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.