Corneal thickness is an important anatomical characteristic of the anterior eye and a useful indicator of corneal health,1 a specific indicator for corneal abnormalities2 and an essential determinant for suitability of refractive surgery.3
An analysis, however, of the literature reporting human corneal thickness over a 30-year period indicated that a wide range of values could be encountered for nominally healthy adults and, at best, values between 473 and 595 μm would be within normal limits.1 Although some of the differences between studies can be attributed to the use of different measuring devices or different underlying optical principles,1,4 no substantial or consistent differences in central corneal thickness in adults seemed to exist for those of Caucasian origin when age, gender, or refractive error was considered. The latter aspect has been confirmed in more recent studies.5
A factor not considered in the meta-analysis undertaken in 2000 on the expected normal values for corneal thickness in adults1 was that of the stature of an adult individual, as assessed by height or body mass. Early, growth-related changes in the cornea, including in its thickness, can be expected to occur in infancy and perhaps into early childhood.6,7 Similarly, age-related changes in eye growth, notably the axial length of the eye, can be expected over the same time period and extending to early adult years with such changes likely being dependent on the refractive error that develops in the growing eye.8,9
The issue of adult body stature and the cornea has been considered in a number of recent population-based assessments, with body height as a factor contributing to interindividual corneal thickness variability10–13 and to differences in radius of curvature of the cornea.14–16 Consideration has also been given to body weight and corneal thickness10,13,14,17–19 or body mass index (BMI) and corneal thickness.18,20–23
The rationale behind these previous population-based studies, and analyses including corneal thickness, seems to have been principally directed towards understanding disease-related changes especially as associated with the onset of severe myopia and other ocular diseases, especially in non-Caucasian individuals. For the most part, individuals over the age of 40 years were those that were studied, and although the outcomes of the analyses have indicated that there could be statistically significant correlations between body height and corneal thickness, for example, the effect size (or magnitude) of any such relationship was not addressed or elaborated upon in most of these studies.10–23 Stated another way, it is unclear how substantial any interdependency between body stature and corneal thickness or other corneal parameters such as anterior corneal radius of curvature (corneal radius) might be in fully grown younger adults, as opposed to growing children or teenagers. Without this specific information on effect size, it is not possible to assess whether such associations are of clinical relevance (e.g. does body height need to be considered in interpretation of pachymetry or keratometry-based findings?).
The purpose of this study was to further investigate the anatomical and statistical relationship between body stature and both corneal thickness and corneal radius in normal healthy young adult Caucasian subjects with a particular focus on the effect size of such relations. The secondary aim was to re-examine the existing literature on the topic. The outcome of this study provides valuable information because the relationship between body stature in adults and ocular dimensions may be useful in understanding the process of emmetropization.15
SUBJECTS AND METHODS
The study adhered to the tenets of the Declaration of Helsinki, and the protocol was approved at the respective institutional ethical review boards at Glasgow Caledonian University and the University of Valladolid. After written informed consent, subjects were asked to complete a version of an Ocular Comfort Questionnaire,7 which includes questions on the eye and general health, on current spectacle and contact lens wear, and medication use. Subjects with active ocular inflammation, previous ocular surgery, and rigid contact lens wear were excluded. Soft contact lens wearers were instructed to remove their contact lenses at least 24 hours before participating in the study. All measurements were taken during waking hours and between 10 AM and 5 PM to minimize the effect of diurnal variations.24,25
Subjects were asked to remove shoes and any jackets or overcoats before obtaining height and weight measurements. Body height was measured using a standard height scale and recorded in meters (m) to 0.01 m accuracy. Body weight was assessed using a calibrated scale and recorded in kilograms (kg) to 0.1 kg accuracy. To obtain a quantifiable index of body height to weight, the body mass index (BMI) was calculated as weight in kilograms divided by the height in meters squared (kg/m2).26
Instrumentation and Ocular Assessments
Subjects underwent an ophthalmic assessment. Habitual visual acuity was obtained using a standard Snellen chart. Slit-lamp biomicroscopy and optical coherence tomography of the anterior segment (Topcon 3D OCT2000; Topcon Corporation, Tokyo, Japan) were carried out to confirm ocular health of the anterior segment. Noncontact specular microscopy of the central cornea (Topcon SP2000; Topcon Corporation) was performed to rule out corneal endotheliopathy or any other notable corneal endothelial abnormalities. The anterior segment was then assessed using the Pentacam Scheimpflug system (Pentacam, Oculus GmbH, Wetzlar, Germany). Two Pentacam measurements of the same eye were performed, with the subjects being asked to blink and reposition between scans and with the automatic release mode used to minimize observer-related variability. Corneal thickness was extracted from topographic maps at 1-mm increments including the apex (central corneal thickness) and peripheral nasal and temporal locations up to 5 mm away from the apex at 11 locations along the horizontal corneal meridian. The mean corneal radius was recorded. The mean of two scans was used for analyses.
