The unseen in COVID-19

The COVID-19 pandemic affected the patients’ vision and eyes in multiple ways. As the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), the causative agent of the pandemic, is a highly infectious virus, the acute and long-term ocular afflictions during the COVID-19 pandemic have been studied with the help of surveys.[1,2] Ocular surface-related symptom of photosensitivity is the most frequent ocular symptom during the acute phase (31%) and may persist for 6 months (20%).[1,2] A recent meta-analysis reported the symptoms of blurring of vision and inability to focus vision in 7%–39% of adults persisting even 3 months after the infection.[3] In this issue of the Indian Journal of Ophthalmology, Sharma et al.[4] have reported color vision deficiency (CVD) in COVID-19 patients. It is the first report of dyschromatopsia in COVID-19 patients. The pathophysiology of the ocular symptoms in the COVID-19 pandemic has not been understood clearly. Most of the information regarding the symptoms is based on surveys with no ophthalmic evaluation during the acute infection. The symptoms of photosensitivity and blurred vision have been hypothesized to be related to ocular surface disorder and fatigue, respectively.[1,2] Similarly, the pathophysiology behind the dyschromatopsia reported in COVID-19 patients is unclear at present and hence can only be explained based on different hypotheses. CVD is a congenital or an acquired condition due to a defect in the retinal photoreceptors called cones (congenital) or can be secondary to a defect anywhere in the visual pathway from the retina to the visual cortex (acquired). Congenital CVD (CCVD) is stable since birth and affects the entire field of vision; in contrast, acquired CVD (ACVD) can fluctuate, is asymmetrical, and may affect only a part of the visual field. ACVD is believed to be highly symptomatic when compared to CCVD.[5] ACVD can be secondary to ocular, neurologic, or systemic diseases. Neuroinflammatory disorders such as multiple sclerosis and neurodegenerative disorders such as Alzheimer’s disease have been associated with a deficit in color vision irrespective of optic nerve dysfunction.[6,7] Dyschromatopsia in these conditions has been attributed to retinal ganglion cell damage.[6,7] COVID-19 is an infectious disease with multiorgan involvement. It has central nervous system involvement too. Angiotensin-converting enzyme 2 receptors, which are the binding site for the SARS-CoV-2, are present in the visual pathway in the central nervous system, neurons, glial cells, retinal photoreceptors, Müller cells, retinal ganglion cells, and retinal vascular endothelial cells.[8] The structural affliction of the visual pathway with the COVID-19 virus has been studied and reported in a recent study using spectral-domain optical coherence tomography to assess the retinal and choroidal changes in patients with coronavirus.[8] The study demonstrated striking qualitative changes in the retinal layers. Hyperreflective bands were noted in different retinal layers. Retinal pigment epitheliopathy was noted in 68.3% of eyes The evaluation of choroidal structure revealed abnormal pachyvessels and significant thickening of the subfoveal choroidal thickness. The authors hypothesized that choroidal thickening in COVID-19 patients may begin with an initial inflammatory phase with increased vascular permeability, followed by a late apoptotic phase with infiltration of the retina by inflammatory cells.[8] Though the study did not find any changes in the thickness of the different retinal layers, hyperreflective lesions in the retinal ganglion cell layer were noted in 18.3% of eyes and in the retinal nerve fiber layer in 11.7% of eyes, suggesting structural changes probably at the cellular level in these layers.[8] The clinical implications of these structural changes in the retinal and choroidal layers during the COVID-19 infection are probably subtle. In conclusion, the dyschromatopsia in COVID-19 is probably subclinical, and despite being an ACVD, the patients are not symptomatic. It may be similar to the subclinical ACVD of MS, Alzheimer’s disease, and other systemic diseases. Further studies are required to confirm these findings. The study done by Sharma et al. is a proactive step in evaluating the clinical implications of the structural changes found in the retina and choroid of patients with an active COVID 19 infection.[4] The scientific merit of the study is its conceptualization and methodology, which enabled the authors to examine a question despite an ongoing pandemic. The major limitation of the study is a lack of complete ophthalmic evaluation of the included subjects due to the implications of the pandemic. The study findings need to be confirmed with future prospective studies evaluating the fundus, color vision, visual fields, and optical coherence tomography of COVID-19 patients after the resolution of acute infection.