The study showed that the effect of both illumination and filters on CS differed for each group based on cause of visual impairment, indicating specific patterns of responses.
Cataract
Increasing illumination with no filter performed poorly in eyes with cataract confirming a study by Smedowski et al [19] who concluded that the mainstay intervention of increasing illumination may rather be detrimental to persons with cataract. The mechanism of contrast sensitivity reduction in cataract is thought to be the result of increased light scatter, especially light of short wavelength, through the thickened opaque lens which reduces the quality of the retinal image and adversely affects the discrimination ability of the visual system especially in bright daylight [20]. Hence, higher levels of lighting increase light scatter and glare perception.
Eperjesi and Agelis [9] explained that since light scattering intensity is inversely related to the wavelength of the light source, blue light, which is of short wavelength is more scattered and contributes most to the loss of contrast sensitivity. This brought about the need for a blue-light blocking tool, with many studies concluding on the yellow filter as a safe contrast enhancing tool with the least alteration to the patient’s color vision [8, 14, 15, 17]. This claim is confirmed by the outcome of this study which showed that by a clear distinction, the yellow filter at the lowest illumination of 100lux gave the highest improvement in eyes with cataract setting it apart as the filter of choice when compared to the pink and orange filters.
Filters interacted significantly with illumination showing that the effect of an increase in illumination is impacted by the type of filter it is combined with. For example, while higher lighting improves CS when combined with the orange filter, it drastically reduces CS when combined with the yellow filter (Fig. 2). The yellow filter at an illumination of 100lux and the orange filter combined with a higher level of illumination such as 700lux are recommended for use in eyes with cataract.
Pseudophakia
Increasing illumination, without any filter, from 100 to 300lux is seen to have a significant impact on CS. Subsequent increments performed similarly to 300lux showing that illumination levels did not necessarily have to be very high to improve CS. Systematic reviews [21–23] have generally concluded on inadequate evidence to support the use of filters in pseudophakic eyes and the results of this study agree to that. Filters, however, have been shown by this study to perform better when combined with high levels of illumination.
Contrast sensitivity loss in pseudophakia is mainly due to the absence of the blue light-blocking ability of the natural crystalline lens [24] and also to degradation of the retinal image as a result of chromatic aberration of intraocular lenses [25, 26]. The natural crystalline lens is a filter which reduces the forward transmission of both ultraviolet B (280-315nm) and A (315-400nm) [27] which are not only harmful to the retina but increase the sensation of glare and therefore, reduce the discrimination ability of the visual system. In pseudophakia, the natural lens being replaced by intraocular lenses which lack this filtering ability, makes the visual system more vulnerable to the effects of ultraviolet radiation and high energy blue lights such as impaired CS. This has prompted many researchers to investigate the effect of a blue-light filtering intraocular lens designed to resemble the natural crystalline lens. So far, the results have been conflicting and inconclusive. Even so, intraocular lenses tinted yellow or orange have become surgeons’ choice in an attempt to restore the visual system to the natural state as much as possible.
Without any filter, increasing illumination, even by a little amount will improve CS significantly in eyes with pseudophakia. Filters however, must be used with some amount of illumination, preferably 1000lux.
Maculopathy
Analysis in eyes with maculopathy showed that at 100lux, only the yellow filter significantly improved contrast sensitivity which improved further at 300lux confirming studies by Ahmad et al. (2017) [7], Caballe-Fontanet et el. (2020) [28] and Wolffsohn et al. (2002) [12] Macular pigments, lutein, zeaxanthin and meso-zeaxanthin have been established to possess blue-light filtering ability, enhancing the quality of central vision by the drastic reduction of the effects of blue light – chromatic aberration and disability glare [29–32]. Renzi and Hammond [33] showed that macular pigment density (MPOD) was positively correlated with contrast sensitivity due to its selective absorption of blue light. Since cone photopigment regeneration is adversely affected in maculopathy [34], contrast sensitivity is markedly reduced even in the presence of adequate visual acuity. This affects everyday activities like facial recognition, reading, and driving. Since macular pigment is yellow in color, researchers have investigated the effect of an external yellow filter in eyes with Age-related Macular Degeneration (AMD), macular scarring and diabetic macular edema, and found positive results which are in agreement with the results of this study.
