Diagnosing protan heterozygosity using the Medmont C‐100 colour vision test

Background : A surprisingly high 15 per cent of women in Caucasian societies are carriers of the genes for abnormal colour vision but there is no clinical method to identify them. It has long been known that heterozygotes for the protan colour vision deficiencies can demonstrate a reduced luminous sensitivity to red light. This is known as Schmidt's sign, which is thought to arise from mosaicism (Lyonisation). The Medmont C‐100 colour vision test measures relative spectral sensitivity using flicker photometry to differentiate protans and deutans. It should be able to diagnose Schmidt's sign. Method : We tested six known protan heterozygotes (four whose sons have a protan colour vision deficiency and two whose fathers are protan) with the Medmont C‐100 test. Results : All six heterozygotes made average settings of ‐1.75 or more negative at the Medmont C‐100 test, settings which are at or beyond the boundary of the distribution of settings made by observers with normal colour vision. There have been two previous cases reported in the literature of protan heterozygotes, who made protan settings on the Medmont C‐100 or its predecessor test, the OSCAR. We also tested six daughters of the known heterozygotes, 50 per cent of whom are likely to be heterozygotes. Four of the six (66 per cent) made protan settings on the Medmont C‐100. The other two made normal 0.0 settings. Conclusion : We conclude that the Medmont C‐100 can be used clinically to diagnose carriers of protan colour vision deficiency.     

The C-100:a new dichotomiser of colour vision defectives

This paper evaluates a new instrument (C-100) which employs flicker photometry or silent substitution to determine the type of colour vision defect (protan or deutan). Specifically, this study addresses the unit's capacity to: 1. detect colour vision defects; 2. differentiate protans from deutans; and 3. produce reliable measurements under different viewing conditions. We find that an average of five readings enables protans to be clearly separated from deutans in all cases (p < 0.0001), but that the distinction between these groups and normals is less clear. Dichromats are not distinguished from anomalous trichromats, so the instrument cannot be used as an index of severity. The results are shown to be robust to most of the test conditions likely to be encountered during normal clinical use. A clinical protocol is suggested that utilises the C-100 for classification of colour defective observers. It is concluded that normal, and some anomalous, trichromat settings are performed using flicker photometry, whereas dichromatic observers appear to utilise silent substitution.     

The new Richmond HRR pseudoisochromatic test for colour vision is better than the Ishihara test

Aim: The Hardy‐Rand‐Rittler (HRR) pseudoisochromatic test for colour vision is highly regarded but has long been out of print. Richmond Products produced a new edition in 2002 that has been re‐engineered to rectify shortcomings of the original test. This study is a validation trial of the new test using a larger sample and different criteria of evaluation from those of the previously reported validation study. Methods: The Richmond HRR test was given to 100 consecutively presenting patients with abnormal colour vision and 50 patients with normal colour vision. Colour vision was diagnosed using the Ishihara test, the Farnsworth D15 test, the Medmont C‐100 test and the Type 1 Nagel anomaloscope. Results: The Richmond HRR test has a sensitivity of 1.00 and a specificity of 0.975 when the criterion for failing is two or more errors with the screening plates. Sensitivity and specificity become 0.98 and 1.0, respectively, when the fail criterion is three or more errors. Those with red‐green colour vision deficiency were correctly classified as protan or deutan on 86 per cent of occasions, with 11 per cent unclassified and three per cent incorrectly classified. All those graded as having a ‘mild’ defect by the Richmond HRR test passed the Farnsworth D15 test and had an anomaloscope range of 30 or less. Not all dichromats were classified as ‘strong’, which was one of the goals of the re‐engineering and those graded as ‘medium’ and ‘strong’ included dichromats and those who have a mild colour vision deficiency based on the results of the Farnsworth D15 test and the anomaloscope range. Conclusions: The test is as good as the Ishihara test for detection of the red‐green colour vision deficiencies but unlike the Ishihara, also has plates for the detection of the tritan defects. Its classification of protans and deutans is useful but the Medmont C‐100 test is better. Those graded as ‘mild’ by the Richmond HRR test can be regarded as having a mild colour vision defect but a ‘medium’ or ‘strong’ grading needs to be interpreted in conjunction with other tests such as the Farnsworth D15 and the anomaloscope. The Richmond HRR test could be the test of choice for clinicians who wish to use a single test for colour vision.     

