Reduced corneal innervation and increased dendritic cell density in the cornea of long COVID patients 20 months after the infection

In this descriptive cross-sectional study, the authors from Spain and Costa Rica used corneal confocal microscopy to investigate corneal innervation in patients with persistent symptoms of long COVID (78% of whom were vaccinated) more than 20 months after acute SARS-CoV-2 infection. The results showed reduced corneal innervation (a decrease in corneal nerve density and branch density), as well as an increase in dendritic cell (DCs) density in patients with long COVID in comparison to the control group.

The WHO has described long COVID as a condition “that occurs in individuals with a previous history of probable or confirmed SARS-CoV-2 infection, usually three months after the onset, with symptoms lasting at least two months that cannot be explained by an alternative diagnosis”. Long COVID could be considered to be a spectrum of disorders. Over 50 symptoms have been reported to be associated with long COVID. Multiple organ systems are affected, including the respiratory, cardiovascular, nervous, and gastrointestinal systems. Fatigue, respiratory distress, and cognitive dysfunction are the most common symptoms.

The cornea is one of the most innervated tissues, receiving heterogeneous sensory nerves from the ophthalmic branch of the trigeminal nerve, and a small number of autonomic sympathetic nerve fibers from the cell bodies of the superior cervical ganglion. Its basal epithelium is populated by resident immune cells, known as dendritic cells, which have function to bridge the innate and adaptive immune responses. They also contribute to corneal nerve homeostasis.

Corneal confocal microscopy is a noninvasive imaging technique that is utilized for direct visualization of corneal structure in vivo, including the corneal subbasal nerve plexus. The loss of corneal nerve fibers, which is indicative of neurodegeneration, may be associated with various diseases. Due to this reason, in vivo confocal microscopy has been used to identify small-fiber neuropathy in immune-mediated small-fiber neuropathies and peripheral neuropathies.

Previous investigations have reported a decrease in corneal nerve fibers and an increase in dendritic cells in patients with acute COVID-19 or long COVID 3–4 months after infection. According to the authors, some previous studies have shown that COVID-19 vaccination may increase dendritic cells in the cornea, which could enhance local immune response. Recent studies have associated long COVID syndrome with small-fiber neuropathy, which is characterized by a selective alteration of small, thinly myelinated nerve fibers such as A-fibers, and unmyelinated C-fibers. Small-fiber neuropathy affects both sensory and autonomic nerves. 

About the study

This descriptive cross-sectional study included a total of 88 individuals; 60 of whom were diagnosed with long COVID, and 28 were controls. The vaccination rate for long COVID patients was 78% (47 individuals).

The patients with long COVID included in this study were older than 18 years. The diagnosis of long COVID was based on the WHO criteria. The long-COVID questionnaire was used to identify persistent symptoms. Individuals with eye diseases, such as ocular surface disease, glaucoma, individuals with a history of systemic disease prior to SARS-CoV-2 infection, contact lenses wearers, and individuals who had undergone eye surgery within the last six months were not included. The control group consisted of volunteers older than 18 years, who did not wear contact lenses, and did not have any systemic or eye diseases.

Corneal confocal microscopy (with the Heidelberg Retina Tomograph II) was used to visualize morphological parameters of the subbasal nerve plexus, including corneal nerve fiber density (number of nerve fibers per square millimeter), nerve fiber length (total length of nerves in millimeters per square millimeter), nerve branch density (number of primary branch points on the main nerve fibers per square millimeter), and nerve fiber total branch density (total number of branch points per square millimeter). Other morphological changes in nerve fibers, including microneuromas, were also recorded. The dendritic cells in the subbasal nerve plexus were numbered by hand using the specialized software. A density of dendritic cells was also calculated (cells/mm2).

The results

Corneal confocal microscopy revealed differences in corneal innervation between patients with long COVID and the controls. Compared to the control group, patients with long COVID had significantly lower corneal nerve fiber densities, shorter nerve fiber lengths, and lower densities of nerve fiber branches. In 15% of Long COVID-19 patients, microneuromas were found.

Patients with long COVID also had a higher density and the larger area of DCs, which could potentially enhance the local immune response. The authors suggested that this immunological response may contribute to the corneal nerve degeneration observed in long COVID patients.

In conclusion, this study found reduced corneal innervation, including a decrease in corneal nerve fiber density, nerve fiber length and nerve branch density, in patients with long COVID more than 20 months after acute infection. The authors noted that the pathophysiological basis for these findings is unclear, but the results support the hypothesis that neuroinflammation is one of the persistent complications in patients with long COVID.

These results also emphasized the potential role of corneal confocal microscopy as an effective non-invasive technique for patients with long COVID.

This article was published in Diagnostics.

Journal Reference

Cañadas, P.; Gonzalez-Vides, L.; Alberquilla García-Velasco, M.; Arriola, P.; Guemes-Villahoz, N.; Hernández-Verdejo, J.L. Neuroinflammatory Findings of Corneal Confocal Microscopy in Long COVID-19 Patients, 2 Years after Acute SARS-CoV-2 Infection. Diagnostics 2023, 13, 3188. (Open Access)