One eye per subject was used for analyses, which were carried out using the Stata SE version 13.1 software (Stata Corporation, College Station, TX). Descriptive statistics including the mean and standard deviation were generated, and the normality of data set distribution was tested using the Shapiro-Wilk test. Appropriate parametric and nonparametric tests were used to assess differences. Spearman’s rank correlation and linear regression analysis were used to assess associations between body stature and ocular measurements. Simple and multiple regression models were applied. Effect size was determined by generating regression and correlation coefficients and the coefficient of determination where appropriate. A subgroup analysis was carried out for two refractive groups, namely myopic and emmetropic subjects. Myopia was defined as spherical equivalent refractive error of <−0.50D and emmetropia as ≥−0.50 ≤ +0.50D. A P value of ≤ .05 was considered statistically significant.
Subject Demographics and Body and Corneal Measurements
One hundred nine eyes of 109 healthy Caucasian subjects (72% female) with a mean (±SD) age of 24 ± 6 years were assessed. The body height measures ranged from 1.54 to 1.86 m and were rather heterogeneous and not normally distributed (P = .003, Shapiro-Wilk test). The mean height was 1.67 ± 0.08 m. The mean body weight was 65.0 ± 12.3 kg, and the resultant mean BMI was 23.21 ± 3.86 kg/m2.
Central corneal thickness ranged from 465 to 629 μm and was normally distributed (P = .89). The mean central corneal thickness for all subjects was 554 ± 33 μm. Corneal thickness increased progressively and asymmetrically from the corneal apex to the periphery with a significantly greater thickness at all nasal locations as compared to the corresponding temporal sites (P < .001, related samples t-test). Corneal thickness increased by 32% to 733 ± 42 μm at 4 mm and by 51% to 833 ± 50 μm at 5 mm nasally from the apex. For the temporal aspect, corneal thickness increased by 21% to 674 ± 41 μm at 4 mm and by 40% to 773 ± 49 μm at 5 mm temporally from the apex. Therefore, the corresponding nasal peripheral corneal thickness at both the 4- and 5-mm locations was on average about 10% greater than the corresponding temporal values. The mean corneal radius was 7.75 ± 0.24 mm (range 7.16–8.49 mm), and the data were normally distributed (P = .17). Corneal power averaged 43.63 ± 1.32D (range 39.75–47.15). Subgroup analysis of myopic and emmetropic individuals revealed that myopic (n = 49) and emmetropic (n = 55) subjects had a similar mean corneal radius with 7.73 ± 0.26 mm and 7.76 ± 0.22 mm, respectively (P = .54, one-way ANOVA), whereas corneal power was 43.73 ± 1.47D for myopic and 43.53 ± 1.21D for emmetropic individuals (P = .46).
Correlations Between Body Height, Corneal Thickness, and Corneal Curvature
Assessments were made of whether or not corneal thickness showed any predictable association with body height (Fig. 1). For central corneal thickness (Fig. 1A), a weak and just statistically significant association was observed with taller subjects having lower corneal thickness values (Spearman’s correlation, P = .04, rho = −0.195). However, this association failed to reach statistical significance when simple linear regression was applied (P = .06, Pearson’s r = −0.180). The regression coefficient, also termed the slope of the regression, was −77.6 μm/m, indicating that for each 0.1 m (10 cm) increase in body height, the central corneal thickness would decrease by approximately 7.8 μm.
