The base-to-base induced-tropia prism test for detection of amblyopia: A pilot study Konstandina Koklanis, PhD, a,b Thong Le, BOrth, OphthSci(Hons), b and Zoran Georgievski, BAppSc(Orth)(Hons) a,c PURPOSE To investigate the accuracy of the base-to-base prism test (BBPT), which entails inducing an esotropia with the use of base-in prisms before each eye, in the diagnosis of amblyopia. METHODS Participants were consecutive patients recruited from a pediatric ophthalmology practice who were able to perform logMAR acuity testing. All participants underwent an orthoptic examination, including logMAR visual acuity testing, and a sensorimotor evaluation. Pa- tients with an interocular difference of 2 or more lines were considered to have amblyopia. Fixation preference was assessed by use of both the vertical fixation test and the BBPT. The agreement between the BBPT and visual acuity was analyzed and compared with the agree- ment between the vertical fixation test and visual acuity. RESULTS Fifty-one consecutive patients aged 4 to 17 (mean, 9.8 years; SD Æ 3.4) were included, of whom 11 were diagnosed with amblyopia. Kappa analysis showed moderate but significant agreement between the BBPT and logMAR (k 5 0.453, p 5 0.001), whereas the vertical fixation test demonstrated only a fair agreement (k 5 0.254, p 5 0.061). The overall sen- sitivity for the BBPT was 72.7% (95% confidence interval [95% CI], 43.4%-90.3%) but only 40% for the vertical fixation test (95% CI, 16.8%-69.7%). For the BBPT, specificity was 80% (95% CI, 65.2%-89.5%); for the vertical fixation test, 78% (95% CI, 63.3%- 88.0%). CONCLUSIONS The BBPT appears to be more sensitive than the vertical fixation test for detecting ambly- opia in our patient population. The 2 tests had similar specificity. (J AAPOS 2010;14:484-487) D etermining monocular visual acuity is a definitive part of the detection, assessment, and manage- ment of amblyopia. However, a quantifiable measurement is not always obtainable in children. Clini- cians therefore often rely on binocular fixation preference to estimate visual function. 1,2 The assessment of fixation preference involves objectively comparing the fixation of each eye such that alternation is indicative of equal vision; a preference to fixate with one eye as opposed to the other is indicative of amblyopia. This method is generally only an option in the presence of manifest strabismus of greater than 10 D . 3 For patients with a micro- tropia or orthotropia, a prism-induced tropia test is used as an alternative to assess fixation preference. The most com- monly used technique involves inducing a vertical strabis- mus using a prism of at least 10 D base down and applying a cover test to grade fixation. 4,5 In a number of studies, the investigators have questioned the ability of an induced tropia test to detect amblyopia. Al- though some suggest the test is reliable, 1,4,6 others have found poor correlation of amblyopia with prism-induced fixation preference. 7,8 This variability is consistent with the lack of agreement between studies in which authors investigated the reliability of fixation preference in individuals with strabismus. 1-3,6,7,9 At the Royal Children’s Hospital in Melbourne, we have been using a prism-induced tropia test designed by one of the authors (ZG), termed the base-to-base prism test (BBPT). The test entails optically inducing a horizontal stra- bismus by the use of loose base-in prisms before each eye. It is thought that this technique may provide a more accurate in- dication of amblyopia because the horizontal movement pro- duced is easier to view in young children as compared with the smaller vertical movement when a 10 D base down is used and may be more valuable in patients with a microtropia, in whom the sensory adaptations occur along the horizontal meridian. This pilot study aims to investigate the accuracy of the BBPT in the diagnosis of amblyopia. Methods The study protocol was approved by the Institutional Review Board of the Royal Children’s Hospital in Melbourne. All partic- ipants were consecutive patients at the Royal Children’s Hospital Author affiliations: a Department of Clinical Vision Sciences, La Trobe University, Australia; b Department of Ophthalmology, Royal Children’s Hospital; c Department & Clinical School of Orthoptics, Royal Victorian Eye & Ear Hospital Submitted June 1, 2010. Revision accepted August 31, 2010. Reprint requests: Zoran Georgievski, Department of Clinical Vision Sciences, La Trobe University, Melbourne Victoria 3086, Australia (email: 2.georgievski@latrobe.edu.au). Copyright Ó 2010 by the American Association for Pediatric Ophthalmology and Strabismus. 1091-8531/$36.00 doi:10.1016/j.jaapos.2010.08.015 484 Journal of AAPOS