Assessment of the Catford drum in visual acuity testing and its use as a measurement of visual performance in low-vision patients.

Abstract

SUMMARY Objective measurements of visual acuity were determined with the Catford drum in 82 eyes of patients in our Low-Vision Clinic who typically suffered from visual loss due to macular disease. The results were compared with subjective measurements of visual acuity by the Snellen chart. The findings indicated a significant overestimation of Snellen visual acuities by the Catford drum in 90-2% of eyes tested by a factor of 1-05 to 20-0, average 4-73. The correlation coefficient for the study was +0-40. This differs from the original results of Catford and Oliver in 1971. In addition, the Catford drum was used on follow-up visits in the same patients to assess 'visual performance'. The initial results showed an improvement in visual acuities when the Catford drum was used in 12 of 15 patients, while the Snellen acuities remained stable when retested after one month of basic instruction and use of standard low-vision aids. This improvement in 'Catford' acuity was by a factor of 0-3 to 10.0, average 4-08. This is thought to represent the patient's ability to learn the use of eccentric viewing or parafoveal retinal areas for vision. It confirms previous intuitive findings and helps to explain why low-vision patients seem to function at a higher level than expected from their Snellen visual acuities.Many of our Low-Vision Clinic patients surprise us by the fact that their daily life performance is often so much better than might be expected on the basis of their Snellen visual acuity. Our current method of assessing visual acuity and presumably performance in low-vision patients is by means of a static subjective test, the Snellen chart.We believe that in addition to the tested Snellen visual acuity fraction every patient has a certain level of 'visual performance' independent of this, which as we define it relates to the patient's ability to function and perform daily life tasks. Furthermore, visual performance is dynamic in that learning and improvement can occur. The majority of our lowvision patients have visual loss secondary to some form of macular disease with normal functioning parafoveal retinal areas. We believe that it is this region of normal surrounding retina and the patient's ability to learn the use of eccentric viewing or fixation Correspondence to J T W van Dalen, MD,

Surveillance of vision and ocular disorders in children with Down syndrome

“…The most common test used in the past 10 years was the Cardiff Acuity Test, 11 but preschool screening in the older children (before the guidelines) sometimes used the outdated Catford drum, 12 and the latter results were not felt to be reliable. Kay Picture Tests 13 and Keeler Cards 14 were used successfully.…”