Efficacy of red contact lens in improving color vision test performance based on Ishihara, Farnsworth D15, and Martin Lantern Test

Abstract

Background

The aim of this study was to evaluate the efficacy of red contact lens in improving color vision test performance based on Ishihara, Farnsworth D15, and Martin Lantern Test (MLT).

Methods

Thirty subjects with a known color vision defect were enrolled after consent. The color vision was assessed using Ishihara test, D15 test, and MLT. The error scores were recorded. The test was repeated after using a red contact lens.

Results

The mean error scores were 7.87 (±1.00), 3.33 (±3.15), and 5.67 (±2.52) on Ishihara, MLT, and D15 test, respectively. The error scores reduced to 3.93 (±3.78), 1.33 (±1.34), and 2.77 (±2.12) Ishihara, MLT, and D15 test, respectively (P value < 0.05, Wilcoxon rank test). None of the subjects were in color perception (CP) II at presentation but 47.5% became CP II with the lens on Ishihara test. Ninety percent could pass the MLT CP III standard.

Conclusion

This study shows that red lens reduced error scores and subjects could pass the color vision tests. Notwithstanding the improvement in performance on color vision tests, lens is not recommended for personnel on active combat duty. The recruiting medical officer needs to be aware of the fact that a subject with defective color vision can pass various tests with a red contact lens.

Introduction

Color vision is an important aspect of visual acuity for day-to-day activities. The importance of normal color vision in certain professions such as aviation and military cannot be overemphasized. The highly sophisticated color-coded working spaces of aircrafts and ships make normal color vision a mandatory requirement for entry into these professions. In addition, color codes are used in signaling of traffic lights and other industrial procedures.

The normal color vision is called trichromatic where three cone systems are responsible for red, green, and blue colors.¹ The prevalence of congenital color blindness varies from place, ethnicity and sex. Various studies² have found the incidence of 8% in males and 0.5% in females.

Color vision defect is a continuous spectrum, and no two color vision defects are alike. The degree of color vision deficiency may vary from mild deficiency to very severe where the patient may not recognize any colors.³ Various methods of color vision assessment are in use.⁴

Color vision assessment is an important aspect of the armed forces medical examination.⁵ The peculiar job requirements of the correct interpretation of signals in aviation and sailing make this assessment more important.

Therapeutic modalities for improvement of substandard color perception are very few. Schmidt in 1976 described the use of red filters for treatment of color-deficient subjects.⁶ Zelter in his monograph on X-chrome lenses described the mechanism of action of these lenses. He compared correction of deficient color vision by red contact lens as correction of myopia by a concave lens.⁷ These contact lenses have been marketed through general advertisements. Various lens systems such as X-Chrome lens or ChromaGen are available. The tint also varies from model to model.⁸ Various studies have tried the utility of colored contact lens on the non-dominant eye and have claimed improvement in color perception. Most of the claims in the improvement of color vision have been made by product manufacturers.

There is a paucity of data in peer-reviewed literature regarding the actual benefit of using a red contact lens in subjects with defective color perception. The testing equipment in this study is the same equipment as used in the screening examination for entry into the armed forces. The possibility of passing the test using the red filter by highly motivated recruits and candidates was also studied.

Materials and methods

This prospective interventional study was performed between December 14 and July 15 on 30 candidates presenting to the eye department of a tertiary care hospital with a diagnosis of defective color perception based on Ishihara Chart. They underwent comprehensive ophthalmological evaluation as per laid down criteria after informed written consent. Subjects with defective color vision more than 16 years of age willing to be part of the study were included. Patients with best corrected visual acuity less than 6/6 with any evidence of macular pathology or optic nerve disease were excluded.

Color vision test equipment used were Ishihara Test Booklet 38 plate Chart, 1990 Ed, Carney Howrah and Co, Tokyo Japan, Martin Lantern Test (MLT) Make Kelvin, Bottomley and Baird Ltd, Glasgow, and Farnsworth D15 manufactured by Richmond Inc, USA, with one reference and 15 colored discs encased in an acrylic transparent case.

Red contact lenses made of poly-2-hydroxyethyl methacrylate (HEMA) was obtained from Classic Contact Lens Pvt Ltd (India). The water content was 67%. Plano contact lens with an overall diameter of 14.0 mm with a central 6-mm red-tinted area covering the pupil was used (Fig. 1). Comprehensive eye examination was carried out as per the protocol. Participants underwent optical coherence tomography (OCT) (Zeiss Cirrus HD-OCT) examination of the macula after dilatation at the end of examination sequence. A fast macular scan was performed for central macular thickness (CMT).

