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

Curr Eye Res. 1999 Mar;18(3):194-202. doi: 10.1076/ceyr.


Purpose: The purpose of this study was to investigate the role that abnormal 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 years), using Regan Repeat Letter charts (RRL) and a logMAR based test (LogMAR Crowded Acuity Test (CAT)) while eye movements were continuously recorded using a commercially available infrared limbal eye tracker (Type 54, Optoelectronic Developments, UK). The eye tracker was controlled via a virtual oscilloscope (Viewdac, Keighly Instruments, UK) on an IBM PC clone (Opus Technology 486).

Results: The mean visual acuity obtained with RRL was significantly higher than that obtained by CAT in the subjects with congenital nystagmus. A significant correlation was found between the root mean square value of the nystagmus waveform and the angular extent of CAT. Linear regression analysis revealed a correlation between the duration of the foveation periods and the linear acuity of the subjects with congential nystagmus. The nystagmus waveforms also demonstrated increased amounts of high frequency components (HFC: >3.0 Hz) when compared to the normal eyes.

Conclusions: The results of this study reveal 1) RRL charts provide a measure of a subjects resolution limit which is less dependent on eye movements; 2) the duration of foveation periods has a significant effect on visual acuity measurements obtained using a linear test chart format; 3) the predominance of high frequency components in the congenital nystagmus waveforms lead to short foveation periods adding to the degree of fixation instability.

MeSH terms

  • Adult
  • Eye Movements
  • Female
  • Fixation, Ocular*
  • Humans
  • Male
  • Nystagmus, Pathologic / congenital*
  • Nystagmus, Pathologic / physiopathology
  • Ocular Motility Disorders / physiopathology*
  • Vision Tests
  • Visual Acuity* / physiology