
8
10      2          --
14      5          --
15
24      3 then 5   3 or 5 only

Table 3 Grading severity with two tests

ISHIHARA PLATES    FARNSWORTH D15     CITY UNIVERSITY
                                     TEST 2nd Edition

Fail              Pass: A circular         Pass
                  diagram with no        No errors
                   crossing lines
Fail               Fail: Up to 5     Fail: Errors on 4
                  crossing lines *   Plates or fewer *

Fail              Fail: 6 or more    Fail: Errors on 5
                  crossing lines *   Plates or more *

ISHIHARA PLATES   SEVERITY OF   AO HRR or RICHMOND
                   RED-GREEN     HRR 4th Edition
                  DEFICIENCY      Grading plates

Fail                SLIGHT         Fail: SLIGHT
                                  Intended Mild
                                     category
Fail               MODERATE        Fail: SEVERE
                                     Intended
                                   Moderate or
                                Severe categories
Fail                SEVERE

* Effective for deutans but not protans
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Article Details
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Title Annotation:1 CET POINT
Author:Birch, Jennifer
Publication:Optometry Today
Article Type:Report
Geographic Code:4EUUK
Date:Jan 17, 2014
Words:2793
Previous Article:Posterior segment disease: Part 4.
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