Fig. 1.

Red contact lens.

After obtaining informed written consent, subjects were assessed for color vision using Ishihara booklet, and Martin Lantern Test as per IAP 4303 Fourth Edition. As per the existing guidelines followed by the armed forces, the color perception (CP) is divided into three categories based on the Ishihara booklet. A subject who reads all the plates is classified as CP II. A subject is classified as CP III if he can read plate no 1 and plates 22–25 correctly. Subjects with CP IV can read only plate no 1. Similarly on testing with MLT, a subject who can see small targets at 6 m is classified as CP I (MLT). Subjects who are not able to identify small colored targets at 6 m are further tested at 1.5 m with larger colored targets. If they correctly identify larger targets, they are classified as CP III (MLT). Farnsworth D 15 test was performed as per the instruction manual supplied by manufacturers.

The test was administered at a distance of 75 cm in good ambient light in a room using tube lights. Subjects were given 2–3 s to respond.

A total of 25 plates were used for assessment. The plates were divided into 10 groups. The error scores were noted out of 10. For example, a patient who reads all plates correctly has error score of zero, whereas a person with defective color vision will not read any of the plates except plate no 1 and hence will have a score of nine. The assessment was repeated with the use of contact lens. The scoring system used is shown in Table 1.

Table 1.

Scoring system based on the Ishihara test.

Group	Plate no.
1	1
2	2, 3, 4, 5
3	6, 7, 8, 9
4	10, 11
5	12, 13
6	14, 15
7	16, 17
8	18, 19, 20, 21
9	22, 23
10	24, 25

MLT was performed using large apertures of red, green, or white shown to subjects in a 1.5-m dark room. The subjects were asked to name the color. If they responded correctly at the first go, the examiner need not instruct them further. In case of incorrect response, they were told once again about the light shown. The response was recorded. A total of 10 targets were shown. The maximum error score of 10 was given to subjects who gave all wrong responses, and zero to subjects who gave all correct responses.

Farnsworth D 15 test was used as per the guidelines given by the manufacturer. The total number of crossings was recorded, and the chart was prepared. A maximum error score of 12 was given based on the number of crossings to reference axis.

All the tests were re-administered to the subjects after inserting the contact lens in the non-dominant eye. The dominant eye was decided using Miles test. The error scores were recorded.

The participants were asked to mark the subjective improvement in the color perception and subjective response to lens comfort in a scale of 1–10 after the use of lenses for 1 h.

The data were recorded in the MS Excel sheet 2013. The analysis was performed using SPSS software, version 18. Descriptive statistics were used for demographic data. Kolmogorov–Smirnov test was used to check for normality. Wilcoxon signed-rank test was used for analysis.

Results

This study enrolled a total of 30 subjects with congenital dyschromatopsia. The sample size was 29 with a prevalence of 1.9% and precision of 5%. All the subjects were males. The average age was 18 years with a range of 16–23 years.

The mean best corrected visual acuity (BCVA) was log MAR 0 (Snellen's = 20/20) at baseline and was 0.023 log MAR (Snellen's = 20/21) with the use of red contact lens. The change in BCVA was not statistically significant under standard testing conditions (p = 0.102, Wilcoxon signed-rank test).

The mean CMT was 254.6 micron with a standard deviation (SD) of 5.917. No structural abnormality of macula was noted.

In all the 30 subjects, the right eye was dominant in 77% of the subjects and the left eye in 23% of the subjects. Of the total 30 subjects with dyschromatopsia, 18 patients had deutranomaly and seven patients had protanomalous color vision defect. Five subjects could not be labeled in to any specific type of color vision defect as tested on Ishihara test (unspecified).

Based on the classification used by Indian Armed Forces for the purpose of entry medical examination of recruits or candidates, of the total 30 patients, 17 were in CP III and 13 were in CP IV. None of the subjects were CP II on Ishihara chart testing (Fig. 2).

Fig. 2.

Ishihara category at baseline (pre-lens) and after using the lens (post-lens).