Normality of colour vision in a compound heterozygous female carrying protan and deutan defects

Background: Inherited red‐green colour vision defects are quite common, affecting one in 12 males, but are less common in women, affecting about one in 250. Because red‐green defects are X‐linked, nearly 15 per cent of females are heterozygous carriers of red‐green colour deficiency. In addition, about one in 150 females are ‘double carriers’, where both of their X chromosomes have L/M gene arrays encoding a red‐green defect. If a woman carries the same type of colour vision defect on each X‐chromosome, she will be red‐green colour deficient, whereas if she carries opposing defects (protan versus deutan) on each X chromosome, she will have normal colour vision, owing to the process of X‐inactivation. These women are referred to as compound heterozygotes, though very few have been reported. Questions remain about whether the colour vision capacity of these women is comparable to that of ‘normal’ trichromats. Methods: We examined a compound heterozygote carrier of both protanopia and deuteranomaly. We also examined male members of her family representing both forms of red‐green defect carried by the female proband. Complete colour vision testing was done, including Rayleigh matches, pseudoisochromatic plates, unique hue measurements and 100‐Hue tests. Flicker‐photometric ERG estimates of L : M cone ratio were obtained, as were Medmont C100 settings. Results: Genetic analyses provided direct confirmation of compound heterozygosity. The compound heterozygote showed Schmidt's sign, consistent with an extreme skew in her L : M cone ratio and usually associated with protan carrier status. Conclusion: Apart from Schmidt's sign, we found the colour vision of the compound heterozygote to be indistinguishable from that of a normal trichromat.     

Color Vision Defects with Variation in the Exon 5 of Red and Green Pigment Genes

Purpose : To investigate correlation of variation in the exon 5 of red and green pigment genes with color vision defects.Methods : Exon 5 of the red and green pigment genes in 11 protans, 19 deutans and 38 normal controls were analyzed by heteroduplux-SSCP analysis.Results : In all 11 protans and 8 of the 19 deutans, defects of the red or green pigment gene could be identified. The C polymorphism (A/C at codon 283) in green pigment gene was present in 8 of 44 trichromats and 5 of 24 dichromats. Specific electrophoretic bands were found in 2 normal controls and a deutan.Conclusions: Variation in the exon 5 of the red and green pigment genes is the most common cause for color vision defects. Heteroduplex-SSCP analysis is a suitable way in screening specific variation in visual pigment genes. Eye Science 1998; 14 : 130 - 133.     

Abnormal colour vision is a handicap to playing cricket but not an insurmountable one

Background: Two studies have reported that abnormal colour vision is under‐represented among cricketers, presumably because cricketers with abnormal colour vision have difficulty seeing the red ball against the green grass of the cricket field and the green foliage around it. We have previously reported on the difficulties of five cricketers with abnormal colour vision but we have also reported that one of Australia’s finest cricketers was a protanope. This survey was undertaken to confirm the under‐representation of abnormal colour vision among cricketers and to ascertain whether those playing tend to be (1) those with a mild colour vision deficiency, (2) bowlers rather than batsman and (3) prefer to field close to the batsman rather than in the outfield. Methods: The colour vision of 293 members of seven Melbourne Premier cricket clubs was tested using the Ishihara test. Those who failed were examined further to confirm their abnormal colour vision, to assess its severity with the Farnsworth D15 test and to classify it as either protan or deutan using the Medmont C100 test. A questionnaire about cricketing ability and problems playing cricket was administered. Results: Twenty‐six (8.9 per cent) of the cricketers had abnormal colour vision, of whom six played in the First Grade (6.7 per cent of First Grade players). The proportion of cricketers with a severe deficiency was significantly less than expected for the First Grade players. There were only two protans. Bowlers were not over‐represented among the colour vision defective cricketers but those preferring to field close to the batsman were significantly over‐represented. Conclusion: Abnormal colour vision is a modest handicap to playing cricket, especially at the higher levels of the game. It may impede batting and the ability to field in the outfield.     