A statistically significant but weak inverse association was observed for nasal peripheral corneal thickness 4 mm from the apex (Fig. 1B, P = .004, r = −0.271), but this relationship failed to reach statistical significance temporally (Fig. 1C, P = .08, r = −0.169). Using Pearson’s regression or applying Spearman’s correlation analyses, central, mid-peripheral, and peripheral corneal thickness were inversely associated with body height at most locations along the horizontal meridian and consistently slightly stronger for nasal corneal thickness as compared to temporal measurements (Table 1). Using Spearman’s correlation, the strongest correlation was observed at 4 mm nasally (P = .003, rho = −0.280).
Statistically significant and positive associations were noted between height and corneal radius, with a taller stature being associated with a flatter corneal curvature (simple linear regression, P ≤ .002, Pearson’s r = 0.351). The effect size (regression coefficient) was +1.09 mm/m, indicating that for each 0.1 m (10 cm) difference in body height, corneal radius would differ by 1.1 mm (Fig. 2). Similar results were observed when applying Spearman’s correlation analysis (P = .002, rho = 0.351).
Applying regression analysis to the myopic and emmetropic subgroups, a slightly stronger effect size was observed between height and corneal radius for the myopic (Fig. 2B; P = .002, r = 0.423) but not for the emmetropic subjects (Fig. 2C; P = .1, r = 0.225). Applying Spearman’s analyses, the association was again confirmed in myopic subjects (P = .003, rho = 0.415), but the relationship failed to reach statistical significance in emmetropes (P = .11, rho = 0.220).
Multiple regression models (all subjects) with age, gender, height, and weight as independent variables returned body height as the only significant factor associated with corneal radius while controlling for the other independent variables (P = .02, R2 = 0.13). Multiple regression also indicated statistically significant interdependencies of body height with central corneal thickness (when adjusted for corneal radius) (P ≤ .02). Corneal thickness and radius were not associated (P = .35, r = 0.091).
The present cross-sectional analyses, on younger Caucasian adults, indicate that body stature could have a small contributory effect in determining corneal thickness of an individual. Perhaps more importantly, this body height–related effect is more pronounced for the progressive increases in thickness from the central cornea to the periphery. Such an association also seems to be linked to corneal curvature so that, overall, thinner and flatter corneas could be predicted for taller individuals and vice versa. The instrument used in the present studies was the Pentacam, which provides repeatable central, mid-peripheral, and peripheral corneal thickness readings, allowing for high-resolution and repeatable assessments of regional (geographic) differences in corneal thickness.27 Corneal thickness and other biometric measurements of the cornea are valuable in clinical assessments of corneal health2 and increasingly important in corneal and anterior segment surgical procedures.14 Assuming that the algorithms for generating corneal thickness profiles and the corneal curvature do provide independent measurement outcomes, the present analyses indicate that body stature could have a slightly greater effect on corneal radius than corneal thickness values.
Recent work has indicated that genetic factors have a greater contribution to the development of refractive error than environmental factors.28 As body height is strongly influenced by genetics,29 it seems appropriate to investigate the link between height and corneal radius in detail. Our study provides further evidence for the complex and multifactorial nature of the process steering emmetropization. The present study provides an important extension of previous research on the possible influence of body stature on corneal metrics, with a particular focus on a less frequently assessed cohort of younger Caucasian adults. However, the present analyses indicate that the effect size of any such relationship in younger adults was small.
Refractive error, and especially the development of myopia, is a considerable public health concern.30 Wu and colleagues discuss the complexity of refractive error development, including the influence of genetic and environmental factors. The results of their biometric study on more than 2000 adult Burmese subjects aged 40 years and older indicate that body stature may be associated with the development of refractive error, in that a moderate association (correlation coefficient 0.302) between body height and axial length was observed.11 The present study supports the notion of a possible association between body stature and refractive development in that body height and corneal radius were correlated.
