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.
Age, gender, and corneal thickness showed no significant association with corneal birefringence measurements (Table 2).
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
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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.
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Rao, H., Yadav, R., Addepalli, U. et al. Corneal birefringence measurements in normal Indian eyes. Eye 26, 1589–1591 (2012). https://doi.org/10.1038/eye.2012.184
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DOI: https://doi.org/10.1038/eye.2012.184