The effect of abnormal fixational eye movements upon visual acuity in congenital nystagmus

Purpose. The purpose of this study was to investigate the role that abnorma l eye movements play in the degradation of visual acuity. Methods. Visual acuity was measured monocularly in 10 normal subjects (26.7 +/- 4.3 years) and 5 subjects with congenital nystagmus (34.9 +/- 8.8 year s), using Regan Repeat Letter charts (RRL) and a logMAR based test (LogMAR Crowded Acuity Test (CAT)) while eye movements were continuously recorded u sing a commercially available infrared limbal eye tracker (Type 54, Optoele ctronic Developments, UK). The eye tracker was controlled via a virtual osc illoscope (Viewdac, Keighly Instruments, UK) on an IBM PC clone (Opus Techn ology 486). Results. The mean visual acuity obtained with RRL was significantly higher than that obtained by CAT in the subjects with congenital nystagmus. A sign ificant correlation was found between the root mean square value of the nys tagmus waveform and the angular extent of CAT. Linear regression analysis r evealed a correlation between the duration of the foveation periods and the linear acuity of the subjects with congential nystagmus. The nystagmus wav eforms also demonstrated increased amounts of high frequency components HFC : >3.0Hz) when compared to the normal eyes. Conclusions. The results of this study reveal 1) RRL charts provide a measu re of a subjects resolution limit which is less dependent on eye movements; 2) the duration of foveation periods has a significant effect on visual ac uity measurements obtained using a linear test chart format; 3) the predomi nance of high frequency components in the congenital nystagmus waveforms le ad to short foveation periods adding to the degree of fixation instability.