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E. B. M. Geukers, S. Schutte, F. C. T. Van der Helm, J. R. Polling, H. J. Simonsz; Proof of Principle of the Delft Assessment Instrument for Strabismus in Young Children (DAISY). Invest. Ophthalmol. Vis. Sci. 2010;51(13):3004.
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The purpose of this study was to investigate the feasibility of an accurate, fast and objective strabismus assessment instrument to measure strabismus angles in young children.
To measure the angle of strabismus, a combination of optical techniques was used: an automated Brückner test was implemented and pupil centers and corneal reflection centers were tracked. Images were acquired using a stereo camera setup on 1m distance from the subject. Corneal reflections were elicited with three infrared (IR) light sources with a radiant power of 200mW. One of the light sources was exactly coaxial with one camera to cause red reflexes with varying intensities, depending on the point of fixation (Brückner test). The angle kappa of the fixating eye was measured in gaze ahead. The gaze directions of both eyes were estimated and the angle of strabismus was assessed, while the subject fixated central and eccentric LED targets. Preliminary validation of the device was performed with 5 orthotropic subjects fixating on 5 fixation points and 5 adult subjects with manifest horizontal strabismus. Angles of strabismus were measured in gaze ahead and were compared with Simultaneous Prism Cover Tests (SPCT) performed by a certified orthoptist. Accuracy was determined by calculating the 95% confidence interval (CI) with the intended gaze directions in the orthotropic subjects and with SPCT results in strabismic subjects.
On repeated measurements the accuracy varied between 0.4° (95% CI) for small rotation angles (< 10°) and 3.0° for larger rotation angles with respect to the camera. This was caused by neglecting aspherity of the cornea. The angle kappa was measured using the 3D location of the pupil centers and corneal reflection centers, which both caused an average variability of 0.5° in the result of the strabismus angle. The algorithm to find the pupil centers performed best on dark coloured irises under IR lighting conditions. All used optical techniques were fast (within a second) and required little cooperation of the subject.
Using a combination of optical techniques, strabismus angles could be measured quickly and objectively. The measurements required little cooperation of a patient. The experimental measurement results show potential for an objective, accurate strabismus assessment for young children.
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