On Ishihara test at baseline, the mean error rates were 7.87 with a SD of 1.008 The mean error rates after the use of lens were 3.93 with a SD of 3.787. The difference of error rates was clinically and statistically significant (p < 0.001, Wilcoxon signed-rank test). After the use of red contact lens, 14 (46.7%) of the total subjects showed a change in the CP category to CP II. Twenty-seven (90%) of 30 of the subjects could read passing plates for CP III. The remaining 3 (10%) subjects showed no change in the CP category. The change in CP category is shown in Fig. 3 (P < 0.001, Wilcoxon signed-rank Test).

Fig. 3.

Ishihara error score at baseline (pre-lens) and after using the lens (post-lens).

Results based on the MLT showed that with the use of lens, 20% more improvement was seen in CP III. As it is evident that only 16.7% (5/30) of the subjects of 36.7% (11/30) of subjects remained in CP IV after the use of red contact lens. Further analysis of data was performed based on the type of dyschromatopsia, and it was found that 60% of the subjects with unspecified color defect in CP IV showed improvement and moved to CP III. Similar beneficial effect was seen in other groups also (Fig. 4).

Fig. 4.

Pre- and post-lens MLT category. MLT, Martin Lantern Test.

On MLT, at baseline, the mean error rate was 3.33 with a SD of 3.15. The mean error rates after the use of lens were 1.33 with a SD of 1.34 (Fig. 5) (p < 0.001, Wilcoxon signed-rank test).

Fig. 5.

Error scores on MLT at baseline (pre-lens) and after using the lens (post-lens). MLT, Martin Lantern Test.

On D15 test, at baseline, the mean error rates were 5.67 with a SD of 2.52 and 95% confidence interval (CI) of 4.72–6.61 with a range of 2–12 errors. The mean error rates after the use of lens were 2.77 with a SD of 2.12 and 95% CI of 1.97–3.56 with a range of 1–10 errors (p < 0.001 Wilcoxon signed-rank test) (Fig. 6, Fig. 7).

Fig. 6.

Error scores on D15 at baseline (pre-lens) and after using the lens (post-lens).

Fig. 7.

Error scores on D15 test at baseline (pre-lens) and after using the lens (post-lens) based on the type of dyschromatopsia.

The mean value of subjective improvement on a scale of 10 was 7.03 with a SD of 1.245.

The difference of subjective improvement was clinically and statistically not significant among the groups of patients with dyschromatopsia (p = 0.867 Wilcoxon signed-rank test).

Mean value of lens comfort while wearing the contact lens on a scale of 10 was 6.63 with a SD of 1.29 (p = 0.06, Wilcoxon signed-rank test).

Discussion

The methods for enhancing color vision in subjects with dyschromatopsia have always fascinated ophthalmologists and optometrists. The error scores noted at baseline and after using a red contact lens in the non-dominant eye were significantly reduced. This is in agreement with the findings of Swarbrick et al.⁸ who also reported similar findings in 13 subjects.

Previous studies on subjects with dyschromatopsia have found macular changes in the form of microstructural variations.⁹ No microstructural abnormalities of macula were noted in this study. This difference could be because of the factor that we enrolled subjects with congenital dyschromatopsia with normal retinal findings. Studies on subjects with acquired dyschromatopsia with macular structural changes as compared with normal subjects were recorded.¹⁰

The most common type of color vision defect found in the study was deuteranomaly (60%). It is in agreement with the previous studies² done on the subject. Some (16.7%) subjects were reported as not fitting into any specific type of dyschromatopsia.

In the study performed by a technical group in the United States using the red contact lens in military personnel, the effect on visual acuity, lateral and vertical phoria, and stereopsis were evaluated.¹¹ The number of plates correctly read improved significantly after using red contact lens. Theoretically, the use of red lens can cause Pulfrich phenomenon and needs further investigation. This inability to identify colored targets when shown rapidly could have implications in combat situations where the dynamic environment changes rapidly. In our study also the subjects could pass the test with large targets on the MLT. The effect of red lens on small targets for CP I was not evaluated as none of the subjects were in CP II on Ishihara Chart initially.

The Ishihara test was used for initial screening, because the same test is used in the armed forces for screening medical examination. This is the first study to consider the effect of red contact lens on categories of color vision⁵ on Ishihara chart as used by Indian Armed Forces.

