Age-related changes in with-the-rule and oblique corneal astigmatism

PURPOSE

To describe the age-related changes in with-the-rule (WTR) and oblique keratometric astigmatism (KA), posterior corneal astigmatism (PCA) and total corneal astigmatism (TCA).

METHODS

We used a Pentacam HR (high-resolution) rotating Scheimpflug camera to determine the KA, PCA and TCA in the right eyes of 710 patients, aged from 20 to 88 years. The age-related changes along the vertical, horizontal and oblique meridians were analyzed with Naeser's polar value method in a cross-sectional study.

RESULTS

In the whole group, all meridional astigmatic powers and polar values were stable in the age groups from 20 to 49 years, followed by a 1.0 dioptre (D) against-the-rule (ATR) change in KA and TCA, and a 0.12 D reduction in against-the-rule PCA. A nasal rotation of the steep meridian in KA and TCA was noted in the 70-88 years old. The PCA averaged approximately 0.25 D ATR in all age groups. Females displayed the same early astigmatic stability as in the whole group, while male eyes demonstrated a linear decay from 1.5 D WTR at 20 years to 0.5 D ATR astigmatism for the oldest patients.

CONCLUSION

Corneal astigmatism is stable until the age of 50 years; thereafter both keratometric and total corneal astigmatism show a 0.25 D ATR change per 10 years. The average 0.25 D ATR PCA compensates the predominant keratometric WTR astigmatism in the younger patients and increases the TCA in the elderly with keratometric ATR astigmatism. The gender-based differences in age-related astigmatism require further studies.

Corneal powers measured with a rotating Scheimpflug camera

BACKGROUND/AIM

Keratometry measures the anterior corneal curvature only. Corneal power is calculated by multiplication with the keratometric refractive index, which takes into account the average negative posterior corneal power. The aim of this study was to calculate and compare various expressions for total corneal power assessed with Scheimpflug camera techniques, which also measure the posterior corneal curvature.

METHODS

We used the Pentacam rotating Scheimpflug camera to measure the equivalent power, total corneal refractive power (based on Snell's law ray tracing), and simulated keratometry (keratometric refractive index=1.3375) over the central 3.0 mm zone in 951 eyes. The keratometric refractive index of the equivalent power and the total corneal refractive power was calculated as the ratio between these values and the anterior corneal curvature.

RESULTS

The equivalent power, total corneal refractive power, and simulated keratometry all differed statistically significantly (analysis of variance, p<0.001) and averaged 42.26 (±1.46) dioptres (D), 42.78 (±1.51) D and 43.42 (±1.49) D. The calculated keratometric refractive indices for equivalent and total corneal refractive power averaged 1.3284 (±0.0009) and 1.3324 (±0.0015). The error of using these calculated keratometric refractive indices rather than the measured values for equivalent and total corneal refractive power averaged 0 (±0.11 D) and -0.01 D (±0.19).

CONCLUSIONS

Pentacam rotating camera assessment of total corneal power over the central 3.0 mm zone differed significantly for simulated keratometry, equivalent power and Snell's law ray tracing.