Abstract
Purpose: :
It is unclear why amblyopia develops in esotropia (ET) since alternate suppression alone should eliminate confusion between mismatched monocular images. Amblyopia may result from asymmetric suppression. This hypothesis is supported by the finding that strabismic monkeys have primarily residual suppressive binocular cortical interactions (Smith et al, J Neurophysiol 78:1353) and that some amblyopic adults have asymmetric dichoptic masking (Norcia et al. Neuroreport 11:1007) and fusional suppression (Hale et al., ARVO 2005). In order to determine whether asymmetric suppression is associated with esotropic amblyopia, we evaluated fusional suppression in amblyopic and nonamblyopic children with ET.
Methods: :
24 children with accommodative ET, 14 children with infantile ET, and 3 normal adults were tested. VEPs were recorded in response to dichoptic vernier multi–bar targets composed of 5 static segments interspersed with 4 oscillating segments that aligned and misaligned at 2 Hz. Offset magnitude was swept in 7 steps from 1 to 10 min during each 7 sec trial. The fellow eye viewed the same target with either 0 or 5 min standing disparity. Fusional suppression was defined as the % amplitude reduction in the 5 min condition. An asymmetry index (ratio of % amplitude reduction for the two eyes, ranging from 0.0 for perfect symmetry to ±1.0 for highly asymmetric suppression) was calculated. Stereoacuity was assessed using the Randot Preschool Stereoacuity Test.
Results: :
In accommodative ET, nonamblyopic children with normal or deficient stereoacuity showed symmetric fusional suppression (0.1±0.4 and 0.1±0.2, respectively), similar to normal adults (0.03±0.3). Those with nil stereoacuity showed little suppression (3±23%). The amount of suppression was correlated with stereoacuity (rs= –0.49; p<0.05). In amblyopic children, fusional suppression was present when the vernier offset was presented to the amblyopic eye (28±17%) but not when it was presented to the nonamblyopic eye (–3±27%). Children with infantile ET showed little fusional suppression whether or not they were amblyopic (11±15% and 10±16% for the amblyopic and nonamblyopic eyes).
Conclusions: :
Data from children with accommodative ET support an association between asymmetric fusional suppression and amblyopia, consistent with the sparing of suppressive interactions observed in monkeys with acquired strabismus. Interestingly, this finding is not present for infantile ET, suggesting that the mechanism for the fusional suppression may be congenitally absent.
Keywords: binocular vision/stereopsis • amblyopia • strabismus