This report is the first multicenter clinical trial to investigate the associations between FA, fundus photograph, and OCT features in DME. The strongest associations between baseline FA variables and other measures were found between baseline fluorescein leakage and baseline total macular volume (by OCT), DME area and DME severity scale (by color photographs) (). Change in fluorescein leakage from baseline to 12 months also had associations with change from baseline to 12 months in total macular volume and DME area (). Cystoid abnormalities seen on OCT were frequently absent on FA () although FA did demonstrate some cystoid abnormalities not evident on OCT. Relatively few eyes had capillary loss by FA (a measure not assessable with OCT), and those that had this feature had minimal involvement. No notable associations were seen between capillary loss and any baseline variables or change in these variables ().
Change in Fluorescein Leakage Area (Square Root Transformation) Within Grid (N=244 eyes)
This multicenter trial employed both film and digital FA, in contrast to the ETDRS which analyzed film images only. There were not large differences in the reproducibility of grading for images obtained with each method. However, this finding does not imply equal validity of the gradings with each method, which would require a study in which each eye was imaged by both methods.
The comparison of OCT and FA variables should be presented with the caveat that the precise area of the macula measured differs considerably between modalities, which presumably weakens any correlations. The ETDRS grid employed in the standard grading of photographic and FAs has a diameter of 4 DD, which equals 7.2 mm (by the modern convention of 1 DD equals 1.8 mm on the retina). In contrast, the Stratus OCT uses a grid with a diameter of 6.0 mm. The area of the ETDRS grid, and each of the 9 photographic subfields, is 44% larger than the corresponding OCT subfield, which adds imprecision to the comparison of characteristics measured by these modalities. A small number of scans in this study (8%) were performed with the OCT 2000 which has a lower scan density than the Stratus OCT (100 versus 128 A scans per B scan in the fast macular map scans). This may have increased variability of measurements as well as decreased the sensitivity of detection of morphology such as cysts.
The Early Treatment Diabetic Retinopathy Study and others previously noted the substantial correlation between fluorescein leakage and extent of retinal thickening as appreciated on color fundus photography in DME, 2, 16, 17
but not all have confirmed this association holds in macular edema from other causes.18
In a series of 30 eyes with DME, Neubauer et al noted that there was a significant correlation between fluorescein leakage severity and specific OCT parameters,19
including central subfield thickness (r=0.46) and mean thickness in the outer subfields (r=0.54) but not the inner subfields. Our study confirms that there are associations between fluorescein leakage and some, but not all, measures of retinal thickening by OCT and photography (). Because FA and color photographs were graded concurrently by the same grader in this study, the conclusions regarding relationship between these two modalities are to be interpreted cautiously because of the potential for bias. Prior publications have qualitatively described good topographic co-localization of FA leakage and the extent of retinal thickening by OCT in DME.20–25
Conversely, the ETDRS and others have noted that fluorescein leakage can occur in macular regions that do not appear to have retinal thickening.1, 18
Others have reported some eyes with DME with increased retinal thickening by OCT in the absence of fluorescein leakage.26, 27
This may be a factor which weakens the correlations between OCT measured thickness and fluorescein leakage in our study.
Some association between change in fluorescein leakage and change in OCT measures of retinal thickness was found (r=0.30 to 0.44). Other studies have reported that qualitative change in fluorescein leakage severity over time correlates with change in retinal thickening by OCT after various interventions.28, 29, 30
Bandello et al evaluated “light” versus “heavy” laser photocoagulation treatment of previously untreated eyes with DME with FA and OCT.31
At 12 months, 19 of 29 eyes had decreased fluorescein leakage and 13 of 29 had a decrease in central subfield thickness by at least 10% (from of their report). Ten of the 13 eyes with decreased central subfield thickness also had decreased fluorescein leakage. The current study also demonstrates that change in fluorescein leakage from baseline to 12 months has associations with some measures of change by OCT and photographically determined retinal thickening (central subfield thickness, total macular volume, DME area, DME severity scale) ().
The association between baseline visual acuity and fluorescein leakage has been noted previously in the ETDRS;2
there was some association in this cohort (r = 0.33, ). In a prior DRCR.net publication analyzing this same data set baseline fluorescein leakage in a multivariate regression model demonstrated a small incremental increase in the prediction of the variance of baseline visual acuity over OCT central subfield thickness in univariate analysis.9
Univariate analysis calculated the association between OCT central subfield thickness and VA to have an r 2
value 23%, and association between FA leakage and VA to be slightly worse with r 2
Because the methodology for measuring fluorescein leakage has changed from an ordinal scale in the ETDRS to a continuous scale in the DRCR.net,8
the reproducibility of grading for FA leakage area and intensity from the ETDRS (kappa 0.43, weighted kappa 0.72) cannot be directly compared to this one (, , and ). However, the inter grader reproducibility of fluorescein leakage evaluation appears roughly comparable between this study and the ETDRS, being only “fair” overall. The grading of lesion characteristics from FA inherently has some measurement variability due to variable image quality and the subjective nature of the assessments which may obscure underlying correlations. In addition, FA interpretation of leakage is made more difficult due to background staining of laser scars, which may mask vascular leakage (8). This potentially affected grading of follow up examinations as well as some fellow eyes which had laser scars at baseline.
The association between cystoid abnormalities by OCT and FA has been noted by several groups which have considered DME with cystoid abnormalities to be more advanced than DME without cystoid abnormalities.20, 23, 33
By histopathology and by OCT the larger petaloid cystoid abnormalities on FA correspond to collections of fluid in the outer plexiform layer, while the smaller “pebbly” cystoid abnormalities correspond to fluid collections in the inner nuclear layer in the extrafoveal regions of the macula.24, 34
In the current study, the association between area of cystoid abnormalities graded on FA and a 5-step scale by OCT was unimpressive (). There was a greater likelihood that cystoid abnormalities are detected by OCT than FA, indicating greater sensitivity of the former. Cases of cystoid abnormalities in DME and in other forms of macular edema where no leakage was seen on FA, but there were cystoid abnormalities by clinical examination and OCT are well documented.20, 26, 27, 34
In addition, cystoid abnormalities observed on FA but not on OCT have been detailed.24
Some cases of cystoid abnormalities without fluorescein leakage may be the result of longstanding cystoid abnormalities with structural alterations in the neural retina.34, 35
A unique attribute of FA assessment is visualization of the small caliber retinal vessels that allows detection and quantification of capillary ischemia (or capillary loss). However, the grading of capillary loss is nearly at the limits of resolution by optical camera systems, and a relatively small degradation of image quality (e.g., poor focus or cataract) can make this variable very difficult to grade. Only 37% of eyes gradable for this feature had capillary loss (likely an underestimate of the true incidence for the reasons mentioned). An association between capillary loss and FA leakage has been noted previously.8
In this study, the association was weak and there were no other notable associations with this feature.
Fluorescein angiography indirectly reflects an important pathophysiologic process (vascular leakage and ischemia) which is not measured by OCT and does not always correlate with retinal thickening. FA might provide evidence of a treatment effect in clinical studies of diseases which feature retinal vascular leakage. However, we did not identify any unique FA variables which correlated with visual acuity outcome better than OCT measures of retinal thickness. This trial did not have an untreated control group or large differences in outcomes between treatment arms, which limits the extent to which these results can be extrapolated to other trials with different designs. Because of the relatively invasive nature of FA and its cost, its implementation in large studies should be balanced against the expected value of fluorescein leakage in support of other endpoints.