Estimation of the thickness of the crystalline lens from on-axis and off-axis Scheimpflug photographs

Agreement between Scheimpflug photography and A-scan ultrasonography in anterior segment ocular measurements in children

PURPOSE

The Scheimpflug principle, as a method of performing biometry on the anterior segment, has not been widely used in the clinical setting compared with ultrasonography. The purpose of this study was to examine the agreement in anterior chamber length and lens thickness measurements between A-scan ultrasonography and Scheimpflug photography.

METHODS

A total of 252 children aged between 6 and 12 years participated in this study. All subjects had instillation of cycloplegic eye drops before measurement. Scheimpflug photography and A-scan ultrasonography were performed in this sequence on the same day. The anterior segment length on the Scheimpflug images was taken as the axial corneal thickness added to the anterior chamber depth. A-scan ultrasonography was performed by placing the probe on the corneas after topical anesthesia. Differences were obtained by subtracting the A-scan ultrasonography measurements from the Scheimpflug measurements. Data were analyzed using difference vs. mean plots.

RESULTS

For anterior chamber length, the mean difference between the two methods was -0.03 +/- 0.16 mm, with 95% limits of agreement of -0.35 to +0.30 mm. However, Scheimpflug readings were greater than that of ultrasonography for smaller measurements, and the opposite was true for larger measurements. For lens thickness, the mean difference was -0.2 +/- 0.10 mm, with 95% limits of agreement of -0.40 to +0.00 mm. For the measurement of lens thickness, adding a correction factor of 0.2 mm to Scheimpflug's readings should replace that of A-scan readings reasonably well.

CONCLUSIONS

The readings obtained from these two methods of biometry must be used with the knowledge of the instruments. We discuss the possible reasons for the difference in readings obtained using these two instruments.

The optics of the eye-lens and lenticular senescence. A review

Although the lens of the eye is structurally a biological tissue, it functions as an optical element providing one third of the refracting power of the human eye, and a variable focus in younger years. Throughout a life-time the optical properties of the eye-lens alter, resulting in changes in function: there is a gradual depletion of the focussing amplitude from infancy to middle age, and a loss of transmittance in the later decades of life. The optical properties of the lens depend on its power, which in turn is determined by its physical dimensions (curvatures and thickness) and its refractive index as well as transmissivity and the organization of its internal components. The power of the functional lens is, however, modifiable by virtue of the lens being attached via the zonule to the ciliary muscle. The contraction and relaxation of the latter respectively increases and decreases lens power in accordance with innervations determined by the physical distance of external objects to be imaged on the retina. This review will consider many of these features and how alterations in any of them may lead to changes in lenticular function. However, as we have recently devoted a detailed study to presbyopia [1] its mechanism will not be considered here.