Visual perception alterations in COVID-19: a preliminary study

Abstract

Aim: To compare the visual perception (color and chromatic-achromatic contrast vision) of a small cohort of COVID-19 patients at the time of infection and after 6mo with that of a healthy population matched for sex and age.

Methods: A total of 25 patients (9 females, 16 males, mean age: 54±10y) with COVID-19 hospitalized in the COVID-19 Unit of the University Clinical Hospital of Valladolid were recruited for this preliminary study. Visual perception, as determined by monocular measurement of contrast sensitivity function (CSF) and color vision was assessed in each patient using the Optopad test. The results obtained were then compared with those of a sample of 16 age- and sex-matched healthy controls (5 females, 11 males, mean age: 50±6y) in which the same measurement procedure was repeated. Statistically significant differences between groups were assessed using the Mann-Whitney U test. Measurements were repeated after a minimum follow-up period of 6mo and statistically significant differences between the two time points in each group were assessed using the Wilcoxon signed rank test.

Results: Discrimination thresholds (color and chromatic-achromatic contrast vision) and their corresponding sensitivity, calculated as the inverse of the discrimination threshold, were evaluated. Analysis of the data revealed higher contrast threshold results (i.e., worse contrast sensitivity) in the COVID-19 group than in the control group for all spatial frequencies studied in the Optopad-CSF achromatic test and most of the spatial frequencies studied in the Optopad-CSF chromatic test for the red-green and blue-yellow mechanisms. In addition, color threshold results in the COVID-19 group were also significantly higher (i.e., worse color sensitivity) for almost all color mechanisms studied in the Optopad-Color test. At 6mo, most of the differences found between the groups were maintained despite COVID-19 recovery.

Conclusion: The present results provide preliminary evidence that visual perception may be impaired in COVID-19, even when the infection has passed. Although further research is needed to determine the precise causes of this finding, analysis of CSF and color vision could provide valuable information on the visual impact of COVID-19.

Keywords: COVID-19; SARS-CoV-2; color vision; contrast sensitivity function; visual perception.

Figure 1. Examples of some plates of the Optopad-Color and Optopad-CSF tests

PDT plate (A), achromatic CSF 1.5 cpd (B), red-green CSF 1 cpd (C), blue-yellow CSF 1 cpd (D). CSF: Contrast sensitivity function; PDT: Protan-deutan-tritan.

Figure 2. CSF (inverse of contrast threshold) values by frequencies for the achromatic mechanism (CSF-A) obtained in each study group

Results obtained at baseline are represented by bars, while results obtained at 6mo are represented by lines. ^aBaseline visit, statistically significant; ^b6-month visit, statistically significant. CSF: Contrast sensitivity function; CSF-A: Achromatic contrast sensitivity function.

Figure 3. CSF (inverse of contrast threshold) values by frequencies for the chromatic red-green mechanism (CSF-T) obtained in each study group

Results obtained at baseline are represented by bars, while results obtained at 6mo are represented by lines. ^aBaseline visit, statistically significant; ^b6-month visit, statistically significant. CSF: Contrast sensitivity function; CSF-T: Red-green contrast sensitivity function.

Figure 4. CSF (inverse of contrast threshold) values by frequencies for the chromatic blue-yellow mechanism (CSF-D) obtained in each study group

Results obtained at baseline are represented by bars, while results obtained at 6mo are represented by lines. ^aBaseline visit, statistically significant. CSF: Contrast sensitivity function; CSF-D: Blue-yellow contrast sensitivity function.

Figure 5. Color sensitivity (inverse color threshold) values obtained in each study group for P: L cone-isolating direction; D: M-isolating direction; T: S-isolating direction; R: Red cardinal direction; G: Green cardinal direction; B: Blue cardinal direction; Y: Yellow cardinal direction

Results obtained at baseline are represented by bars, while results obtained at 6mo are represented by lines. ^aBaseline visit, statistically significant; ^b6-month visit, statistically significant.