We read with interest the excellent article entitled ‘Late-onset visual decline following successful treatment of subfoveal choroidal neovascularisation with photodynamic therapyapos; by Bhatnagar and Musadi (Eye 2006; 20: 491–493).1 This observational study of four patients developing late increasing size of fibrotic scarring after photodynamic therapy (PDT) is very valuable for clinicians who manage choroidal neovascularisation.
It caught our attention that the visual acuity data were presented in several formats namely: number of lines lost, number of letters lost, and the reciprocal of logMAR (Figure 1). However, nowhere in the paper is the actual logMAR visual acuity shown. Most ophthalmologists (and especially those treating patients with PDT) are familiar with logMAR visual acuity units. If they were not then the reciprocal would surely leave them even more baffled. The only possible explanation was that the authors wished to present declining visual acuity with a negative slope.
LogMAR visual acuity is the accepted scientific means of presenting visual acuity in journals and for carrying out statistical analyses. Many feel that it should become the accepted means of testing vision in clinical practice.2 Snellen eye charts suffer from well-known limitations of unequal changes in visual angle per line, letters of differing legibility, and different numbers of letters per line. Snellen charts are insensitive to changes in acuity at the top end of the chart where changes in visual acuity for patients with choroidal neovascularisation are important. The Bailey–Lovie or ETDRS chart was designed to overcome these deficiencies3 and is more reproducible and reliable.4 The use of the Snellen chart to define the threshold for PDT has also been questioned. Patients had to achieve 34 letters on a modified ETDRS chart to receive treatment under treatment of age-related macular degeneration with photodynamic therapy (TAP) study inclusion criteria. This is said to be equivalent to 6/60 Snellen, but in fact agreement is variable.5 We agree with the authors' avoidance of Snellen acuity.
All four patients reported by Bhatnagar are rightly considered to have had a disappointing result from PDT. Nonetheless, it is interesting to note that one of the four would meet the criterion for success according to such studies as the TAP study by virtue of having lost only 14 letters on the logMAR chart.6 Figure 1 in their paper illustrates concern that this criterion for success is of course partly governed by how much vision the patient still has to lose.
Therefore, we feel that journals should be promoting the clear presentation of logMAR visual acuity data. Familiarity with logMAR acuity values should be promoted not least in the field of TAP where logMAR visual acuity is an accepted means of monitoring progress.
References
Bhatnagar A, Musadiq M, Yang YC . Late-onset visual decline following successful treatment of subfoveal choroidal neovascularisation with photodynamic therapy. Eye 2006; 20: 491–493.
Hussain B, Saleh GM, Sivaprasad S, Hammond CJ . Changing from Snellen to LogMAR: debate or delay? Clin Exp Ophthalmol 2006; 34: 6–8.
Bailey I, Lovie J . New design principles for visual acuity letter charts. Am J Optom Physiol Opt 1976; 53: 740–745.
Laidlaw DAH, Abbott A, Rosser DA . Development of a clinically feasible logMAR alternative to the Snellen chart: performance of the compact reduced logMAR visual acuity chart in amblyopic children. Br J Ophthalmol 2003; 87: 1232–1234.
Talks SJ, Setty R, Clarke L . The incidence and outcome of photodynamic therapy for macular degeneration in the Northern region of the UK. Eye 2004; 18: 588–594.
Bressler NM . Photodynamic therapy of subfoveal choroidal neovascularization in age-related macular degeneration with verteporfin: two-year results of 2 randomized clinical trials-tap report 2. Arch Ophthalmol 2001; 119: 198–207.
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Doyle, E., Casswell, A. The presentation of data relating to visual acuity. Eye 21, 284–285 (2007). https://doi.org/10.1038/sj.eye.6702532
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DOI: https://doi.org/10.1038/sj.eye.6702532
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