Accommodation and vergence response gains to different near cues characterize specific esotropias

Abstract

Aim: To describe preliminary findings of how the profile of the use of blur, disparity, and proximal cues varies between non-strabismic groups and those with different types of esotropia.

Design: This was a case control study.

Methodology: A remote haploscopic photorefractor measured simultaneous convergence and accommodation to a range of targets containing all combinations of binocular disparity, blur, and proximal (looming) cues. Thirteen constant esotropes, 16 fully accommodative esotropes, and 8 convergence excess esotropes were compared with age- and refractive error-matched controls and 27 young adult emmetropic controls. All wore full refractive correction if not emmetropic. Response AC/A and CA/C ratios were also assessed.

Results: Cue use differed between the groups. Even esotropes with constant suppression and no binocular vision (BV) responded to disparity in cues. The constant esotropes with weak BV showed trends for more stable responses and better vergence and accommodation than those without any BV. The accommodative esotropes made less use of disparity cues to drive accommodation (p = 0.04) and more use of blur to drive vergence (p = 0.008) than controls. All esotropic groups failed to show the strong bias for better responses to disparity cues found in the controls, with convergence excess esotropes favoring blur cues. AC/A and CA/C ratios existed in an inverse relationship in the different groups. Accommodative lag of > 1.0 D at 33 cm was common (46%) in the pooled esotropia groups compared with 11% in typical children (p = 0.05).

Conclusion: Esotropic children use near cues differently from matched non-esotropic children in ways characteristic to their deviations. Relatively higher weighting for blur cues was found in accommodative esotropia compared to matched controls.

Figure 1

The remote haploscopic videorefractor. (A) Motorized beam. (B) Target monitor. (C) Upper concave mirror. (D) Lower concave mirror. (E) Infra-red ‘hot’ mirror. (F) Image of participant’s eye where occlusion takes place. (G) Plusoptix SO4 PowerRef II. (H) Headrest. (J) Raisable black cloth screen. Clown and difference of Gaussian targets illustrated lower right; much of the high resolution detail of the clown has been lost in this reduced reproduction.

Figure 2

Response gain profiles (error bars = standard error of the mean) of the different study groups showing between–group differences in profiles. Dark bars show vergence response, pale bars show accommodation response. A response gain of 1 indicates the full response to target distance change. Low response gains were always due to indicate under-response for near (lag). High response gains indicate over-response for near (lead). Strabismic groups show less strong bias towards best responses to disparity-containing cues and the convergence excess accommodative esotropes show strongest bias towards cues containing blur.

Figure 3. AC/A vs CA/C ratios

AC/A (dark shading) and CA/C (pale shading) ratios (error bars = standard error of the mean), showing the broadly reciprocal relationship between the two measures. As we use MA/D units (which correct for differences in IPD), the typical adult AC/A ratio shown here of 0.56MA:1D is equivalent to a clinical prism cover test AC/A ratio of 3.4Δ:1D in a patient with an IPD of 6cm. Strabismic, and particularly accommodative strabismic, groups show high AC/A and low CA/C ratios (more vergence to blur cues than accommodation to disparity cues).