In this cross-sectional study, significantly worse dry eye tests including tear break-up time and Schirmer-1 test values were observed in psoriasis group compared with controls. No statistically significant difference was found between psoriasis group and controls in terms of bulbar and corneal epithelial thicknesses measurements. Corneal stromal thickness values were significantly higher in psoriasis group, especially in patients with nail involvement compared with controls. Schirmer-1 test values were negatively correlated with corneal stomal thickness in psoriasis groups with and without nail involvement.
Although the psoriasis is a chronic auto-immune disease, cornea and conjunctiva structures are not extensively studied. Some strong associations between keratoconus, progressive thinning of the cornea and autoimmune diseases were found [13]. Varma et al. [14] reported peripheral corneal melting in psoriasis. The others focused on the DED related ocular surface disturbance in psoriatic patients [2–9, 15–17].
Her et al. [6] reported 36.6% of the psoriatic patients experienced dry eye. The pathogenesis of increased risk of dry eye in psoriasis is not fully understood. Similar to T- cells mediated inflammatory responses leading ocular surface epithelial metaplasia in DED [6], T- cells in psoriatic skin keratinocytes were addressed as one of the main cause of psoriasis [1]. Using conjunctival impression cytology, Karabulut et al. [4] observed higher rates of squamous metaplasia of conjunctival epithelial cells in patients with psoriasis. Furthermore, more cell alteration and decreased goblet cell density in conjunctiva were observed in psoriasis [6]. Best of our knowledge, OCT measurements of ocular surface epithelial thicknesses in psoriasis have not been reported. Moreover, the OCT studies [18–20] evaluating the ocular surface epithelial thickness have somewhat conflicting results even in dry eye. Francoz et al. [20] reported a significantly thicker bulbar conjunctival epithelium (BCE) thickness in DED and a direct relationship between ocular surface disease severity and BCE thickness but the central corneal epithelial thickness did not differ between DED and controls. Gumus et al. [19] reported no statistically significant differences in BCE and central corneal epithelial thicknesses between aqueous-deficient type DED, evaporative-type DED, and controls. Although, the BUT and Schirmer values did not correlated with both the temporal BCE and central corneal epithelial thicknesses, negative correlations between tear osmolarity and both the temporal BCE and central corneal epithelial thicknesses were observed [19]. Cui et al. [21] reported the superior corneal epithelium in DED was significantly thinner than controls. They did not observe any differences in central corneal epithelium. The difference between central and peripheral cornea was attributed the immune and angiogenic privilege of central cornea. Liang et al. [18] reported significantly thicker bulbar conjunctival epithelium thickness in DED compared with controls but the central corneal epithelial thickness did not differ between the groups. Kanellopoulos et al. [22] reported significantly thicker central corneal epithelial thicknesses in DED compared with controls and concluded the augmented corneal epithelial thickness might be used as an objective clinical indicator of dry eye. Conversely, Edorh et al. [23] reported significant corneal epithelial thinning in DED compared with the controls. In accordance with the studies concerning DED [19–21], we observed no significant differences in BCE and central corneal epithelium thicknesses in patients with psoriasis having significantly worse dry eye test values compared with controls. The differences in epithelial thickness measurements between the studies concerning DED might be attributed to selected area differences (ie, limbal, bulber, central or peripheral), manual positioning of the caliper on selected scan to measure epithelial thickness, status of inflammation related to tear hyperosmolarity, exclusion or inclusion of the precorneal film, and precence of autoimmune diseases (eg, psoriasis). Further studies concerning cell morphology (ie, confocal microscopy) are needed to differentiate the possible causes of epithelial thickness involvement.
Using ultrasonic pachymeter [10] and OCT [11], central corneal thickness (CCT) measurements did not differ between psoriasis group and controls. Corneal biomechanical properties including corneal hysteresis (CH) and corneal resistance factor (CRF) were evaluated in psoriasis patients [10, 11]. Edris et al. [11] reported a significant negative correlation between PASI and CH and concluded that the CH might be played a role as an indicator of disease activity. In accordance with the Edris et al. [11], disease activity in the present study was negatively correlated with corneal stromal thickness and reached statistically significance in nail psoriasis group having higher PASI values. Our data supports the theory concerning the altered immune-response and T lymphocytes in psoriatic patients led subsequent weakening of corneal proteoglycans and glucosaminoglycans. However, this theory needs to be clarified with electron microscopic studies.
The involvement of corneal biomechanics in DED somewhat conflicting. Using ocular response analyzer (ORA), Fırat et al. [25] reported no differences in biomechanics of the cornea. More recent studies using ORA[26] and Corneal Visualization Scheimpflug Technology (CorVis ST) [27] measurements in patients with Sjogren's disease suggested that the biomechanics of the cornea are effected in patients with Sjogren’s disease.
There are some limitations of the current study. First, relatively small numbers of psoriasis patients were included. Second, we did not evaluate the serum levels of cytokins that take part in pathogenesis of psoriasis. Third limitation was only the infero-temporal conjunctiva and central cornea were evaluated. Studies with larger patient cohorts, take into account the serum levels of inflammatory cytokines, and evaluating the different parts of both conjunctiva and cornea are now required to detect the ocular surface involvement more precisely.
In conclusion, Psoriasis is associated with the DED. This is the first report of epithelial and the stromal thickness changes of both the conjunctiva and cornea in psoriasis. Further studies are needed to clarify the causes of central corneal stromal thickening in psoriasis.