Increasing illumination with no filter, however, had no significant main effect. The line chart (Fig. 4) showed an improvement in CS at 300lux and 700lux but a slight reduction in CS as lighting was increased from 700 to 1000lux. In the absence of the glare-reducing ability of macular pigments, it is postulated that high intensities of light may induce glare which override the positive impact of illumination on visual discrimination. More research on this is recommended.
Considering the protective effects of the yellow filter and its similitude to the natural macular pigment, this study recommends it to be prescribed in clinical practice, preferably to be used with low levels of lighting such as 300lux for eyes with maculopathy.
Glaucoma
All three filters at 100lux did not significantly improve CS in eyes with glaucoma disagreeing with Ding et al. (1997) [35] and Ahmad et al. (2017) [7] who found significant improvements with the yellow filter. Increasing illumination did not significantly affect the performances of the filters. Since patients with glaucoma experience high levels of glare and haloes due to pressure build-up in the eye, it was posited that filters, in reducing forward transmission of certain wavelengths, could improve the quality of the retinal image. The results of this study, however, showed a disagreement to that assertion. Even though measured visual acuity may be good, persons with glaucoma often complain of vision difficulties such as poor navigation in dim light. Contrast sensitivity is adversely affected even in early glaucoma making it a more reliable estimation of visual performance (Hawkins et al. 2003) [36]. The mechanism of contrast sensitivity loss remains unclear but have been linked to loss of retinal ganglion cells in the macular which is responsible for the quality of central vision.
On the other hand, eyes with glaucoma showed significant improvement in CS with increased lighting and no filter agreeing with earlier studies [37, 38]. Lighting as an intervention for persons with glaucoma have been shown to improve general quality of life, postural gait and mobility [39], all of which are positively correlated with good contrast sensitivity. The results of this study show that making environmental changes with regards to lighting will impact positively on the ability of glaucomatous eyes to perceive contrast and hence home lighting adjustments should be advised.
There is still a lot of controversy concerning these interventions, especially the effect of filters, in persons with low vision. Much of this could be attributed to differing population characteristics, experiment set-up and choice of analysis. This study did not match age, visual acuity and gender in the selection of participants and significant differences existed in the mean age and visual acuity of the different groups. This was accounted for by employing these two variables as covariates in order to ensure that the results are not confounded. Researchers must take this into account.
This study was delimited to only eyes with cataract, pseudophakia, maculopathy and glaucoma. Other contrast sensitivity-reducing ocular conditions such as retinitis pigmentosa, cornel opacities, and optic atrophy were excluded due to difficulty in obtaining adequate numbers to make sound analysis. The study was also delimited to the purposive sampling method as only a limited number of persons in the society could serve as primary data sources.
Clinical Applications
Illumination is currently being employed in low vision care in the form of general adjustments to home and workplace lighting. Most magnifiers also come along with a source of illumination for focused tasks such as reading. Persons with glaucoma and pseudophakia have been shown to benefit from increased lighting and therefore can be prescribed such tools. Depending on the outcome of needs assessment and task analysis, different levels of lighting can be prescribed. Changes to the home and workplace environment should also be advised to augment the effect of increased lighting.
Filters, however, are uncommon in the Ghanaian clinical practice. The yellow filter has been shown to be of benefit in persons with cataract and maculopathy. A clinical test of the effect of the yellow filter on contrast sensitivity can be integrated into routine clinical procedures. For patients who are shown to potentially benefit from the filter, a subjective trial in the real world can be carried for a period of time and a permanent prescription given to those who report a positive impact on their visual performance.