This cross-sectional study of Persian school children provides IOP and CCT reference values for this population and demonstrates that IOP is correlated with CCT. The IOP and CCT for each age group are also provided, but the relationship between CCT and age remains largely unknown. Mean CCT for all included children was 513.47 ± 34.51 μm in the OD and 513.93 ± 33.88 μm in the OS.
Our CCT measurement was lower than those reported for children of other ethnicities, including Japanese, Singaporean, Black, Chinese, African–American, Turkish, and Malay children [5–12]. This is important to know because a thin central cornea can indicate an underestimation of IOP and possibly impede physicians from making a glaucoma diagnosis [12, 13].
Ultrasound pachymetry has been considered as the current gold standard for quantifying CCT. Studies have shown specular microscopy measurements of CCT are considerably lower than ultrasound pachymetry measurements of CCT [7, 14]. In our study, corresponding to parents’ consent only a single contact measurement was taken, and therefore, the authors decided to measure IOP with gold standard Goldmann applanation tonometer and CCT measurement was documented with noncontact specular microscope. Since we used noncontact pachymetry to measure CCT, we were not able to compare current CCT findings with those of others who used ultrasound pachymetry to measure CCT. While comparing our CCT measurements with those from a similar study carried out on children from Malaysia [12] which used a similar methodology and noncontact instrumentation, this study documented thinner CCT.
We found that IOP measured with GAT increased by 0.8 and 1.2 mmHg in OD and OS, respectively, for 100 μm rise in CCT. Interestingly, this IOP increased was 3.3 and 2.9 mmHg in OD and OS, respectively, for every 100 μm rise in CCT when IOP was measured with NCT. A recent study from Northern China (The Jinan City Eye Study) [14] and Malaysia [12] presented a 3.2 mmHg increase in IOP for every 100 µm increase in CCT. Additionally, Muir et al. showed a positive association between IOP and CCT meaning that for every 100 μm increase in CCT, the IOP raised by 2.2 mmHg [6]. However, this finding is somewhat controversial because Haider et al. [4] did not find a statistically significant relationship between IOP and CCT in either African–American or White children.
The GAT is a reliable and consistent technique for measuring IOP. The IOP measurements made with NCT were in fair agreement with those made with GAT in the OD, but in good agreement with those made with GAT in OS. Shah et al. [15] revealed a significant association between PT100 air-puff and GAT measurements (Pearson’s correlation = 0.715, P < 0.01) as a recent study in Turkey showed that NCT and Tono-Pen® XL (Reichert Technologies, Depew, NY, USA) IOP measurements were both comparable to GAT, the gold standard measurement, in normotensive eyes [16].
There are a number of limitations that affected the outcomes of this study. First, the nature of our methodology as the cross-sectional study provided only one time measurement for each subgroup. Nevertheless, the absence of a relationship between age and gender in IOP and CCT does not necessarily signify that this link is nonexistent. On the other hand, younger subjects below 6 years of age, can often be uncooperative during regular ocular assessments, especially in IOP and CCT measurements under local anesthesia. The authors therefore decided to examine children of 6 years and older in this study. The authors decided to document values using noncontact pachymetry, without using general anesthesia similar to methodology of former studies in which they used noncontact pachymetry in children [7, 12, 15, 17]. This may be considered as a limitation as the ultrasound pachymetry is still considered as the gold standard in the literature. Furthermore, the purpose of this research was not to correlate measurements comparing adult and children values or study those measurements taken over a specific period of time.
As a pioneer study in Persian children, examination of a uniform population consisting of various age groups, straightforward associations between NCT and GAT values of IOP made our outcomes significant.
It has been established that knowledge of normal ocular structures in different ethnicities may deliver a significant reference value and may contribute in the identification of diseases including glaucoma [18]. It is remarkable that the members of the same racial background who live in different countries or even different states could behave contrariwise. It is interesting to know that individuals who live in urban areas present with thicker corneas than those who live in rural areas [19].
Biomechanical characteristics of the cornea are altered by metabolic and environmental factors; particularly by UV exposure [20], therefore, environmental exposure may influence the biophysics of the eye. Therefore, further study should focus on subjects with similar racial background who live in other states with different environmental exposure as well as other countries in Middle East region. Since our analysis only focused on children with normal ocular examinations, further study is required to find the correlation between CCT and IOP in subjects with diseased corneal or pseudophakia, children who have diagnosed as congenital glaucoma or positive past medical history of glaucoma. It is our belief that longitudinal studies following the subjects from infantile to maturity would prove particularly beneficial for improving our perceptive of ocular development. A systematic review and meta-analysis on CCT and IOP values in normal children, in addition to study of the variations in corneal biometric and their relationship to other ophthalmic considerations, such as corneal curvature, axial length, and endothelial cell density, could improve our basic knowledge of the ocular health in Persian children and pediatric population who live in Middle East region.