The effect of illumination-microscope angle on slit lamp estimate of the anterior chamber depth

The anterior chamber depth can be estimated by the slitlamp without additional attachments. The method involves increasing the length of a horizontal slit focused on the cornea until the corneal and iris/lens images appear to just touch. For a slitlamp angle of 60 degrees, multiplying the just-touching slit length (JTSL) by 1.4 gives an estimate of the anterior chamber depth as measured by ultrasonography. The purpose of this study was to determine whether the slitlamp angle affects the accuracy of the procedure.

The anterior chamber depth of 63 adult male and female subjects aged between 19 and 44 years was measured by A-scan ultrasonography and estimated by the slitlamp technique. The angle between the illumination and microscope systems was varied at 10 degrees intervals from 30 degrees to 70 degrees.

The average ultrasonographic anterior chamber depth for all subjects was 3.23 +/- 0.38 mm. The average JTSL was as follows: 30 degrees = 1.5 mm, 40 degrees = 1.7 mm, 50 degrees = 2.1 mm, 60 degrees = 2.3 mm, and 70 degrees = 2.5 mm. The conversion ratio between the ultrasonographic anterior chamber depth and the average JTSL was as follows: 30 degrees = 2.2, 40 degrees = 1.9, 50 degrees = 1.5, 60 degrees = 1.4, and 70 degrees = 1.3. The ultrasonic anterior chamber depth was best estimated at 60 degrees. The 95% limits of agreement between the estimated and measured anterior chamber depth were as follows: 30 degrees = -0.76 to +0.84 mm, 40 degrees = -0.75 to +0.63 mm, 50 degrees = -0.36 to +0.46 mm, 60 degrees = -0.26 to +0.28 mm, and 70 degrees = -0.45 to +0.41 mm.

These results experimentally confirm Smith's suggestion that the optimum slitlamp angle at which to accurately estimate the anterior chamber depth is 60 degrees. At this angle, estimates of the anterior chamber depth are minimally deviated from the gold standard (A-scan ultrasonography). At a slitlamp angle of 60 degrees, the JTSL can be converted to an estimate of the ultrasonic anterior chamber depth by multiplying it by 1.40.