The effect of cycloplegia on measurement of the ocular components

The purpose of this study was to examine the effect of cycloplegic agent on the measurement of refractive error and the ocular components.

We compared two commonly used topical cycloplegic agents, 1% tropicamide and 1% cyclopentolate, for their effect on the measurement of refractive error (by Canon R-1 autorefraction), accommodative response (by Canon R-1 autorefraction and by the conventional, subjective "pushup" method), crystalline lens power (by video phakometry and by calculation), and axial ocular dimensions (by A-scan ultrasonography) in 20 emmetropic to moderately hyperopic children.

Comparison of refractive error at each drug's reported time of maximum cycloplegia (30 minutes for tropicamide and 60 minutes for cyclopentolate) showed that distance autorefraction in the vertical meridian differed by +0.20 +/- 0.30 diopters (D) (P = 0.008). The average difference was +0.07 +/- 0.10 mm for anterior chamber depth (P = 0.004), -0.03 +/- 0.05 mm for crystalline lens thickness (P = 0.025), -0.65 +/- 0.69 D for phakometrically measured crystalline lens power (P < 0.001), +0.03 +/- 1.55 D for calculated crystalline lens power (P = 0.94), and -0.09 +/- 0.19 mm for vitreous chamber depth (P = 0.062, all paired t tests; positive signs denote greater values with cyclopentolate). Residual accommodation was 0.47 and 0.67 D greater with tropicamide when measured by autorefraction and the pushup method (P = 0.013 and 0.08 respectively, paired t test). All significant differences were consistently in the direction of poorer cycloplegia with tropicamide.

Although tropicamide, as expected, showed poorer cycloplegia compared to cyclopentolate, the degree of difference appeared to be small, with minimal effect on the measurement of distance refractive error and the ocular optical components.