[Clinical investigation on AC/A ratio in intermittent exotropia coexisting with ametropia]

To investigate AC/A ratio and coexisting ametropia in intermittent exotropia. To discuss the relation between AC/A ratio and the development of intermittent exotropia.

The medical records of 135 patients who had an exotropia were retrospectively reviewed. Patients were divided into 3 groups based on the type of ametropia: exotropia without ametropia (-0.50-+3.00 D between the age of 3-6 years old, -0.50-+2.00 D between the age of 7-40 years old) , exotropia coexisting with myopia (-0.50-7.75 D) and exotropia coexisting with hypermetropia ( ≥ + 3.00 D of 3-6 years old , ≥ + 2.00 D of 7 ∼ 40 years old). AC/A ratios of all patients were assessed using synoptophore method. Distance and near deviations were assessed using prism cover test. AC/A ratios and deviation angles of 3 groups were compared. The relationship between AC/A ratios and ametropia/age were investigated.

There were obvious differences in AC/A ratios(2.686 ± 1.372, 1.773 ± 1.110, 4.581 ± 1.552, F = 36.323, P < 0.001) and in near deviation angles (44.473(Δ) ± 19.008(Δ), 53.621(Δ) ± 20.749(Δ), 34.455(Δ) ± 13.292(Δ), F = 8.762, P = < 0.001) between 3 groups of patients, no obvious differences were seen in distance deviation angles (40.333(Δ) ± 19.474(Δ), 44.052(Δ) ± 23.722(Δ), 35.590(Δ) ± 11.143(Δ), F = 1.444, P = 0.24). AC/A ratios were negative linear correlated with refractive powers for patients of intermittent extropia coexisting with myopia or with hypermetropia (r = 0.320, P = -0.469 and r = -0.046, P = 0.036, respectively) . A trend of decline with age was found for AC/A ratios only in patients of intermittent extropia without ametropia (r = -0.320, P = 0.019; r = -0.023, P = 0.865; r = 0.246, P = 0.296 for the other 2 groups, respectively).

Myopia and hypermetropia, when coexisting with abnormally low or high AC/A ratio, can destroy the balance between extraocular muscles and induce exophoria to be manifest.