Reliability and interexaminer agreement for induced tropia test: is normal always normal?

The purpose of this study was to evaluate the reliability of the fixation preference testing with 10 and 20 diopter prisms (Δ) in children without strabismus in order to extrapolate its utility to preverbal children and to determine interexaminer agreement. Fourty children (24 girls, 16 boys) aged between 5 and 16 years with normal ophthalmological examination except refractive errors were included in the study. The visual acuity, refractive errors, and orthoptic findings were recorded. The base-down prism fixation test was performed by two experienced examiners with 10Δ and 20Δ. The presence of corrective movement was recorded as positive result separately for each eye. Best-corrected visual acuity was 1.0 bilaterally for all patients. All patients had binocular single vision and stereopsis. First examiner noted bilateral fixation movement in 65 % patients with 10Δ and in 57.5 % with 20Δ, second examiner in 50 % with 10Δ, and in 37.5 % with 20Δ. Interexaminer agreement was 42.5 % for 10Δ and 54.5 % for 20Δ (p < 0.01). Both examiners observed the fixation movement to be less frequent in older children. The fixation movement can be absent or seen asymmetrically even in children with equal visual acuity and binocular vision. The rate of fixation movement seemed to be reduced with age possibly due to increased concentration and cooperation of the child. In addition, there was a remarkable interexaminer variability in both tests. It should be borne in mind that the findings of prism-guided fixation preference tests may be misleading in preverbal children.