The Ishihara chart is based on the principle of confusion, in which numbers are embedded in the background.³ Our results are in agreement with those of Zuleyha et al. who enrolled 30 subjects in the comparative study of efficacy of red filter for color vision deficiency and found that all 30 subjects could read Ishihara plates without any errors.¹² The red contact lens may help in improving the performance on various testing modalities by allowing preferential transmission of a particular wavelength of light which could alter the cues used for identification of various colors.

MLT is the confirmatory test used in the Armed Forces for entry into aviation and navy.¹³ At presentation, 19 (63%) of the subjects were in CP III but with the use of this lens, the percentage improved to 83% (25 subjects). As it is evident that only 16.7% (5/30) remained in CP IV out of 36.7% (11/30) after the use of red contact lens.

The reduction of error scores in this study may be different from actual combat situations where other variables of reduced illumination, fog and visibility, play an important role.¹⁴

Apart from dyschromatopsia, this lens was used in 46 subjects with dyslexia by Harris et al.¹⁵ in 1999, and they found that the reading speed was better with the use of tinted lenses in 41 subjects. The lenses were also used by Park et al. (2004) for reduction of glare in subjects with cone disorders. The subjects reported improvement in visual acuity and reduction in glare and photophobia.¹⁶

In one of the earlier studies by Zuleyha et al., 30 subjects were enrolled in the comparative study of efficacy of red filter for color vision deficiency, and they found improvement in all 30 subjects.¹²

During World War II, various devices were used to pass the medical examination by highly motivated recruits. It included color coaching and training by professionals along with the use of tinted goggles and lenses. The use of red contact lens may help the individual to pass a particular test but in no way it can be authenticated that his color vision has become normal.¹⁷ The American Ophthalmological Association extensively looked into the issue and concluded that no method can correct the physiological deficiency of color-defective subjects.¹⁷ Any attempt to correct color perception can decrease safety in transportation and reduce efficiency in industry. Joshi et al. reported a case of a highly motivated candidate who was declared medically unfit on initial medical examination on account of substandard color perception. He could pass the Ishihara test when he reported for appeal medical examination. On detailed evaluation, it was found that he was wearing a red contact lens on the left eye.¹⁸

The red lens may potentially aid in functional rehabilitation of those personnel who are already trained and have joined service based on an erroneous initial medical evaluation and are later detected to have a substandard color perception not meeting the occupational requirements.

Results of this study are in agreement with those of Welsh et al.¹⁴ who found variable results on error scores based on the duration of use of contact lens. The findings of this study are contrary to those of Swarbrick et al.⁸ who evaluated the performance of red contact lens on 14 color-defective subjects. Error rates at 2 weeks after the use of this lens were lesser as compared with errors at baseline on Ishihara. The vision in dim light with red lens was described by Zeltzer. He advised not to use these lenses in dim illumination.7, 19, 20 Matsumoto et al.¹¹ also found that use of red lens in dark led to problems to participants and cautioned against its use during night.

Zuleyha et al. studied 30 subjects with dyschromatopsia on efficacy of red filter for color vision deficiency, and they proposed that red filter can be used to diagnose malingerer in color blindness.¹² Moreover, in our setup also, cases of malingering keep reporting for change of trade or to get relieved of more strenuous duties.

In this study, only one shade of contact lens was used because red-green color blindness is the most common deficiency. Multiple tint lenses can be offered for individual color vision defects, which may further improve color vision.²¹

This study has used Indian Armed Forces medical standards and testing conditions for the first time, consequently making it pioneer and unique in peer-reviewed literature. The study has addressed the issue of misuse of red contact lens for passing color vision tests by aspirants for entry into armed forces.

Conclusion

In this prospective study, performance of the red contact lens was evaluated in 30 subjects with defective color vision. The error scores after the use of red contact lens reduced significantly based on Ishihara test, MLT, and Farnsworth D15 test. The improvement in performance based on various tests of color perception enabled most of the subjects to pass the Ishihara test. This fact must be borne in mind by medical officers. Red contact lens can be used in rehabilitation of armed forces personnel who are later detected to have a substandard color perception not meeting the occupational requirements.

This lens may help highly motivated recruits in passing the initial Ishihara screening test. Medical officers should be careful about tinted spectacles or contact lenses while performing medical examination. As a therapeutic modality, red contact lens can be offered for rehabilitation of color-defective cases in armed forces who surface up with delay and do not require good color vision for professional duties. They should be cautioned about altered visual function with lenses.

Conflicts of interest

All authors have none to declare.