Protan colour vision deficiency and road accidents

Background : Protans are precluded from holding a commercial driver's licence in Australia because they have a substantially reduced ability to see red lights and have more road accidents involving signal lights. This exclusion has been in place since 1994 but is likely to be abandoned following a current review of medical standards for commercial drivers. This paper reviews the level of risk of road accidents due to protan colour vision deficiency. It also addresses the question of whether it is fair to regard all protans as having a higher risk of road accident because some protans might have a sensitivity to red light that is as good as that of some people with normal colour vision. Methods : Data of two studies by Verriest and co‐workers are re‐analysed to estimate the degree of overlap of the protan and colour normal distributions of sensitivity to red light. Results : Field trial data show that protans have a very reduced visual range for red signals compared to colour normal observers but there is considerable variability among both classes of observers and the distributions do overlap. However, some variability is due to differences in observers' choices of a detection criterion, their speed of response and the measurement method. A laboratory study of the spectral sensitivity of protan and colour normal subjects that largely removes these sources' variability shows that all protans have a sensitivity to red light that is less than that of the least sensitive colour normal. Conclusion : It is reasonable to conclude that all protans, regardless of the severity of their defect, have a lesser ability to see red signals than colour vision normal observers and for that reason will have a higher risk of road accident.     

Molecular basis of congenital color vision defects in Chinese patients.

Applying Southern blot hybridization, the structures of the red pigment gene (RPG) and the green pigment gene (GPG) were analyzed in 43 Chinese patients with red-green color vision defects, including 3 female cases of deuteranopia. The same analysis was carried out in 4 normal relatives and 3 carriers from 3 affected families, as well as in 11 normal controls. Among the 43 patients, abnormalities of the RPG were detected in all 19 protans, and abnormalities of the GPG were found in 14 of the 24 deutans. In about 80% of the protans and deutans, an alteration of exon 5 in RPG or GPG was discovered. All 19 protans had anomalous RPG and in one protan the normal RPG was replaced by a 5' red-3' green hybrid gene. However, no protans showed deletion of the whole RPG. Some deutans had no GPG; some had a 5' green-3' red hybrid gene with or without the GPG. The exon 5 of RPG and GPG was amplified by polymerase chain reaction (PCR) and the amplified fragments were further analyzed by RsaI digestion. The results of PCR were identical to those of nucleic acid hybridization. PCR will be a useful tool in prenatal diagnosis and genetic counseling.     

Visibility of protective helmets worn by forestry workers

Background : High visibility helmets must be worn by Forestry workers in New Zealand for protection and as conspicuous ‘clothing’ to alert workers to the presence and location of other workers. The colours yellow-green (fluorescent yellow-green) and ‘water melon’ (fluorescent pink) are used and both appear to be conspicuous. To solve controversy, we investigated which helmet colour is more visible for use in a forest setting for workers having normal or defective colour vision. Method : We obtained threshold angular sizes for two-millimetre square samples met material presented against a textured background containing colours representative of those found in the foliage and bark of the most common forest type (Pinus Radiata). Observers with normal colour vision (n = 22) and with deutan (n = 8 and protan (n = 6) defects participated. Subjects with mild colour vision defects were excluded. Results : The yellow-green colour was significantly more visible than the pink for the normal (p < 0.001) and protan (p < 0.05) observers. For the deutan observers the pink helmet colour was significantly more visible (p < 0.01). The median equivalent out-door detection distances were for normal observers 400 m (pink) and 500 m yellow-green); for protan observers 185 m (pink) and 500 m (yellow-green); and for deutan observers 550 m (pink) and 450 m (yellow-green). Conclusions : The yellow-green helmet can be detected at large distances by all observers. The yellow-green helmet has greater reflectance and therefore greater luminance contrast. The pink helmet colour can be confused with green forest background colours by observers with protan defects. For some observers with a protan colour vision defect, detection distances for the pink helmet colour are less than half of normal detection distances.     

New investigations concerning the relationships between congenital colour vision defects and road traffic security

New extentive experiments demonstrated that: (a) protan observers are more deficient than deutan ones with regard to perception distances of some traffic panels, of vehicle red stop lights, of vehicle red rear-position lights and of white, yellow and red reflectors. Contrarily, deutan observers are more deficient than protan ones for the distinction of differently coloured traffic lights and vehicle rear lights; (b) protan and deutan drivers are nevertheless not responsible for more traffic accidents than drivers with normal colour vision; (c) this apparent contradiction is due to psychological compensation mechanisms. The practical conclusions are: (a) that persons with defective colour vision need not to be excluded from non professional road traffic; (b) that it is nevertheless useful that they should be aware of their handicap; (c) that the red traffic signal has to be larger than the other ones; and (d) that the stop and red position lights of vehicles must be sufficiently intense and that the filters transmitting only pure red should be avoided in them.     