Even though correlation does not imply causality,31 the present study adds a new and interesting perspective to the issue of refractive error development, specifically in younger Caucasian adults. Previous research has linked corneal radius with refractive error and corneal radius has been shown to be independently associated with refractive error even though the balance between structural components is the key determinant for the development of myopia. A recent study by Richdale and coworkers also indicated that a steeper corneal radius can be linked with increasing myopic refractive error in adults aged 30 to 50 years. The magnitude of the effect size (i.e. the regression coefficient) was −0.16, which was slightly lower compared to the effect size for the association between height and corneal radius noted in the present study (0.35).32
These findings and those of the present study support the sentiment that many factors including ocular variables, body stature, and environmental factors contribute jointly to the ultimate refractive error of an individual. Extending the concept of multi-causality, it could be anticipated that each of the contributing elements accounts for a limited proportion of the variability in refractive error only, leading to comparatively small effect sizes, even when the number of study participants is substantial.
Body stature (as assessed by measurements such as body height and weight) has been included in various analyses undertaken in a number of population-based studies to identify correlates of body development and risk factors for various ocular and systemic diseases.10–14,17,18,21 These studies provide little or no indication of the effect size and/or the clinical significance of body stature on corneal parameters (e.g. how much of a difference in body height would be needed to have a clinically important impact on corneal thickness or curvature). This type of outcome is different to simply assessing whether or not statistically significant effects or interactions were present.
In such studies, assessments of the relationship between body stature and corneal thickness or corneal radius have been undertaken, albeit not with such analyses being the primary study aim. Multiple regression models have been devised with corneal thickness or corneal radius as interdependent parameters alongside body stature. For example, body height has been considered as a factor contributing to differences in corneal thickness10–13,17,19,22 and radius of curvature of the cornea,14–16 but usually with inconsistent detail being provided to indicate the effect size or even the overall predictability of these effects. For most of these previously published cross-sectional studies, positive correlation coefficients were generally only around 0.1,10,11,17,18 although one study that included younger adults reported a correlation coefficient of 0.44 for the association between body height and corneal thickness.12 No body height effect was noted in other reports18,33 despite a significant effect for body weight being noted.18 These publications have not usually included examples of regression plots that could be derived from univariate analysis and which could indicate important characteristics such as the distribution of the data. Similarly, in most of the publications on this topic, little indication has been provided on the proportionality of the predicted interactions, especially as based on the effect size generated from regression analyses. Scatterplots and effect size analysis are provided in the present studies and help to highlight the tenuous nature of any possible correlations between body stature and corneal metrics.
In the cohort evaluated in the present studies, the relationship between body height and corneal thickness was inverse, whereas a positive relationship was noted between height and corneal radius. This is consistent with the outcome of a recent study reporting on older Caucasian subjects, where application of multiple regression analyses, including age as a factor, also indicated a negative correlation could exist between corneal thickness and body height.13 Multivariate analyses indicated that each 10 cm increase in body height would be associated with a 3.18-μm decrease in central corneal thickness. The outcome of the present study is in agreement with these results, i.e. taller people might have slightly thinner corneas and with the overall effect being of 8 μm/0.1 m (10 cm) height difference. In other multivariate analyses, an effect was noted,19 but no details were provided (especially as to whether the effect was positive or negative), whereas in other reports, no predictable relationship was evident.10–12,17 The effect size for the relationship between body stature and corneal radius was similarly small.
Most of the previously published studies were conducted on older adults with the minimum age usually being 40 years and extending to at least 80 years. In such studies, any contribution of stature (body height) to corneal parameters should be considered to be a residual effect of body and eye growth in infancy and childhood. The same limitation applies to the present cross-sectional studies on young adults, and it would be useful for longitudinal studies to be undertaken on body stature and corneal thickness during early childhood years alongside measures of axial length and refractive error.
Overall, although numerous studies have indicated that central corneal thickness in adults has a wide range, there does not seem to be a substantial influence of body stature on central corneal thickness. Although customizing refractive surgery based on an ever-increasing number of metrics could improve surgical outcomes, body stature is unlikely to be a significant parameter because effect size in this and in previous studies was consistently small. The same conclusion can be applied to IOP assessments, where the magnitude of any corneal difference is likely well below that which might affect tonometry outcomes.1 This seems to apply to different age and ethnic groups. Our structured review of the literature indicated that a detailed analysis on the topic does not seem to have been undertaken recently. However, earlier studies also show that larger sample sizes, such as those of population-based studies, while allowing for useful complex multivariate regression models and the controlling of a wide range of possible confounding variables, do not lead to any better predictability or to a larger effect size.
