Videographic Hirschberg measurement of simulated strabismic deviations

Purpose. To demonstrate the potential use of subpixel image processing methods to perform automated Hirschberg measurements of strabismic deviations using relatively inexpensive personal computer hardware; to determine if the method might allow screening for strabismus using full-face video images obtained from a distance of 1 meter. Methods. Strabismic deviations (< 25 prism diopters) were simulated by means of induced asymmetric fixation. A ring of coaxial infrared light-emitting diodes (LED) were used to generate first Purkinje reflexes. Computerized image analysis with subpixel processing was used to measure the locations of the first Purkinje reflexes and pupil centers of video images of 10 normal subjects, following the technique of the clinical Hirschberg test. The apparent strabismic deviation was calculated from the relative asymmetry of the center of the corneal reflex ring to the pupil center in each eye. Results. In 10 normal subjects, there was a statistically significant linear correlation of Hirschberg horizontal reflex deviation with asymmetric fixation pseudo-esotropia (0.85 ≥ r² ≥ 0.99, P < 0.05). Conclusions. The Hirschberg test is used manually to detect strabismus in infants and children but requires a highly skilled examiner. The image processing method described here requires no operator interpretation and may make the test more applicable. The results suggest that this technology may be appropriate for a screening instrument.