Spectral characteristics of early receptor potential in congenital red-green color blindness

The spectral response curve (amplitude versus wavelength) of the R₂ of the early receptor potential (ERP) was studied in normal, protan, and deutan subjects. The R₂ amplitude peaked at 520nm in most normal subjects. The R₂ at long wavelengths was smaller than normal in protans and larger than normal in deutans when the maximum amplitudes were normalized to 100% at the peak. The ratio of the R₂ amplitude at 460 nm to that at 600 nm clearly differed between protans and deutans. The ERP and the rapid off-response, which is mainly due to the cessation of the late receptor potential, were recorded in the same subjects. The ratio of the sensitivity of the rapid off-response at 500 nm to that at 600 nm was correlated with the ratio of the R₂ amplitude at 460 nm to that at 600nm (correlation coefficient, 0.823, p < 0.001). This study, in conjunction with our previous study, indicates that the abnormality is in the outer segments of the cones in protans and deutans.     

Efficiency of the Ishihara test for identifying red-green colour deficiency

The Ishihara test is the most widely used screening test for red-green colour deficiency. Results obtained by 401 people with red-green colour deficiency show that the combined sensitivity of the Transformation and Vanishing plates of the 38 plate Edition of the Ishihara plates is 95.5% on eight errors, 97.5% on six errors and 99.0% on three errors. The Hidden digit designs only identified approximately 50% of colour-deficient subjects. The protan/deutan classification plates were found to be more effective for deutans than for protans. No classification was obtained for 18% of protanopes and 3% of deuteranopes who saw neither figure on classification plates; 40% of protanomalous trichromats and 37.5% of deuteranomalous trichromats saw both classification figures and were classified on the relative luminance (clarity) of these figures. The specificity of the Ishihara test was determined in a previous study (Birch and McKeever, 1993) and the results combined with the present data to obtain the overall efficiency of the Ishihara plates for a representative cross section of colour-deficient subjects.     

Cone interaction and color substitution as revealed by pattern ERG

Transient electroretinograms to a reversing color-contrast checkerboard pattern (P-ERG) were recorded in a protanomalous, a deuteranomalous, and a normal observer. Alternate monochromatic checks were of constant wavelength (630 rim. red - 531 nm green), while the relative energies were varied systematically. When changing the radiance ratio 630 nm-531 nm of the stimulus, the normal subject exhibited a P-ERG to all stimuli with only a relative amplitude minimum at a distinct radiance ratio, whereas the color-deficient observers failed to show a P-ERG at some color contrast 630 nm-531 nm, the radiance ratio of which was different in the protan and deutan. From the radiance ratio of color contrast for the smallest potential in the normal observer, we conclude that the green- and red-sensitive cone mechanism provides a difference signal which generates the response. The data from the color-deficient observer support the view that color discrimination in protans and deutans is reduced because the input of one type of photoreceptor is missing.     

Spectral characteristics of electroretinography in congenital red-green color blindness

There are few conclusive electroretinography (ERG) studies comparing the spectral characteristics in deutans and normals in contrast to protans and normals. The difficulties of research on deutans were thought to be due to problems in detecting the very slight differences in the spectral characteristics between deutans and normal subjects. To record monochromatic ERG responses accurately in deutans, our time-locked scanning method was improved as follows: We used 12 interference filters for stimulus lights with narrow half widths (4-6 nm) and wavelengths of peak transmission arranged at intervals of 10 nm between 520 nm and 600 nm. Each stimulus light was strictly adjusted to an equal energy and checked simultaneously with ERG recordings. Contact lens electrodes were reformed for comfortable fitting to subjects' corneas. The time interval between each stimulation was set at 300 msec and one scanning of all stimulations took only 3.9 sec. ERG bp-waves were recorded in congenital color blindness by scanning monochromatic light stimuli, and spectral responses obtained could be evaluated as a spectral pattern. Different spectral patterns of responses from those of normal subjects and shift of the peak in the spectral response curves were obtained for congenital color blind subjects. The maximal responses were recorded at around 540 nm in protans and at 570-580 nm in deutans under white adaptation. Differences in the response curves were not found between dichromats and anomalous trichromats. Moreover, selective chromatic adaptation disclosed the separate responses of green cone and red cone systems. In normal subjects the peak of the spectral response curves was shifted to around 540 nm by red adaptation and to around 580 nm by blue adaptation. The spectral patterns changed so that they looked like the patterns under white adaptation of protans and deutans, respectively. But in protans and deutans the same spectral response patterns and almost the same wavelengths of the peak in the spectral response curves as those obtained under white adaptation were recorded under chromatic adaptation. This method provides the possibility of differentiating between red and green color blind subjects and normal subjects by the ERG. Defects or marked abnormality in the red cone system in protans and the green cone system in deutans can also be detected. Monochromatic ERGs of deutans were recorded under more intense red adaptation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Anomaloscope examination: scotopization (the luminance fall).

