Screening for diabetic retinopathy : aspects of photographic methods

Abstract

Diabetic retinopathy (DRP) is a major cause of acquired blindness and visual impairment among people of working age as well as those aged 65 years or more. About 3-4% of the population has diabetes mellitus, 35-65% of the diabetic patients have some type of DRP and 10-35% have sight-threatening retinopathy. Regular eye examinations with a sensitive method are important in order to detect the treatment needing, usually asymptomatic, lesions in time. Only then laser treatment reduces the risk of visual impairment by 50-90%.

Forty-five degree photography from one or two fields using colour transparencies or Polaroid pictures has so far been the predominant photographic screening method. Field definitions, especially for the macular field, have varied according to different recommendations. Sixty degree wide-field photography offers large field coverage and might, despite less magnification, improve detection of diabetic retinopathy abnormalities. Furthermore, the detectability of vascular structures and red lesions might improve when using red-free light at photography. The technique of using a monochromatic green filter enhances the contrast of retinal blood vessels and haemoglobin containing structures.

In this methodological study varying photographic screening methods for the detection of DRP were evaluated. Using wide-field 6C degrees cameras it was studied whether DRP was more easily detected from red-free film based or digital black-and-white pictures as compared to corresponding colour transparencies. Furthermore, it was evaluated whether two 60 degrees photographic fields were needed or whether one 60 degrees field was enough for screening purposes. The field coverage of one and two 60 degrees fields was compared with that of the Gold Standard (30 degrees seven-field photography). We also studied how retinal neovascularizations (NVEs) were detected from one and two 45 degrees fields and compared the results with that of one- and two-field 60 degrees photography.

Furthermore, in order to find out whether any of three varying 45 degrees macular fields was superior in detecting NVEs, the number of NVEs detected in each of them was compared with the number detected from the 60 degrees fovea-centred field.

Our results show that especially early DRP lesions (red dots) but also intra-retinal microvascular abnormalities and venous beading, both indicating severe DRP, are more easily detected from monochrome red-free digital images and photographs, compared to colour transparencies. This is important as the detection of the first abnormalities as well as of the severe DRIP lesions influences both future screening intervals and decisions for referral or treatment. "White lesions", e.g. cotton wool spots, were the only abnormalities which were less easily detected with the red-free technique.

For further prognosis these lesions are, however, not considered as important as the former ones. Single-field 60 degrees photography can be advocated only when the finding in this field is normal otherwise severe lesions can be missed. One and two-field 60 degrees photography covers 60% and 80%, respectively, of the areal coverage of that of 30 degrees seven-field photography (Gold Standard). One- and two-field 45 degrees photography disclosed 53% and 77%, respectively, of the NVEs which were detected from two-field 60 degrees photography. Of the 45 degrees macular fields investigated, the field centred most temporally turned out to disclose NVEs most appropriately.