The Effect of Font Size on Reading Performance in Strabismic Amblyopia: An Eye Movement Investigation

Abstract

Citation: Kanonidou E, Gottlob I, Proudlock FA. The effect of font size on reading performance in strabismic amblyopia: an eye movement investigation. Invest Ophthalmol Vis Sci. 2014;55:451-459. DOI:10.1167/iovs. 13-13257 PURPOSE. We investigated the effect of font size on reading speed and ocular motor performance in strabismic amblyopes during text reading under monocular and binocular viewing conditions. METHODS. Eye movements were recorded at 250 Hz using a head-mounted infrared video eye tracker in 15 strabismic amblyopes and 18 age-matched controls while silently reading paragraphs of text at font sizes equivalent to 1.0 to 0.2 logMAR acuity. Reading under monocular viewing with amblyopic eye/nondominant eye and nonamblyopic/dominant eye was compared to binocular viewing. Mean reading speed; number, amplitude, and direction of saccades; and fixation duration were calculated for each font size and viewing condition. RESULTS.Reading speed was significantly slower in amblyopes compared to controls for all font sizes during monocular reading with the amblyopic eye (P ¼ 0.004), but only for smaller font sizes for reading with the nonamblyopic eye (P ¼ 0.045) and binocularly (P ¼ 0.038). The most significant ocular motor change was that strabismic amblyopes made more saccades per line than controls irrespective of font size and viewing conditions (P < 0.05 for all). There was no significant difference in saccadic amplitudes and fixation duration was only significantly longer in strabismic amblyopes when reading smaller fonts with the amblyopic eye viewing.CONCLUSIONS. Ocular motor deficits exist in strabismic amblyopes during reading even when reading speeds are normal and when visual acuity is not a limiting factor; that is, when reading larger font sizes with nonamblyopic eye viewing and binocular viewing. This suggests that these abnormalities are not related to crowding.

Slow reading in children with anisometropic amblyopia is associated with fixation instability and increased saccades

“…Slow reading was related to ocular motor dysfunction. The relationship of reading rate and the number of saccades is consistent with previous studies of reading in strabismic amblyopia 2,3,6 and with simulated central scotomas. 20 The increase in saccades during reading in our study may be due to a suppression scotoma associated with anisometropic amblyopia.…”

“…Data showing that slow reading in strabismic amblyopia is related to making more saccades than controls support this hypothesis. 2,3,6 Other studies of eye movements in normally sighted 14 and visually impaired 15,16 adults have reported an association between fixation instability and slower reading. However, no studies have assessed the relationship between ocular motor dysfunction and reading in amblyopic children without strabismus (ie, anisometropic amblyopia).…”

Amblyopic children read more slowly than controls under natural, binocular reading conditions

“…This finding is consistent with prior reports of more saccades during reading in adults with strabismic amblyopia than in normal controls. 10 Similar increases in the number of saccades have been observed with simulated small central scotomas 17 ; therefore, the ocular motor changes observed in the current study may be the result of a monofixation scotoma associated with mild esotropic amblyopia. 18,19 Small central scotomas reduce the visual span, resulting in poor saccadic accuracy and increased number of saccades during binocular reading.…”

“…Only two of the three studies assessed eye movements and the results were inconsistent despite being conducted by the same research group; one reported more regressive saccades and longer fixations than controls, 9 but the other reported more forward saccades than controls. 10 Another limitation of earlier studies 9–11 is that only patients with both strabismus and amblyopia were enrolled, making it impossible to address whether the reading impairment was a consequence of the strabismus, amblyopia, or both. In the present study subjects read a grade-appropriate paragraph of text silently under natural conditions of binocular viewing and at the child’s habitual reading distance.…”

Functional limitations recognised by adults with amblyopia and strabismus in daily life: a qualitative exploration

“…A range of lower and higher order visual and motor deficits have been identified in those with amblyopia and strabismus; these include impairments in contour, shape, motion and depth perception, contrast sensitivity, eye‐hand coordination, prehension skills, visual search and positional uncertainty . In addition, experimental studies have demonstrated functional impacts in the performance of real‐life tasks such as reading, driving and reaching and grasping . However, it is not clear whether these deficits translate into functional limitations that are recognised by individuals themselves in normal everyday situations.…”