Masking Colour Blindness: A Case Report

ABSTRACT

Colour vision is an important aspect of visual function that might help individuals in doing daily activities. Some occupations require and test for good colour discrimination. We describe a case of a 20-year-old man who was referred to our centre to establish if he had colour vision deficiency (CVD). He had been tested for this twice as part of his assessment to enter the police force. At the first examination, he had normal colour vision, while the second examination revealed CVD, thus the patient was referred for confirmation. Colour vision tests using the Ishihara plates showed normal results with each eye, while a Roth test revealed an unspecified CVD in the right eye and deuteranopia in the left eye. During the evaluation, we noticed he was using a red-tinted contact lens in the right eye, and was wearing a red mask with transparent red plastic in the upper part. After removal of the contact lens and mask, he was asked to repeat the examinations and it revealed deuteranopia in both eyes. A tinted contact lens is a corrective device that can help to enhance colour discrimination in CVD subjects. However, in this case the tinted contact lens was used inappropriately to manipulate the colour vision examination. We highlight the case to raise awareness that the use of red contact lenses and red filters can mask CVD.

Introduction

Colour vision deficiency (CVD) is estimated to affect 1.6%−13.99% of the population worldwide, which varies among countries. It is inherited in an X-linked recessive pattern and affects 8% of men and 0.5% of women.¹ The most common type is red-green deficiency, which accounts for 95% of CVD, with different types and severity among cases.²

Subjects with CVD have difficulty in colour discrimination, which can affect daily tasks including watching television, cooking, or driving. Therefore, colour enhancement devices, such as tinted glasses or contact lenses have been developed in order to help those suffering from CVD to improve their quality of life.²

In some safety critical occupations, such as in transport, aviation, and the police normal colour vision is required and screened for on enrolment.³ We report the case of a 20-year-old who masked CVD using a tinted contact lens in order to enter the police force.

Case report

A 20-year-old man was referred to Cicendo Eye Hospital by the police force for colour vision testing. He was a police cadet, who had previously been examined for CVD and had a normal result. However, he was retested during a naked physical examination and was found to have CVD. There was no previous history of ocular disease, hospitalisation, or trauma.

His uncorrected visual acuity was 20/20 in each eye. He had normal intraocular pressures. He was tested for colour vision using the Ishihara 38 Plates book and the Roth 28 Hue Test. He scored 21/21 on the Ishihara test with each eye. The Roth test revealed an unspecified CVD in the right eye and deuteranopia in the left eye (Figure 1a). Anterior segment examination using slit-lamp biomicroscopy revealed that he was wearing a red-tinted contact lens in the right eye, which was not seen clearly without any additional light towards the eye (Figure 2). The rest of the anterior segment and fundus examination was within normal limit. He was asked to remove the contact lens, and the colour vision tests were repeated. On the second examination, he seemed to refix his mask position many times, and we realised that he wore a special mask with a transparent red filter in the upper part that was covered by a medical mask. With the mask now removed he could only read the first number in the 22^nd, 23^rd and 24^th plates and one correct plate with the right eye, and the first number in 22^nd, 23^rd and 25^th plates and three correct plates with the left eye, leading to the diagnosis of deuteranopia. The repeated Roth test also revealed deuteranopia (Figure 1b).

Figure 1.

(a) Roth 28 Hue test result using the red-tinted contact lens in the right eye showing an unspecified colour vision defect, and in the left eye without a contract lens showing deuteranopia. (b) Repeat test without the tinted contact lens showed deuteranopia in each eye. L = left eye; R = right eye.

Figure 2.

(a) Red-tinted contact lens. (b) Red-tinted contact lens in the right eye. (c) Mask with transparent red filter in the upper part. (d) Red mask with the transparent red filter, which was covered by a medical mask.

Upon further questioning, he revealed that he had obtained the tinted contact lens and red mask filter over the internet. He was unable to continue as a police cadet.

Discussion

Colour vision is important for some occupations that need good colour discrimination. Identification of colours requires normal function of photoreceptors containing visual pigment responsible for short (blue), medium (green), and long (red) wavelengths, which are the S-cones, M-cones, and L-cones, respectively. Normal colour vision is known as trichromacy.⁴ However, if one of this photoreceptors is absent or defective, dysfunction in colour perception will be present. Anomaly of a photoreceptor is known as anomalous trichromacy, absence of one of the photoreceptor cones is called dichromacy, and absence of two of the photoreceptor cones is called monochromacy. Anomalous trichromacy can cause tritanomaly, deuteranomaly, or protanomaly. Dichromacy can cause tritanopia, deuteranopia, or protanopia. Monochromacy, which is caused by the absence of red and green cones, is called blue cone monochromacy. The absence of all cones is called achromatopsia or total colour vision loss.^1,2,5 Red-green colour deficiency is the most prevalent form of CVD. Red-green CVD is caused by the absence of M-cones or L-cones, causing deuteranopia and protanopia, respectively.⁶ In this condition, there are overlapping of green and red wavelength bands received by cone photoreceptors, causing abnormality of deutan and protan colour perception.

CVD is diagnosed through colour vision testing, including Ishihara colour plates, Farnsworth-Munsell and Roth tests. The Ishihara plates are pseudochromatic plates consisting of dots forming shape of numbers or figures against a specific background. The colour of the figures and backgrounds have hues that are difficult to distinguish by those with CVD, thus they will find no pattern or an alternative pattern.⁶ However, the Ishihara plates are only sensitive to identify those with red-green deficiency. The Farnsworth-Munsell and Roth tests are more accurate at identifying the type of CVD.. The principle is to discriminate colour tablets based on the most adjacent hues, thus the patient is asked to arrange the tablets with the most similar hues next to each other.

Currently, there is no cure for CVD. However, some methods have been developed to aid in enhancing colour discrimination, including the use of coloured filter visual aid devices. Some of available modalities are tinted spectacles and tinted contact lenses. These devices have filters that will absorb specific problematic wavelengths (520–580 nm), thus the red-green band will not be stimulated and colour perception will be improved.² Previous studies have shown that these types of filter in spectacles can improve colour contrast and perception of those affected with CVD. However, the spectacles are bulky and impractical for use in daily activities, thus tinted lens have been developed to overcome the limitations of spectacles. Studies have shown that the tinted contact lenses significantly reduce Ishihara error rate, especially for subjects with deuteranopia , as in our patient.⁷ However, the Roth test using the tinted lens in the right eye in out patient showed an unspecified CVD, probably because the filter disrupted the hue of the tablets seen by the patient, while the left eye without any filter showed deuteranopia. Also, the tinted lens can decrease distance and near visual acuity, as well as static and dynamic contrast sensitivity at some special frequencies.^5,8 This might explain why our patient did not use the tinted contact lens on the left eye in order to preserve vision and perform visual acuity tests correctly. As an alternative, the patient wore a mask with a red transparent filter as an aid for the left eye, even though the material of the filter and its efficacy to enhance colour vision is not known.

Conclusion

CVD is one of the visual abnormalities that might affect daily tasks, especially in some specific fields of occupation. Despite the benefit of tinted contact lenses to help subjects with CVD, this case indicates that they may be used inappropriately to manipulate colour vision testing. Therefore, ophthalmologists and other health care professionals who perform colour vision testing need to pay careful attention to ensure that the examination is carried out without any artificial aids.

Funding Statement

The authors reported there is no funding associated with the work featured in this article.

Disclosure statement

No potential conflict of interest was reported by the authors.