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

Figures & data

Table 1 Patient demographics

	All patients	Males	Females	P-value
Sex
Males	26 (31%)	—	—	—
Females	58 (69%)
Age (years)
Mean±SD	26.9±4.7	25.9±3.8	27.4±5.0	0.214
Range	19–38	22–34	19–38
Race				0.006*
Caucasian	44 (52%)	10 (38%)	34 (59%)
African American	10 (12%)	3 (12%)	7 (12%)
Asian	30 (36%)	13 (50%)	17 (29%)
Sphere (diopters)				0.643
Mean±SD	−3.31±1.59	−3.38±1.64	−3.28±1.58
Range	−0.50– −7.25	−0.50– −7.25	−0.75– −7.25
Cylinder (diopters)
Mean±SD	0.57±0.28	0.54±0.26	0.59±0.29	0.450
Range	0.25–1.25	0.25–1.25	0.25–1.25
Flat K (diopters)				0.010**
Mean±SD	43.34±1.22	42.98±1.21	43.50±1.20
Range	40.50–47.00	40.50–45.00	40.75–47.00
Steep K (diopters)
Mean±SD	44.20±1.31	43.69±1.30	44.43±1.26	0.002***
Range	41.25–48.25	41.25–46.25	41.25–48.25

Notes: Data represented as mean±standard deviation (SD). Range (min–max). *chi-square test. **Student’s t-test. ***Mann-Whitney Rank Sum test.

Abbreviation: K, keratometry.

Table 2 Central corneal epithelial and stromal thickness

Central 3 mm region: grouped by eye	All M±SD Range CV	OD M±SD Range CV	OS M±SD Range CV	P-value (OD vs OS)
Epithelial thickness (µm)	54.3±2.8 48–64 5.2%	54.4±2.9 48–63 5.2%	54.2±2.8 49–64 5.2%	0.798
Stromal thickness (µm)	475.4±31.9 412–550 7.0%	475.3±32.1 412–550 6.7%	475.5±31.9 413–550 6.7%	0.977
Total corneal thickness (µm)	529.7±32.8 465–611 6.2%	529.7±32.9 465–609 6.2%	529.7±33.0 465–611 6.2%	0.998
Central 3 mm region: grouped by sex		Males M ± SD Range CV	Females M ± SD Range CV	P-value (M vs F)
Epithelial thickness (µm)		55.8±2.9 50–63 5.2%	53.6±2.8 48–64 5.2%	<0.001*
Stromal thickness (µm)		481.1±31.4 417–550 6.5%	471.5±32.6 412–544 6.9%	0.131
Total corneal thickness (µm)		540.5±30.2 496–611 5.8%	525.2±33.0 465–594 6.3%	0.020*

Notes: Data represented as mean±standard deviation (SD). Range (min–max). *Mann-Whitney rank sum test.

Abbreviation: CV, coefficient of variation.

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.

Table 3 Corneal epithelial and layer offset values for thin vs non-thin corneas

	All mean±SD	Thin mean±SD	Non-thin mean±SD	P-value (T vs NT)
Layer offset thickness
SN-IT (2–5 mm)	23.4±11.1	22.8±12.0	23.6±10.9	0.801
S-I (2–5 mm)	19.1±12.0	16.9±11.9	19.8±11.9	0.101
Min (mm)	522.8±32.9	478.1±12.3	536.0±24.3	<0.001*
Min–Median (mm)	−23.5±4.2	−22.4±3.9	−23.9±4.3	0.042*
Min–Max (mm)	−63.2±13.5	−60.4±12.2	−64.1±13.7	0.141
Epithelial statistics within central 5 mm
S (2–5 mm)	54.3±3.3	53.8±3.2	54.5±3.3	0.350
I (2–5 mm)	55.4±3.1	54.8±2.8	55.5±3.1	0.341
Min (mm)	51.7±3.3	50.5±3.9	52.1±3.0	0.025*
Max (mm)	57.5±3.4	57.1±3.1	57.6±3.5	0.750
SD	1.3±0.6	1.5±0.7	1.3±0.6	0.010*
Min–Max (mm)	−5.8±2.6	−6.7±3.6	−5.5±2.2	0.009*

Notes: Data represented as mean±standard deviation (SD). *Mann-Whitney Rank Sum test.

