Corneal birefringence measurements in normal Indian eyes

Sir,

Studies investigating the corneal birefringence measurements are largely restricted to Caucasian population.^{1, 2, 3} We therefore evaluated the corneal birefringence measurements in normal subjects of Indian origin.

Case report

In a cross-sectional study, 140 eyes of 73 subjects of Indian origin (37 men and 36 women) underwent corneal birefringence measurements with scanning laser polarimetry (GDxPRO, Carl Zeiss Meditec Inc., Dublin, CA, USA). Inclusion criteria were best-corrected visual acuity of 20/30 or better, spherical refraction within ±5.0 D and cylinder correction within ±2.0 D. Subjects were excluded if they had any ocular surface, corneal or macular pathology, or a history of previous corneal or intraocular surgery. Measures of corneal birefringence obtained were corneal polarization axis (CPA) and corneal polarization magnitude (CPM).⁴ Corneal scans were retaken if the quality score was <8. Central corneal thickness was measured using the ultrasound pachymetry.

Table 1 and Figure 1 show the distribution of CPA and CPM. Median CPA was 3° nasally downwards with 85% of the eyes having a CPA value between 9° nasally downwards and 5° nasally upwards. The mean CPM was 96 nm (95% CI: 92.4–99.0). There was a significant (P<0.001) correlation between the fellow eyes both for CPA (R²=0.57) and CPM (R²=0.72) measurements (Figure 2). Figure 3 shows that CPA and CPM were significantly (R²=0.12; regression coefficient, β=−0.91; P<0.001) associated with each other.

Table 1. Demographic and characteristic features of the study subjects (140 eyes of 73 subjects).

	Mean±SD	Range
Age (years)a	41 (22, 54)	19–68
Central corneal thickness (μm)	527±29	457–611
Corneal polarization magnitude (nm)	96±20	57–140
Corneal polarization axis (degrees)a	3.0 (−0.7, 6.1)	−42.3–23.4

^aMedian with first and third quartiles.

Figure 1.

Distribution of corneal polarization axis (a) and magnitude (b) in the study cohort.

Figure 2.

Scatterplot showing the correlation between (a) corneal polarization axis (CPA) and (b) corneal polarization magnitude (CPM) measurements of the right and left eye of 67 subjects whose both eyes were imaged.

Figure 3.

Correlation between corneal polarization axis and magnitude.

Age, gender, and corneal thickness showed no significant association with corneal birefringence measurements (Table 2).

Table 2. Results of linear mixed model (after accounting for the correlations between fellow eyes of the same subject) showing the effects of age, gender and corneal thickness on corneal birefringence measurements.

Parameter	Age		Gender		CCT
	β (SE)	P	β (SE)	P	β (SE)	P
CPA	0.03 (0.06)	0.54	−1.08 (1.78)	0.55	0.003 (0.01)	0.84
CPM	−0.02 (0.15)	0.90	4.74 (4.83)	0.33	−0.01 (0.03)	0.83

Abbreviations: CCT, central corneal thickness; CPM, corneal polarization magnitude; CPA, corneal polarization axis; β, coefficient; SE, standard error.

Comments

Wide variability in CPA and CPM has been reported in Caucasian eyes too.^{1, 2, 3} However, CPA values in Caucasians were mostly between 10° and 20° nasally downwards, which were significantly different from that found in Indian eyes. Wider range of CPM values was found in the Caucasian eyes, with 20% of eyes showing CPM values lower than 40 nm and higher than 140 nm.² CPM reported by Weinreb et al.³ also ranged between 22 and 150 nm. These differences in corneal birefringence properties may indicate differences in the internal structure of the cornea, which may be responsible in part to the differences in the prevalence and progression of corneal ectatic conditions like keratoconus between ethnicities.⁵

Rao HL and Garudadri CS are consultants at Allergan. Garudadri CS is also a consultant at Merck and Alcon. All other authors declare no conflict of interest.