Autofluorescence distribution along the corneal axis in diabetic and healthy humans

Corneal autofluorescence, as measured with a commercial scanning fluorophotometer (lambda(exc): 415-491 nm; lambda(em): 515-630 nm), is increased in patients with diabetes mellitus. However, such fluorophotometers register an average fluorescence signal over all corneal layers as a consequence of their limited axial resolution of 0.5 mm. In order to determine the location of the fluorophores responsible for the increased corneal autofluorescence measured in diabetics, an attempt was made to measure in vivo the distribution of autofluorescence along the optical axis of the cornea with a modified slitlamp. Fluorescence excitation and emission filters identical to those of the scanning fluorophotometer were fitted to a slitlamp equipped with a slow scan CCD camera. Corneal autofluorescence intensity profiles were obtained with the slitlamp in five patients with severe diabetic retinopathy and compared to those of age-matched healthy controls. Corneal autofluorescence was also measured with the scanning fluorophotometer for comparison. The resolution of the CCD camera for measurement of fluorescence along the corneal axis was 0.1 mm. The corneal autofluorescence intensity of the patients and the healthy controls gradually decreased by about the same amount from the endothelium to the epithelium (57% mm(-1)+/-6 s.d. and 52% mm(-1)+/-5 s.d., respectively). The area under the fluorescence intensity curve was significantly greater for the patients than for the healthy controls (factor 2.4+/-1.0 s.d., P<0.001) and was proportional to the corneal fluorescence measured with the scanning fluorophotometer (r=0.92, P<0.001). The results show that (1) the distribution of autofluorescence along the corneal axis can be measured in vivo in humans, (2) the fluorophores involved are distributed throughout the cornea, and (3) the relative distribution of fluorescence is similar in diabetic patients and healthy controls.