The frequency of non-pathologically thin corneas in young healthy adults

Abstract

Purpose: Measurement of normal corneal thickness and corneal epithelial thickness is important in keratorefractive surgery, glaucoma, following extended contact lens wear, and in patients with corneal disease. Clinically, a central corneal thickness less than 500 µm is considered to be moderately-to-extremely thin. The purpose of this study was to compare biological differences in patients with clinically thin compared to normal corneal thickness values in healthy young adults using Fourier domain optical coherence tomography. Patients and methods: In total, 168 eyes from 84 patients aged 19-38 years were scanned using an Avanti optical coherence tomographer. To eliminate circadian effects on corneal thickness, all patients were scanned within a 4-hour window. Corneal thickness was measured across the central 6 mm of the cornea. Total central corneal thickness, corneal epithelial thickness, and corneal stromal thickness were compared between males and females and tested for correlations with age, use of systemic hormones, degree of myopia, and corneal curvature. Results: The average central corneal thickness for males and females was 540.5±32.0 μm and 525.2±33.0 μm, respectively (P=0.020). Thirty-eight eyes had corneal thickness measurements below 500 µm; 12% (6 eyes) from males and 28% (16 eyes) from females (P=0.008). All women with corneas below 500 μm were bilaterally thin. This finding differed for men. Corneal thinning was not associated with age, use of systemic hormones, or degree of myopia. Females had steeper keratometry (K) readings (P=0.01 for flat K, P=0.002 for steep K) than males. No differences in layer offset values between normal thickness corneas and thin corneas were evident, suggesting that the reduced thickness was not pathological. Conclusion: The results of this study indicate that a subpopulation of healthy young adults have non-pathologically thin corneas, well below 500 μm; and that these thinner corneas are more frequent in females. This underscores the importance of accurate corneal thickness measurements prior to keratorefractive surgery and when evaluating intraocular pressure in glaucoma.

Keywords: OCT; cornea; epithelial; stroma; thickness.

Figure 1

Epithelial and stromal thickness within 2–5 mm zone (5 mm) and the 5–6 mm zone (6 mm) measured across all eight regions. (A) In both the 5 mm and 6 mm corneal zones, the corneal epithelium was thickest inferiorly and inferior nasally and thinnest superiorly (*P<0.001 for N compared to ST, S, and T; **P<0.05 for IN compared to ST, S, T, SN, IT, and N; ***P<0.05 for I compared to ST, S, T, SN, IT, N, and IN; ****P>0.05 for IT compared to ST, S, and T; *****P<0.05 for SN compared to ST, S, and T; One-way ANOVA, Student-Newman-Keuls post hoc multiple comparison test). Epithelial thickness values at all regions were significantly different between the 5 mm and 6 mm zones (P=0.009). (B) Unlike the corneal epithelium, corneal stromal thickness was thickest superiorly and thinnest in the inferior temporal region (*P<0.001 for S compared to all other regions except SN; **P<0.001 for SN compared to all other regions; ***P<0.001 for N compared to IT, T, and I; ****P<0.001 for ST compared to IT, T, and I; *****P<0.05 for IN compared to IT, T, and I; and ******P<0.05 for I compared to IT and T; One-way ANOVA, Student-Newman-Keuls post hoc multiple comparison test). Overall stromal thickness was uniformly thinner within the 5 mm zone compared to the 6 mm zone (P<0.001). Data presented as mean±standard error. Abbreviations: I, inferior; IN, inferior nasal; IT, inferior temporal; N, nasal; S, superior; SN, superior nasal; ST, superior temporal; T, temporal.

Figure 2

Corneal epithelial and stromal thickness in males vs females. (A) Epithelial thickness within the 5 mm zone was significantly thicker in the inferior and inferior nasal region (*P<0.05 for IN compared to ST, T, and S; **P<0.05 for I compared to ST, T, and S; Two-way ANOVA, Student-Newman-Keuls post hoc multiple comparison test). Across all regions, the epithelium was thicker for males compared to females (P<0.001). (B) Stromal thickness within the 5 mm zone was thickest superiorly (*P<0.05 for S compared to IT, T, I, IN, and ST; **P<0.05 for SN compared to IT, T, I, and IN; ***P<0.05 for N compared to IT, T, and I; ****P<0.05 for ST compared to IT and T; Two-way ANOVA, Student-Newman-Keuls post hoc multiple comparison test). Stromal thickness was greater in males than females in all regions (P<0.001). (C) In the 6 mm zone, the epithelium was also thickest in the inferior and inferior nasal region (*P<0.05 for IN compared to ST, T, and S; **P<0.05 for I compared to ST, T, and S; Two-way ANOVA, Student-Newman-Keuls post hoc multiple comparison test). Similar to the 5 mm zone, the epithelium was thicker for males than females across all regions (P<0.001). (D) Stromal thickness within the 6 mm zone was also thicker superiorly, with a greater difference between the superior and inferior stroma (*P<0.001 for S compared to all other regions except SN; **P<0.05 for SN compared to all other regions; ***P<0.05 for N compared to all other regions except T, IT, and I; ****P<0.05 for ST compared to T, IT, and I; *****P<0.05 for IN compared to T and IT; Two-way ANOVA, Student-Newman-Keuls post hoc multiple comparison test). Again, males had a greater stromal thickness than females across all regions (P<0.001). Data presented as mean±standard error. Abbreviations: I, inferior; IN, inferior nasal; IT, inferior temporal; N, nasal; S, superior; SN, superior nasal; ST, superior temporal; T, temporal.

Figure 3

Distribution of central corneal thickness across all subjects. Data is plotted as the normalized number of males and females per total number in each group.