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Cristina Llerena Law, Matt Siu, Patricia A Modica, Benjamin T Backus; Stimulus Characteristics Affect the Assessment of Pupil Defects in Amblyopia. Invest. Ophthalmol. Vis. Sci. 2014;55(13):440. doi: https://doi.org/.
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There is no clear consensus as to the frequency or clinical significance of pupillary response defects for amblyopia (Portnoy et al, 1983, Firth, 1990, Miki et al, 2008). New technology that is sensitive to subtle changes in pupil behavior may shed light on the true incidence of this defect in amblyopes. Pupillary responses have striate and extra-striate cortical influences (Kardon et al, 1993, 1998, Barbour et al, 1998, 2004), and by designing stimuli that highlight these contributions, one might better be able to identify pupil defects in amblyopes when they exist.
Steady state and dynamic pupil response measures using various stimulus conditions known to isolate cortical and pre-cortical contributions to pupil responses were acquired with a binocular infrared pupillometer (Konan-USA) on 15 amblyopic subjects (anisometropic and small angle strabismic) and 11 normally sighted age matched controls. One full field white flash stimulus (330 cd/m2) and three small annuli of varying contrast levels (0.3, 0.6, and 1.8 x above background) were presented alternatively for a stimulus duration of 100 msec and an inter-stimulus duration of 2000 msec. Each sequence was repeated until 9 valid pairs of data from each eye were obtained with each stimulus. Subjects were dark adapted for five minutes before each set of stimuli.
All stimulus conditions tested found a greater degree of variability between the right and left eye’s afferent pupil responses in amblyopes compared to controls (p<0.01). However, on average when compared to normal subjects, afferent responses of the amblyopic eye were not significantly smaller than that of the fellow eye for the full field flash. For low contrast annular stimuli, the mid-contrast (0.6) stimulus exhibited the most statistically significant pupil response difference between the amblyopic and fellow eyes of amblyopes (p<0.04).
Small low contrast targets select for cortical contributions to the pupil response, and may be more useful for detecting mild pupil defects present in amblyopic patients. Proper documentation of the nature of pupil responses in amblyopes and further characterization of the potential mechanism involved in this deficit may elucidate cortical neural mechanisms responsible for amblyopia. Pupil analysis could prove useful if it is found to have a diagnostic or prognostic value for differentiating which amblyopes will respond best to treatment.
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