Abbreviations: I, inferior; IT, inferior temporal; S, superior; SN, superior nasal.

Table S1 Comparison of OD vs OS for all geographic regions within the 5 mm zone

Region	Epithelial thickness (µm)			Stromal thickness (µm)
	OD M±SD Range CV	OS M±SD Range CV	P-value	OD M±SD Range CV	OS M±SD Range CV	P-value
Superior	54.2±3.3 46–64 6.2%	54.5±3.2 47–63 5.9%	0.400	510.1±35.8 441–609 7.0%	509.4±35.5 443–600 7.0%	0.900
Superior nasal	54.7±3.2 47–65 5.9%	54.7±3.2 47–64 5.9%	0.853	509.4±35.1 447–605 6.9%	507.4±34.8 441–598 6.9%	0.721
Nasal	54.8±2.9 48–63 5.3%	54.9±3.1 48–64 5.6%	0.921	501.3±33.4 436–580 6.7%	499.6±34.3 434–583 6.9%	0.735
Inferior nasal	55.3±2.9 49–62 5.3%	55.4±3.1 50–64 5.6%	0.947	495.7±32.6 431–566 6.6%	494.3±34.2 431–576 6.9%	0.755
Inferior	55.5±3.1 50–63 6.0%	55.2±3.1 50–63 5.6%	0.619	489.5±32.7 421–563 6.7%	489.3±33.8 420–574 6.9%	0.970
Inferior temporal	54.7±3.1 49–64 5.7%	54.3±3.0 50–63 5.6%	0.534	484.7±32.9 411–560 6.8%	485.9±32.9 410–564 6.8%	0.812
Temporal	53.9±3.1 47–62 5.7%	54.0±3.2 48–62 5.9%	0.728	486.3±33.8 422–566 7.0%	487.1±33.3 420–562 6.8%	0.693
Superior temporal	53.7±3.3 45–63 6.2%	54.1±3.3 47–63 6.1%	0.498	498.7±34.8 433–588 7.0%	498.7±34.5 432–580 6.9%	0.988

Abbreviations: OD, right eye; OS, left eye; C, coefficient of variation.

Table S2 Comparison of OD vs OS for all geographic regions within the 6 mm zone

Region	Epithelial thickness (µm)			Stromal thickness (µm)
	OD M±SD Range CV	OS M±SD Range CV	P-value	OD M±SD Range CV	OS M±SD Range CV	P-value
Superior	54.0±3.5 47–63 6.5%	53.9±3.4 48–64 6.3%	0.993	546.7±39.1 475–656 7.2%	545.7±38.4 470–645 7.0%	0.866
Superior nasal	54.9±3.4 47–65 6.3%	54.9±3.5 48–64 6.4%	0.885	543.1±39.6 455–657 7.3%	539.0±36.7 470–645 6.8%	0.488
Nasal	55.3±2.9 50–64 5.3%	55.4±3.0 49–64 5.4%	0.718	530.1±35.8 462–622 6.8%	527.2±36.2 459–623 6.9%	0.603
Inferior nasal	55.7±3.0 50–64 5.4%	56.1±3.3 50–65 5.9%	0.403	522.7±34.2 452–607 6.5%	521.3±36.0 453–607 6.9%	0.791
Inferior	56.2±3.4 51–65 6.0%	56.0±3.2 51–64 5.8%	0.625	516.1±33.8 445–595 6.5%	515.8±35.3 445–606 6.8%	0.957
Inferior temporal	55.3±3.2 50–64 5.7%	55.0±3.1 50–65 5.7%	0.437	506.9±34.1 429–585 6.7%	507.8±34.4 425–583 6.8%	0.867
Temporal	54.3±3.0 48–64 5.6%	54.0±3.1 49–64 5.7%	0.646	506.2±34.1 439–590 6.7%	508.0±34.5 437–582 6.8%	0.738
Superior temporal	53.9±3.1 47–63 5.8%	54.1±3.4 48–64 6.2%	0.581	528.0±36.9 458–624 7.0%	527.3±36.3 456–614 6.9%	0.909

Abbreviations: OD, right eye; OS, left eye; C, coefficient of variation.