Quantitative cataract measurement with the Scheimpflug technique

Abstract

Cataract is the most prevalent blinding eye disease. Scheimpflug photography is an objective method for documentation of lens nuclear opacity. Using the Scheimpflug slit-lamp technique, a photograph, in focus from the anterior surface of the cornea to the posterior surface of the lens, can be obtained. Measurement of the lens image density provides quantitative information of nuclear opacity. However, the methods for evaluation of lens opacity with Scheimpflug photography are still under improvement.

A method for the calibration of measurements in a Scheimpflug photograph was established. There is a significant variation of density among films and among photographs within a film. Therefore, each photograph should be calibrated individually. The calibration curve can be expressed as a third-order polynomial. The precision of the calibration was estimated to 2%.

A common lens nuclear area (CNA) was defined in order to optimize detection of lens nuclear opacity and to standardize the procedure for quantitative measurements in Scheimpflug images. The CNA was determined based on analysis of variance of the location and shape of the nuclear zone among individuals. The risk to wrongly include the anterior and posterior cortex in the CNA is 2.5%, respectively.

The density distribution in a vertical cross section of the normal lens was assessed. There is a significant variation of nuclear density in a direction perpendicular' to the anatomical axis of the lens. This variation can be modeled as a second-order polynomial. The regression coefficients of the polynomial can be used as indices for evaluation of nuclear cataract development.

A universal opacity standard was developed for comparison of measurements from different studies using Scheimpflug cameras. An aqueous suspension of polystyrene spheres was selected as the universal standard. The relationship between the concentration of polystyrene spheres and the gray scale unit of the Topcon camera and the Nidek system, respectively, was established. It was found that a second-order polynomial is sufficient to convert the measurement from the camera to the concentration of the suspension. The precision of the conversion for the Topcon camera and the Nidek system is 8 and 5%, respectively.

The least number of meridians which are needed to optimize detection of lens nuclear opacity was determined. A possible angular change, caused by the instrument, was examined by using an aqueous suspension of polystyrene spheres as a standard. There, is a significant angular change of back scattered light from the human cataract nucleus. The variation of the back scattered light at different angles is partly caused by the variation of the camera. It is suggested that one meridian at 90' is sufficient for estimation of lens nuclear cataract.

In a clinical study a possible side effect on the lens nuclear opacity, caused by a cholesterol-lowering drug simvastatin, was investigated using the established method. No statistically significant difference was found between the simvastatin and placebo groups. However, there was a big variation among individuals and the samples were limited in size.