CATARACT CLASSIFICATION

Opacifications of the eye lens - generally defined as cataracts - deve lop in various different parts of the lens. Therefore, one has to diff erentiate the types of opacities. For epidemiological studies it is pr erequisite to classify the cataracts according to their localization w ithin the lens as well as to the size and intensity of the opacified a rea. Two approaches have been used in the past: 1) subjective methods of lens observation (based on slit lamp microscopy) and 2) objective m ethods with measurements of lens transparency or lens opacity respecti vely based on slit image documentation according to the Scheimpflug pr inciple combined with the retroillumination technique. With ageing, th e light transparency of the lens is subjected to considerable changes. Even without the formation of an opacity the transmission of the wave lengths in the UV-B/UV-A and the visible range is diminished. The sing le lens layers are affected by this phenomenon to different degrees. T hese changes which might also indicate an early stage of 'cataract for mation' cannot be discerned by subjective methods. The densitometric i mage analysis of Scheimpflug slit images, however, allows the exact me asurement of the light scatter in the single lens layers and enables t he early recognition of disturbances in transparency which is of cruci al importance particularly in cataract epidemiology. In view of our pr esent knowledge the evaluation of risk factors which might be of impor tance in multifactorial cataract processes will hardly be possible by carrying out prevalence and/or incidence studies involving a single ex amination of the population. In this case follow-up studies (cohort st udies) with repeated examinations are prerequisite. The 'objective met hods' for classification alone are able to ensure the necessary reprod ucibility and the possibility to measure transparency changes in the l ens before visible (and therefore subjectively recognizable) opacifica tions occured. The methodical procedure with respect to an epidemiolog ical study on the involvement of UV-B radiation in the processes of ca taract formation in man requires the application of objective methods for cataract classification.