Video controlled M-mode biometry

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.

B-mode-guided vector-A-mode versus A-mode biometry to determine axial length and intraocular lens power

PURPOSE

To compare prospectively the reproducibility and accuracy of B-mode-guided biometry with those of A-scan biometry using a conventional A-mode probe to calculate intraocular lens (IOL) power.

SETTING

Department of Ophthalmology, Hôtel-Dieu de Paris, France.

METHODS

The axial length (AL) in 87 eyes of 72 candidates for cataract surgery was determined by B-mode-guided vector-A-mode and A-mode biometry using an Ophthascan S Ultrasound imager. Patients were assigned to one of two groups based on the B-mode biometry: nonmyopic (AL < 24.5 mm; n = 54) or myopic (AL > 24.5 mm; n = 33). Postoperative refractive results were compared with attempted values.

RESULTS

Mean AL variance was significantly greater when using the A-mode than the B-mode: 0.157 mm +/- 0.260 (SD) versus 0.015 +/- 0.018 mm in the myopic group (P < .001) and 0.024 +/- 0.024 +/- 0.045 versus 0.009 +/- 0.011 mm in the nonmyopic group (P < .001). More eyes having B-mode biometry achieved a final refraction within +/- 0.50 diopter (D) of the attempted refraction (63 and 43%, respectively; P < .05). No deviation greater than 1.60 D was observed with the B-mode in the myopic or nonmyopic group. Three cases with a such a deviation (up to 2.24 D) would have been observed had A-mode-based biometry been chosen for the IOL power calculation. In the myopic group, attempted postoperative refraction was within +/- 0.50 D in 78% of eyes having B-mode biometry compared with 65% having A-mode. This difference was not statistically significant. CONCLUSION> These results suggest that the reproducibility and accuracy of AL measurements are significantly better with B-mode-guided A-mode biometry than with A-mode biometry in myopic and nonmyopic eyes.