PURPOSE: The evaluation of a criterion for the detection of pathologic scotopization in routine anomaloscope examination. METHODS: Fifty congenital protan subjects, 50 congenital deutan subjects, 30 autosomal recessive congenital achromats, and 25 (44 eyes) acquired type I red-green defective subjects were selected. The anomaloscope examination was according to the Linksz procedure. The luminance fall was calculated as the slope quotient SQ: Y units luminance fall per X units width of the matching range. RESULTS: The mean SQ was -0.01 for congenital deutan subjects, -0.40 for congenital protan subjects and -1.30 for congenital achromats. There was no overlap between the three groups. Pathologic scotopization was found in 98% of the eyes presenting with an acquired type I colour vision defect. CONCLUSION: Calculation of the slope quotient SQ is helpful for the detection of pathologic scotopization in acquired colour vision deficiency.     

Colour vision defects in asymptomatic carriers of the Leber's hereditary optic neuropathy (LHON) mtDNA 11778 mutation from a large Brazilian LHON pedigree: a case-control study

AIMS: To determine if asymptomatic carriers from a previously identified large pedigree of the Leber's hereditary optic neuropathy (LHON) 11778 mtDNA mutation have colour vision deficits. METHODS: As part of a comprehensive analysis of over 200 members of a large Brazilian LHON pedigree spanning seven generations, colour vision tests were obtained from 91 members. Colour vision was tested one eye at a time using the Farnsworth-Munsell 100 (FM-100) hue colour vision test. The test was administered under uniform conditions, taking into account: ambient light levels, daylight colour temperature of 6700 kelvin, and neutral uniform background. Tests were scored using the FM-100 MS-Excel computer scoring program. Defects were determined and categorised as tritan, deutan, or protan. Categorisation of each dyschromatopsia was based on review of demonstrated axis computer generated plots and age adjusted error scores which coincided with Verriest 95% confidence intervals. Only the axis with the greatest magnitude error score was used to classify the defect. 55 of the 91 test subjects were LHON mtDNA 11778 J haplotype mutation carriers, proved by mtDNA analysis. The remaining 36 subjects were age matched non-blood relatives (off pedigree), who served as controls. RESULTS: 27 of 55 carriers (49.10%) were shown to have colour vision defects in one or both eyes. 13 of the 27 (48%) abnormal tests in the carrier group were tritan defects and the remaining 14 (52%) were deutan defects. Nine of the 27 (33%) abnormals in the carrier group were identified as having bilateral defects. Six of these were deutan, and the remaining three were tritan dyschromatopsias. Only six of the 36 (16.66%) age matched controls were found to have any type of dyschromatopsia. Five (83.3%) of these were deutan defects. The remaining one was a tritan defect. The difference between the two groups using a chi(2) test with one degree of freedom was statistically significant with a p value less that 0.001. CONCLUSIONS: Until now, LHON has always been characterised by a sudden, devastating vision loss. Asymptomatic carriers, those without vision loss, were considered unaffected by the disease. It now appears that asymptomatic carriers of the LHON mutation are affected by colour vision defects and may manifest other subtle, yet chronic, changes.     

Electrodiagnosis of red-green colour deficiency

The electroretinographic rapid off-response and the early receptor potential were studied in congenital red-green colour deficiency. The sensitivity of the rapid off-response was low in protans and high in deutans at long wavelengths, and high in protans and low in deutans at short or medium wavelengths, as compared with normal subjects. The ratio of the sensitivity at 480 nm to the sensitivity at 620 nm (S480/S620) was higher in all cases of protans and lower in all cases of deutans so far tested than in normal subjects. The amplitude of the rapid off-response to white stimulus light did not differ among normals, protans and deutans. The S480/S620 was abnormal in some protan-carriers, deutan-carriers and Farbenamblyopie. The mean amplitude of the early receptor potential (R2) was small in protans at long wavelengths, and small in deutans at short or medium wavelengths. A significant correlation was found between the amplitude ratio of the R2 and the sensitivity ratio of the rapid off-response at short and long wavelengths.     

