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| LETTER TO EDITOR |
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| Year : 2006 | Volume
: 54
| Issue : 3 | Page : 214 |
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Snellen chart may be preferable over early treatment diabetic retinopathy study charts for rapid visual acuity assessment
Hem Kumar Tewari, Vandana Kori, Parul Sony, Pradeep Venkatesh, Satpal Garg
Dr. Rajendra Prasad Centre for Ophthalmic Sciences, All India. Institute of Medical Sciences, New Delhi, India
Correspondence Address:
Pradeep Venkatesh
Dr. Rajendra Prasad Centre for Ophthalmic Sciences, All India. Institute of Medical Sciences, New Delhi
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/0301-4738.27087
How to cite this article:
Tewari HK, Kori V, Sony P, Venkatesh P, Garg S. Snellen chart may be preferable over early treatment diabetic retinopathy study charts for rapid visual acuity assessment. Indian J Ophthalmol 2006;54:214 |
Dear Editor,
Visual acuity (VA) measurement with early treatment diabetic retinopathy study (ETDRS) charts is considered as the universal standard and is used in most of the clinical research trials.[1] The ETDRS charts have numerous advantages over the Snellen chart.[1],[2] However, our clinical impressions suggest that reading an ETDRS chart requires more time as compared to the Snellen chart. To the best of our knowledge, there is no study in literature to support our belief. We conducted a study to compare and evaluate the time required for recording the best-corrected visual acuity (BCVA) with ETDRS and Snellen charts.
Our study included 102 eyes of 51 patients. All patients underwent a record of VA uniocularly and with best refractive correction, using Snellen chart and the ETDRS charts. Time required for reading the charts was noted by an independent observer with a stop watch.
The Snellen VA was recorded under standard illumination condition, from a 6 meter distance. For illiterate patients, 'E' chart was used. Recording of VA with ETDRS charts was preformed under the recommended illumination condition, at a 4 meter distance. The right and left eyes were tested with separate charts provided, with the ETDRS vision recording system. For illiterate patients, Landolts 'C' chart was used.
Statistical analysis was performed using SAS commercial statistical software package (SAS institute, Inc, Cary, NC). Paired 't' test was used to compare the difference in the time required for reading the two charts. A ' P ' value of <0.05 was considered statistically significant.
Our study included 28 males and 23 females, with a mean age of 34.52±12.34 years. The mean time required for reading was 13.17±12.6 seconds and 30.9±13.31 seconds with Snellen and the ETDRS charts, respectively ( P <0.001). The ETDRS charts required 2.4 times higher time for VA recording, compared to the Snellen chart. The mean BCVA in decimal notation was 0.97±0.15 and 0.97±0.18 with the Snellen and ETDRS charts, respectively ( P =0.78).
Resolution VA is a commonly used indicator of visual function. A variety of letter charts have been introduced for measuring the resolution VA.[1],[2],[3] Snellen chart is a simple, most popular and familiar examination method for measuring VA. However, it has various disadvantages, like each letter line does not follow a geometric progression i.e. the change in the letter size is not uniform and the letters in different rows have different legibility or difficulty scores. These problems were overcome with introduction of the ETDRS charts, which have equal number of letters in each line, similar and uniform legibility of each line and offer an ease of conversion of VA to a logarithmic scale.[4],[5] The ETDRS charts are now considered as the most accurate and standard method of VA recording, when precise data on VA measurement is required.[3],[4],[5]
Despite various advantages, the ETDRS acuity assessment appears to be more time- consuming. Our observations clearly showed that the Snellen charts offer a relatively rapid method for VA assessment, with significantly lower time requirement when compared to the ETDRS charts. We also observed that decimal notation as recorded with Snellen charts, compared well with those obtained with the ETDRS charts.
The ETDRS charts are superior to the Snellen chart in various respects and are commonly employed for VA assessment in clinical and research trials.[1],[2],[3] Our study indicated that the Snellen chart provides a rapid assessment of VA, without compromising its accuracy when decimal notation is concerned. One of the drawbacks of our study is small sample size and that we recorded the time required for reading these charts with BCVA only and time requirement for reading charts with uncorrected VA was not recorded. However, we feel that in situations requiring visual assessment of a large number of patients in a limited duration of time, such as in high-load tertiary referral hospitals or mass eye camps at the community level in a developing country like ours, the use of Snellen chart seems more appropriate.
| References | |  |
| 1. | Kniestedt C, Stamper RL. Visual acuity and its measurement. Ophthalmol Clin N Am 2003;16:155-70.  [ PUBMED] |
| 2. | Bailey IL, Lovie JE. New design principles for visual acuity letter charts. Am J Optom Physiolo Opt 1976;53:740-5. [ PUBMED] |
| 3. | Sloan LL. New test carts for the measurement of visual acuity. Am J Ophthalmol 1959;48:808-13. [ PUBMED] |
| 4. | Holladay JT, Prager TC. Snellen equivalent for Bailey- Lovie acuity chart. Arch Ophthalmol 1989;107:955. [ PUBMED] |
| 5. | Holladay JT. Visual acuity measurements. J Cataract Refarct Surg 2004;30:287-90. [ PUBMED] [ FULLTEXT] |
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