Factors Contributing to Discrepancy Between Visual Acuity Fractions Derived From a Snellen Chart and Letter Scores on the Early Treatment Diabetic Retinopathy Study Chart.

PURPOSE: To report factors influencing the relationship between visual acuity (VA) fractions measured on Snellen chart and letter scores on the Early Treatment Diabetic Retinopathy Study (ETDRS) chart. DESIGN: This was a retrospective review from a single ophthalmology outpatient clinic. METHODS: All patients had routine consecutive VA testing in the right eye using a Snellen chart (1-6 m) and the ETDRS chart (4 m and/or 1 m), by the same optometrist, using a standardized testing protocol for each chart. Both acuity fractions and letter scores were converted to their equivalent logarithm of minimum angle of resolution (logMAR) for comparison. Multiple regression analysis was performed. RESULTS: A total of 237 patients with a wide range of ocular disease and VAs were enrolled. Mean age was 63 years (range, 18-95 years). Recorded VA (logMAR) was better on Snellen chart by a mean (95% limits of agreement) of -0.07 (-0.33 to +0.18, P < 0.001). There was a trend for subjects with VA worse than 6/36 to read 3 to 6 letters more and those better than 6/36 to read 4 to 5 letters less on the ETDRS chart. Visual acuity level and amblyopia contributed to a small portion (17%-26%) of the total variance in the difference between logMAR equivalents derived from the 2 charts. CONCLUSIONS: The discrepancy in VA derived from Snellen and ETDRS charts was nonuniform across VA range. This has implications on interpretation of published studies converting Snellen fractions to logMAR for analysis and reporting of VA outcomes.

The effect of the first application of verteporfin photodynamic therapy on lesion growth in choroidal neovascularisation and its potential impact on combination therapy.

PURPOSE: To evaluate the impact of lesion size on the observed growth of the choroidal neovascularisation (CNV) following the first application of photodynamic therapy (PDT). METHODS: A retrospective study of consecutive patients with subfoveal classic CNV or predominantly classic CNV undergoing first verteporfin photodynamic therapy (VPDT) according to treatment of age-related macular degeneration with photodynamic therapy (TAP) protocol between June 2005 and September 2005. Patients were classified into two groups according to baseline greatest linear dimension (GLD) at the time of VPDT monotherapy. Group 1 comprised patients with lesion GLD less then 2000 microm and Group 2, patients with lesion GLD 2000 microm or larger. Difference between the mean GLD at baseline and at three months post-treatment were investigated for clinical significance using the paired t-test. RESULTS: Group 1 (n = 16) showed a mean change in GLD from 1380.25 microm to 2031.25 microm, while Group 2 (n = 26) showed a mean change of 2909.26 microm to 3023.07 microm. Data of mean percentage change in GLD for Group 1 showed a 51.97 per cent increase in the lesion size [95% CI, 22.93 - 81.01%] in comparison to Group 2, which showed only 5.8 per cent increase in the lesion [95% CI, -9.65 - 21.61%]. Comparison of the mean percentage change in GLD between the two groups showed a statistically significant difference (p = 0.003). CONCLUSIONS: Our study demonstrates that following the first application of VPDT, smaller lesions enlarge disproportionately more than larger lesions. As we have entered into the era of using angiostatic agents in combination with VPDT, it may be important to evaluate this effect with regards to the timing of VPDT commencement.