This study adds to the literature by providing a detailed review of the literature of the topic, a detailed consideration of effect sizes, and the addition of new data relating to the refractive error development specifically in younger Caucasian adults. The outcome of this study has relevance to the correction of myopia, including the surgical correction of refractive error, in particular laser refractive surgery. Studies investigating the effect of corneal radius on laser ablation depth have shown that the effective ablation depth decreases with an increasing corneal radius.34 Based on the outcome if our study, taller patients are likely to have flatter corneas and therefore require lesser ablation depths for a given surgical correction of their refractive error.
The study is potentially limited by the relatively small sample and by the restriction to one ethnic group. Further research is needed to assess whether or not body stature and corneal parameters are related in populations and ethnic groups with a high prevalence of myopia.
In summary, the evidence for meaningful interdependence of body height and corneal parameters seems to be weak and ambiguous. The same applies to considerations of interrelated metrics such as body weight and BMI. The effect sizes of any such relationships are relatively small with no more than 13% of variability in corneal parameters being accounted for by body height, while controlling for variations in age, gender, and weight. The outcome of the present studies and the objective analysis of the literature do not support the notion of including body stature in routine clinical practice such as preoperative assessments.
Vision Sciences, Department of Life Sciences
Glasgow Caledonian University
Glasgow, G4 0BA
This work was supported by funding from Santander Universities UK (SJ) and a travel grant by Birmingham Optical Group.
The authors have no financial interest in any of the products, materials, or methods mentioned in this article.
Received April 13, 2016; accepted October 4, 2016.
1. Doughty MJ, Zaman ML. Human corneal thickness
and its impact on intraocular pressure measures: a review and meta-analysis approach. Surv Ophthalmol 2000;44:367–408.
2. Pflugfelder SC, Liu Z, Feuer W, et al. Corneal thickness
indices discriminate between keratoconus and contact lens-induced corneal thinning. Ophthalmology 2002;109:2336–41.
3. Cummings AB, Cummings BK, Kelly GE. Predictability of corneal flap thickness in laser in situ keratomileusis using a 200 kHz femtosecond laser. J Cataract Refract Surg 2013;39:378–85.
4. Villavicencio O, Belin MW, Ambrosio R, et al. Corneal pachymetry: new ways to look at an old measurement. J Cataract Refract Surg 2014;40:695–701.
5. Ortiz S, Mena L, Rio-San Cristobal A, et al. Relationships between central and peripheral corneal thickness
in different degrees of myopia. J Optom 2014;7:44–50.
6. Müller A, Doughty MJ. Assessments of corneal endothelial cell density in growing children and its relationship to horizontal corneal diameter. Optom Vis Sci 2002;79:762–70.
7. Doughty MJ, Blades KA, Ibrahim N. Assessment of the number of eye symptoms and the impact of some confounding variables for office staff in non-air-conditioned buildings. Ophthalmic Physiol Opt 2002;22:143–55.
8. Selović A, Juresa V, Ivankovic D, et al. Relationship between axial length of the emmetropic eye and the age, body height
, and body weight
of schoolchildren. Am J Hum Biol 2005;17:173–7.
9. Gardiner PA. The relation of myopia to growth. Lancet 1954;266:476–9.
10. Tomidokoro A, Araie M, Iwase A, et al. Corneal thickness
and relating factors in a population-based study in Japan: the Tajimi study. Am J Ophthalmol 2007;144:152–4.
11. Wu HM, Gupta A, Newland HS, et al. Association between stature, ocular biometry and refraction in an adult population in rural Myanmar: the Meiktila eye study. Clin Exp Ophthalmol 2007;35:834–9.
12. Nangia V, Jonas JB, Matin A, et al. Body height
and ocular dimensions in the adult population in rural central India. The Central India Eye and Medical Study. Graefes Arch Clin Exp Ophthalmol 2010;248:1657–66.
13. Elflein HM, Pfeiffer N, Hoffmann EM, et al. Correlations between central corneal thickness
and general anthropometric characteristics and cardiovascular parameters in a large European cohort from the Gutenberg Health Study. Cornea 2014;33:359–65.
