Corneal densitometry is accepted to reflect both corneal clarity and health. Light scattering is minimal in the normal cornea thanks to its internal structure [7]. Increased corneal light backscattering has been reported in various corneal disorders such as bacterial keratitis, keratoconus, pseudoexfoliation syndrome, Fuchs endothelial dystrophy, and rheumatoid arthritis [7–11]. Light entering the cornea scatters either in a forward or backward direction. Backward light scattering results in a reduction in the amount of light reaching the retina [1]. Increased backward scattering decreases the quality of vision and degrades the retinal images [4]. We wanted to evaluate corneal light backscattering in healthy eyes with various refractive values by using corneal densitometry measurements. A literature survey did not reveal any other study that investigated the morphological structure of the cornea and optical densitometry values in the presence of various refractive errors in children. Our aim in the current study was to investigate whether a relationship was present between corneal densitometry values and refractive values as well as other ocular structure parameters and thus to identify whether these factors contributed to the optical status.
The main corneal scattering of light occurs at the interfaces between air and the tear film, and between the tear film and the cornea. The anterior superficial corneal epithelial cell layer and the posterior corneal endothelium are the major sources of light backscattering with the superficial epithelial layer making the highest contribution. The corneal stroma provides the cornea’s transparency with the regular arrangement of collagen fibrils, the homogenous interfibrillar distance, and the uniform diameter [1, 12]. The posterior stroma with a more regular structural organization has more corneal transparency than the anterior stroma with a weaving structural pattern. In concordance with this, we found hyperopic cases had lower corneal optical density (COD) values in the 0–2 mm and 2–6 mm areas compared to both the myopic and emmetropic cases in the posterior layer, and lower densitometry throughout the total diameter was also seen in the anterior, posterior and total layers. This indicates a lower degree of scatter in the total diameter in all layers except the central corneal layer in hyperopia.
The zones where the hyperopic cases were not different than the myopic cases but had lower COD values than the emmetropic cases were the central 0–2 and 2–6 mm zones considering the total of these all corneas, this difference was significant in the 2–6 mm zone. These results indicated that the decisive corneal zones in terms of COD were the corneal apex (0- to 2-mm zone) and the pericentral cornea (2–6 mm zone) in eyes with refractive values. A small number of studies investigating corneal densitometry values in normal healthy corneas have reported a wide range of results in various ages without differentiating between refractive errors [5, 13–15]. Although comparison of the studies is difficult, Cankaya et al. found the lowest COD values to be in the total 0–12 mm total diameter of the posterior layer and the highest backscattering value to be in the anterior layer [5]. They reported the lowest COD value to be in the central 0–6 mm zone and to show a gradual significantly increase while advancing towards the limbus [5].
We observed higher COD values in the corneal apex, and pericentral cornea instead of the peripheral zones of 6–10 mm in all corneal layers for each group. However, the annular zone of 10–12 mm diameter in all layers for each group had the highest mean COD value. Furthermore, the total corneal layer of the 10–12 mm diameter was statistically significantly lower in the hyperopic group. Due to the non-homogenous distribution of endothelial cell density in the cornea, the highest value is found in the 10–12 mm annulus [14], which means more corneal transparency because of the water pumping and barrier function of the endothelium. So, we can speculate that endothelial cell density in the 10–12 mm annulus has the highest value in hyperopic eyes. On the other hand, analysis of the 10–12 mm zone by using Scheimpflug devices had the lowest repeatability and reproducibility [15]. The assessment of COD values according to the position of the limbus and sclera during the measurement, especially in cases with corneas smaller than the 12-mm zones, can result in increased COD values in the peripheral zones. This could explain the negative statistically significant correlation between the WTW diameter and the COD values in all annular zones except the 0–2 mm and the 2–6 mm zones in all three groups in this study.
