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

Aim. To describe 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. 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. 13 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. The constant esotropes with weak binocular vision (BV) showed trends for more stable responses and better vergence and accommodation than those without any BV, although even those with constant suppression Horwood Accom & vergence in esotropia Version 1 07.04.2012 3 still responded to disparity cues. 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. As expected, all esotropic groups did not 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 a broadly reciprocal relationship in the different groups. Accommodative lag was common in esotropia. Conclusion. Esotropic children use near cues differently from matched non-esotropic children in ways characteristic to their deviations. Relatively higher weighting for blur cues in accommodative esotropia compared to matched controls may explain both etiology and why treatment with spectacles can be so effective. INTRODUCTION In 2008 we presented a novel laboratory method of assessing vergence and accommodation responses to the three main cues to near vision (blur, disparity and proximity/size change/looming) (Horwood & Riddell, 2008; Horwood & Riddell, 2009) We hypothesized that the weighting and response profiles of these cues, both alone and in combination, might characterize different clinical diagnoses. In our laboratory, we consistently find that in typical children and adults blur and disparity are not used equally as cues to drive responses to target distance (Horwood & Riddell, 2008). Both convergence and accommodation to any target containing disparity cues are much better than to those where disparity is excluded, with blur playing a lesser role and proximity playing a minor part. However, infants appear to respond best to proximal cues in their first weeks, then use all cues relatively equally in “middle infancy” (10-26 Horwood Accom & vergence in esotropia Version 1 07.04.2012 4 weeks of age). By 5 years of age children behave similarly to adults, with best responses to targets containing disparity cues. Blur retains similar weighting from infancy to adulthood ( Horwood & Riddell, 2009) (and manuscript in preparation). In the case of childhood concomitant strabismus, disparity detection might be primarily defective, secondary suppression might disrupt it, or refractive error or abnormal AC/A (accommodative convergence to accommodation) relationships disrupt use of blur cues; so atypical patterns of cue weighting would be predicted. Development might be arrested at the level it had reached before onset of the deviation or new weightings emerge. Intact cues (such as blur or proximal /looming) might “make up the difference” to drive an appropriate response to approaching targets, or still only drive the same (reduced) responses that that cue would in drive in typical children, resulting in lag for near. Strabismic individuals might only respond to disparity cues if some binocular vision (BV) is retained via abnormal retinal correspondence (ARC) or peripheral fusion is retained. We might also predict that children with accommodative esotropia and a high AC/A ratio (which by definition implies that response to blur drives extra convergence) might use blur cues to drive both accommodation and vergence more than children without accommodative esotropia. This preliminary paper explores these predictions using data from small groups of patients with different clinical diagnoses to illustrate differences in response profiles in comparison to matched control groups. MATERIALS AND METHODS The study adhered to the Declaration of Helsinki and was allowed to proceed by UK National Health Service and institutional ethics committees. We studied seven groups of participants (see Table 1), four groups of esotropes (constant with weak BV, constant without Horwood Accom & vergence in esotropia Version 1 07.04.2012 5 any BV, fully accommodative esotropia and convergence excess esotropia) and three control groups (typical emmetropic young adults, and two non-strabismic groups matched by age and refractive error to the strabismic children so that strabismic and nonstrabismic responses could be compared as much as possible while controlling for age and refractive error). Some of the nonaccommodative constant esotropes had residual esodeviations following surgery. All participants who had been prescribed spectacles wore a full cycloplegic refractive correction for testing, so accommodation started from the same refractive baseline for distant fixation. n Matched with Age Deviation BV Ref Horwood Accom & vergence in esotropia Version 1 07.04.2012 6 Table 1 Study groups and clinical characteristics (yrs) (PCT) at 33cm (unless stated otherwise) Error (MSE) mean range Mean range Group 1 (control) Emmetropic non-strabismic adults 27 20.2 18-24 0-3らexo <120” TNO +0.3D -0.25D-+0.5D Group 2 (control) Non-strabismic children 13 Individually matched to constant esotropes in Group 3&4 5.05 4.06.3 0-3らexo <120” TNO +2.09D 0.0D-+6.0D Group 3 Non-accom ET Weak BV 7 Individually by age & refractive error (within 1.0D MSE) with Group2 children 4.85 4.55.0 2-35ら eso >400”, +ve Bagolini Gls or Lang stereotest +1.75D 0.0D-+5.5D Group 4 Non-accom ET No BV 6 Individually by age & refractive error (within 1.0D MSE) with Group2 children 5.16 4.56.3 5-25ら eso None, + constant suppression on Bagolini Gls and Worths Lights and synoptophore +2.41D 0.0D-+6.0D Group 5 (control) Non-strabismic hypermetropic 10 Group matched with Group 6 by age and refractive error 5.9 5.0 8.5 2らexo-2ら eso <120” TNO +4.02D +2.5D-+6.0D Group 6 Fully accomodative ET 16 Group matched with Group 5 by age and refractive error 5.79 4.38.0 1-8ら with gls, 15-45 ら without gls <120” TNO with gls +3.9D +2.25D-+6.0D Group 7 Convergence Excess ET 8 6.4 4.57.1 20-45 ら eso Nr, 0-6 ら eso Dist <120” TNO with +3.0 add +2.18D 0D-+4.75 Horwood Accom & vergence in esotropia Version 1 07.04.2012 7 The laboratory method has been described in detail elsewhere(Horwood & Riddell, 2008), but briefly all participants watched the target being presented via a two-mirror optical system, while a PlusoptiXSO4 PowerRefII photorefractor collected simultaneous eye position and refraction measurements (Fig. 1). Targets moved between five different fixation distances (0.33m, 2m, 0.25m, 1m, 0.5m) in a pseudo-random order.