Abstract
Purpose: :
Intermittent exotropes can have tenacious proximal fusion which prevents them from manifesting their true near deviation. Among Asian children with divergence excess intermittent exotropia, the prevalence and characteristics of true- versus pseudo- divergence excess are not well reported. This study aims to elucidate distance-near relationships pre- and post- monocular occlusion and the AC/A (accommodation convergence - accommodation) ratio in this group of intermittent exotropes.
Methods: :
Children with intermittent exotropia and a distance deviation exceeding the near deviation by at least 10Δ were recruited consecutively. In addition to prism and cover test measurements at 6m and 1/3m, calculation of the AC/A ratios using the heterophoria method and the gradient method at 6m and 1/3m with -2.00DS and +3.00DS lenses respectively were undertaken. The habitually deviating eye was then patched for 1 hour. The AC/A ratios using the heterophoria and gradient methods based on post-occlusion near measurements were then calculated. We defined a high AC/A ratio as greater than 8:1 and 6:1 for the heterophoria and gradient methods respectively.
Results: :
38 children with a mean age of exotropia onset at 47.5 months were recruited. The mean difference between distance and near deviation was 22.2Δ (range 10 to 45Δ) pre-occlusion and 13.2Δ (range -15 to 36Δ) post occlusion (p<0.001). 36.8% of patients had a less than 10Δ distance-near difference post-occlusion. Based on the heterophoria method, all patients had a high AC/A ratio pre-occlusion; of which 27 (71.1%) continued to have a high AC/A ratio post-occlusion, with 28.9% having a pseudohigh AC/A ratio. Based on the gradient method using a +3.00DS lens, 22 patients (57.9%) had a high AC/A ratio pre-occlusion; of these, 13 (59.0%) continued to have a high AC/A ratio post-occlusion, with 41.0% having a pseudohigh AC/A ratio.
Conclusions: :
The rate of pseudodivergence excess occurred in slightly over a third of our patients, with a near deviation approximating or equal to the distance deviation post-occlusion. More patients were diagnosed as having a high AC/A ratio with the heterophoria method as accommodative and proximal convergence may be present, compared to the gradient method which keeps proximal convergence constant by performing both measurements at the same testing distance. This has implications on counselling patients pre-operatively for requirement of bifocals post-operatively.
Keywords: strabismus • strabismus: diagnosis and detection