Evaluation of the correlation between the spherical equivalent values and COD values revealed a statistically significant correlation only in the hyperopic group. A significant but weak negative correlation was present between the spherical equivalent and corneal densitometry values in hyperopic cases, especially in the central 0–2 mm zone of the anterior, central and total layers. However, we saw that the COD value increased as the spherical equivalent value increased when the measurements were advanced towards the limbus and periphery (central layer of 10–12 mm) and towards deeper areas (posterior layer of 6–10 mm, 10–12 mm). This could result in decreased image quality on the retina in high hyperopic cases. In addition, the lower mean COD over the 12 mm diameter area in the anterior, posterior and total layers in only the hyperopic eyes suggested that this could have an effect on the balance of the formation of the optical qualities of the cornea in hyperopic children. In a study where high myopic values (> -6 dioptric (D) spherical refractive errors) were compared with age- and gender-matched < 5 D spherical cases, a high COD value was found in the 10–12 mm peripheral zone, while the 0–12 mm total diameter had lower corneal densitometry values in the other zones and in total in the high myopic group [13]. Although the groups we compared were different, we observed in our study that the myopic cases were not different than the hyperopic cases, but had lower COD values in the central and posterior 6–10 mm pericentral zone compared to emmetropic cases. However, high-diopter and pathologic cases were not included in our study.
We found a negative correlation between the central corneal thickness (CCT) and COD for the all depth layers of the central and pericentral annular zones in myopic cases in this study. Unlike our study, Dong et al. have found a positive correlation between the central corneal thickness and total corneal densitometry in highly myopic cases [13]. Cankaya et al. have found no correlation between CCT and corneal densitometry values [5]. We observed that the corneal densitometry values for all corneal depth layers increased only in the 6–10 mm zone as the CCT value increased, thus causing more corneal backscattering in hyperopic cases. This has raised the question of whether the COD values of the zones that are flattened during refractive surgery have a possible effect on the resultant image quality as the mid-peripheral zone is targeted in hyperopic eyes while the central zone is targeted in myopic eyes.
COD values increasing with age have been found in studies conducted with subjects at an advanced age where corneal densitometry values in various refractive errors were evaluated in healthy corneas [5, 14, 15]. Changes occurring in the structure of the cornea with age were presented as the reason for this result, due to the following changes: thickening of the epithelial basal membrane, reformation of the stromal collagen fibers increasing the thickness of Descemet’s membrane, and decreased endothelial cell function accompanied by increases in stromal hydration. A COD value increasing with age was only found in emmetropic cases in the age group of < 18 years in our study, but no correlation was found between the age and corneal densitometry values in myopic and hyperopic cases. The contrasting results in the literature may be due to the investigation of different age groups and different refractive errors. The emmetropization process seems to depend on the visual input being normal, and the lack of a normal input could disturb the process, resulting in refractive errors [16–19]. This made us consider that COD values may contribute to the development of refractive errors due to the lack of clear visual input.
The presence of COD values in various zones in myopic and hyperopic cases that are different than emmetropes may be balanced by various optical factors in eyes with different refractive errors. The main part of the emmetropization process is known to be completed within the first 6 years of life [16]. Considering that the major part of ocular development had already been completed in our study that included pediatric patients above the age of 6 years, it would be appropriate to investigate with longitudinal studies how light backscattering in the 0–6 years age range can change during the emmetropization process.
The ocular biometric parameters including axial length (AL), anterior chamber depth (ACD), and lens thickness (LT) are known to be significantly correlated with refractive errors. These parameters have been found to be lower in hyperopic eyes than in myopic and emmetropic eyes [20]. Tomomatsu et al. have shown that the degree of hyperopia inversely correlates with the AL [21]. The increase in axial length during ocular growth in myopic eyes has been shown to be balanced by the decrease in the lens thickness [22]. Lee et al. have found the deepest ACDs in myopic eyes and the shallowest ACDs in hyperopic eyes in children [23]. Similarly, we observed ACD and anterior chamber volume values to be lowest in the hyperopic cases and highest in the myopic cases in our study, while both parameters were significantly different than in the emmetropic cases. Again similar to the same study [23], we encountered a shallower ACD in hyperopic eyes as the degree of hyperopia increased.
In conclusion, increased light backscattering results in the reduction of the amount of light reaching the retina and impairs the quality of vision. In the present study, the COD values were significantly lower in hyperopic eyes than in myopic and emmetropic eyes in children. The COD values also increased with age in emmetropic eyes. We believe that further and longitudinal studies are needed to support the significance of this result in clinical practice and its relevance for the optical system.