Of three smaller studies that measured the association between visual acuity and central foveal thickness in patients with retinitis pigmentosa and macular cysts, one found a significant relationship
8 and two did not.
7 9 In the present study we confirmed that visual acuity is inversely related to central foveal thickness in eyes with macular cysts; ETDRS acuity declined by an average of 1.7 letters for each 100-μm increase in thickness. We previously reported an 11-letter average decline in ETDRS acuity for each 100-μm decrease in central foveal thickness due to cell loss in eyes of patients with retinitis pigmentosa without macular cysts.
1 By taking a ratio of these two figures (i.e., 11 letters/1.7 letters), we find that for a given change in central foveal thickness the impact of cell loss on visual acuity appears to be 6.5 times the impact of edematous swelling on acuity in this disease.
Remarkably, we found in our patients with macular cysts that ETDRS acuity declined due to increases both in central foveal thickness and in the mean retinal thickness within an outer ring spanning an eccentricity of 5° to 10° from the foveal center (see
Fig 4 , top and bottom). The retinal thicknesses in the bottom graph likely underestimate the magnitude of edema at that location, since patients with retinitis pigmentosa generally have marked parafoveal cell loss, as evidenced by the tomograms of eyes without macular cysts.
1 Thus, much of the retinal thickness data in the bottom graph reflect the net result of cell loss and swelling.
Although the dependence of visual acuity on edema at the foveal center, where acuity is measured, is intuitive, the basis of its dependence on edema in a parafoveal region is not obvious, especially given that these two dependencies appear to be
independent according to our analysis. That is, our data indicate that the loss of acuity due to edema represents the sum of the effect of edema in the foveal center and the effect of edema in the parafovea. It may be relevant to note that in some patients with retinitis pigmentosa and cystoid macular edema, marked reductions in retinal thickness within the fovea after treatment with a topical carbonic anhydrase inhibitor
14 or an intravitreal steroid
15 were not associated with commensurate improvements in visual acuity. It is possible that these eyes had reduced acuity due to foveal cell loss before the edema or developed irreversible functional damage as a result of the edema itself,
8 and treatment benefit was therefore limited by a
ceiling effect. However, it is also possible that the treatments did not effectively reduce parafoveal edema (as was evident in one illustration
15 ). Our results suggest that, in evaluating the benefit of any treatment for macular edema in retinitis pigmentosa, its effect on both the foveal and parafoveal retina should be considered.
The authors thank Bernard Rosner, PhD (Professor of Medicine and Biostatistics, Harvard Medical School) for guidance regarding the use of SAS.