14. Yuen LH, He M, Aung T, et al. Biometry of the cornea and anterior chamber in Chinese eyes: an anterior segment optical coherence tomography study. Invest Ophthalmol Vis Sci 2010;51:3433–40.
15. Wong TY, Foster PJ, Johnson GJ, et al. The relationship between ocular dimensions and refraction with adult stature: the Tanjong Pagar Survey. Invest Ophthalmol Vis Sci 2001;42:1237–42.
16. Eysteinsson T, Jonasson F, Arnarsson A, et al. Relationships between ocular dimensions and adult stature among participants in the Reykjavik Eye Study. Acta Ophthalmol Scand 2005;83:734–8.
17. Zhang H, Xu L, Chen C, et al. Central corneal thickness
in adult Chinese. Association with ocular and general parameters. The Beijing Eye Study. Graefes Arch Clin Exp Ophthalmol 2008;246:587–92.
18. Su DH, Wong TY, Foster PJ, et al. Central corneal thickness
and its associations with ocular and systemic factors: the Singapore Malay Eye Study. Am J Ophthalmol 2009;147:709–716.e1.
19. Nishitsuka K, Kawasaki R, Kanno M, et al. Determinants and risk factors for central corneal thickness
in Japanese persons: the Funagata Study. Ophthalmic Epidemiol 2011;18:244–9.
20. Suzuki S, Suzuki Y, Iwase A, et al. Corneal thickness
in an ophthalmologically normal Japanese population. Ophthalmology 2005;112:1327–36.
21. Tomoyose E, Higa A, Sakai H, et al. Intraocular pressure and related systemic and ocular biometric factors in a population-based study in Japan: the Kumejima study. Am J Ophthalmol 2010;150:279–86.
22. Tan DK, Chong W, Tay WT, et al. Anterior chamber dimensions and posterior corneal arc length in Malay eyes: an anterior segment optical coherence tomography study. Invest Ophthalmol Vis Sci 2012;53:4860–7.
23. Zhou Q, Liang YB, Wong TY, et al. Intraocular pressure and its relationship to ocular and systemic factors in a healthy Chinese rural population: the Handan Eye Study. Ophthalmic Epidemiol 2012;19:278–84.
24. Aakre BM, Doughty MJ, Dalane OV, et al. Assessment of reproducibility of measures of intraocular pressure and central corneal thickness
in young white adults over a 16-h time period. Ophthalmic Physiol Opt 2003;23:271–83.
25. Kida T, Liu JH, Weinreb RN. Effect of 24-hour corneal biomechanical changes on intraocular pressure measurement. Invest Ophthalmol Vis Sci 2006;47:4422–6.
26. Keys A, Fidanza F, Karvonen MJ, et al. Indices of relative weight and obesity. J Chronic Dis 1972;25:329–43.
27. Jonuscheit S. Data extraction and reporting strategies of studies assessing non-central corneal thickness
by Pentacam: a review. Cont Lens Anterior Eye 2014;37:323–30.
28. Chua SY, Ikram MK, Tan CS, et al. Relative contribution of risk factors for early-onset myopia in young Asian children. Growing Up in Singapore Towards Healthy Outcomes Study Group (GUSTO). Invest Ophthalmol Vis Sci 2015;56:8101–7.
29. Jelenkovic A, Sund R, Hur YM, et al. Genetic and environmental influences on height from infancy to early adulthood: an individual-based pooled analysis of 45 twin cohorts. Sci Rep 2016;6:28496.
30. Morgan IG, Ohno-Matsui K, Saw SM. Myopia. Lancet 2012;379:1739–48.
31. Armstrong RA, Eperjesi F, Gilmartin B. The use of correlation and regression methods in optometry. Clin Exp Optom 2005;88:81–8.
32. Richdale K, Bullimore MA, Sinnott LT, et al. The effect of age, accommodation, and refractive error on the adult human eye. Optom Vis Sci 2016;93:3–11.
33. Pan CW, Li J, Zhong H, et al. Ethnic variations in central corneal thickness
in a rural population in China: the Yunnan Minority Eye Studies. PLoS One 2015;10:e0135913.
34. Mrochen M, Seiler T. Influence of corneal curvature on calculation of ablation patterns used in photorefractive laser surgery. J Refract Surg 2001;17:S584–7.