Clinical use of the City University Test (2nd Edition)

The City University test (TCU test) aims to identify people with significant colour deficiency and to classify the type of defect. 222 people with congenital red-green colour deficiency, diagnosed with the Nagel anomaloscope, were examined with the TCU lest (2nd Edition), All deuteranopes and 44% of deuteranomalous trichromats failed the TCU test. Deutans who failed could be subdivided into two categories of severity depending on whether errors were made on five or more plates. 96% of protanopes and 26% of protanomalous trichromats failed. Protans made fewer errors than deutans and subcategories of severity could not be distinguished according to the number of errors made. The Farnsworth D15 test was found to be more effective than the TCU test in identifying significant protan colour deficiency. Detection and classification rates varied on all the plates of the TCU test. Mixed protan and deutan classification errors were made by 61% of subjects with the majority result correct in 80%. The most efficient plates are identified and recommendations are made for the optimum use of the TCU test in clinical practice.     

Impairment of colour contrast sensitivity and neuroretinal dysfunction in patients with symptomatic HIV infection or AIDS.

Ophthalmic and neurological complications are frequent findings in patients with AIDS. Little is known about neuroretinal dysfunction in patients with HIV infection. The purpose of this study was to measure and evaluate colour vision in patients with HIV infection or AIDS. Colour contrast sensitivity tests were performed on 75 patients (150 eyes) in different stages of HIV infection. A highly sensitive computer graphics system was used to measure tritan, deutan, and protan colour contrast thresholds. Patients were classified into three clinical groups: (a) asymptomatic HIV infection, (b) lymphadenopathy syndrome or AIDS-related complex, and (c) AIDS. Overall, tritan (p < 0.0001), deutan (p = 0.003), and protan (p = 0.009) colour contrast sensitivities were significantly impaired in patients with HIV infection compared with normal controls. Colour thresholds in patients with asymptomatic HIV infection (mean tritan threshold: 4.33; deutan: 4.41; protan: 3.97) were not impaired compared with normal controls. Colour vision was slightly impaired in patients with lymphadenopathy syndrome or AIDS-related complex (tritan: 6.25 (p < 0.0001); deutan: 4.99 (p = 0.02); protan: 4.45 (p = 0.05)). In patients with AIDS the impairment was even more marked (tritan: 7.66 (p < 0.0001); deutan: 5.15 (p < 0.0009); protan: 4.63 (p = 0.004)). Analysis of covariance controlling for age demonstrated a close association between impairment of tritan colour contrast sensitivity and progression of HIV disease (p < 0.0001). Following Köllner''s rule, our study suggests that neuroretinal dysfunction occurs in patients with symptomatic HIV infection or AIDS. This is emphasised by the finding that the relative impairment in tritan vision compared with deutan/protan vision might reflect the difference in the number of cones or receptive fields. Measurement of tritan colour contrast sensitivity appears to be an appropriate and easily applicable method to detect early neuroretinal dysfunction in patients with HIV disease.     

Orienteers with poor colour vision require more than cunning running

Background: Highly detailed colour coded maps are used in the sport of orienteering to enable competitors to navigate from one check point to another and to provide guidance on the nature of the terrain to be traversed. The colours are defined by the International Orienteering Foundation (IOF) and are said to have been chosen so they will not be confused by competitors who have abnormal colour vision. However, there are anecdotal reports that individuals with colour vision defects do have problems with the colour coding. Method: A Minolta Spectrophotometer CM‐503i was used to measure the CIE x,y chromaticity co‐ordinates and the reflectances of the standard colours recommended by the IOF for the colour coding of orienteering maps, as well as the colours on two maps used in orienteering events. Results: Four pairs of IOF standard colours are likely to be confused by protan observers and four pairs by deutan observers. There were three pairs of colours likely to be confused by both deutan and protan observers on one of the competition maps and one pair likely to be confused by protan observers on the other map. Some of the colours on the actual competition maps differed noticeably from the standard IOF colours. Discussion: Orienteers with more severe forms of abnormal colour vision are likely to be disadvantaged by their inability to differentiate some colours used on orienteering maps. The IOF should choose different colours that are less likely to be confused or should employ a redundant code (such as a pattern or texture). There is need for better quality control of the colours of competition maps to ensure they do conform to the IOF standard colours.