The agreement between the Irvine 4 diopter prism test and assessment of ocular fixation in microtropia with identity.

Microtropia describes a primary ocular deviation of less than 10 prism diopters associated with harmonious anomalous retinal correspondence and reduced stereopsis. It is routinely accepted that children with microtropia are less likely to achieve equal vision following occlusion therapy than those with bifoveal fixation. The most commonly used methods of diagnosing a microtropia are the 4 diopter prism test (4PT) and assessment of ocular fixation. This study examines the agreement between the two tests. One hundred and twelve typically developing children without a manifest strabismus who were able to undertake a linear visual acuity test and had two or more lines of anisoacuity following refractive adaption to their full cycloplegic correction underwent assessment of the 4PT and ocular fixation along with their routine orthoptic examination. One hundred and twelve children (46 boys and 66 girls) attending the Orthoptic department who fitted the above criteria were included in the analysis. The mean age at examination was 6 years. 80.3% had anisometropia of at least 1.25 diopters. The 4PT indicated a microtropia in 74 cases, whereas assessment of fixation indicated a microtropia in 68 cases. In 88 cases (78.6%), the results of the two tests agreed. Analysis found only moderate agreement between the two tests ( = 0.540 (CI 0.379-0.700)). Logistic regression analysis comparing those children where the two tests agreed with those where they disagreed found no difference in the level of anisoacuity ( = 0.7823), degree of anisometropia ( = 0.9385), the vision in the worst eye ( = 0.5260), the refractive error in the "worst" eye ( = 0.865), or the age at the time of testing ( = 0.4485) between the two groups. When assessing for a microtropia, it was found that not all children who elicit a central suppression response on the 4PT are found to be fixing eccentrically and vice versa. This could potentially have implications for the treatment of their amblyopia. It is important not to rely on just one test at one time to make the diagnosis of microtropia. Rather, if one or other test indicates a microtropia the first time they are attempted, this should be reassessed regularly as treatment progresses and certainly before treatment is stopped and suboptimal visual acuity is accepted.