Purpose:
To investigate how small pupil size may reduce the extent of its movement, blunting the efferent portion of the pupil light reflex. The results from this study may facilitate a more reliable interpretation of pupil responses as an indicator of light processing by the retina and brain in health and disease.
Methods:
Pupil responses were recorded using a dual channel binocular eye frame pupillometer (Arrington Research, Scottsdale, AZ), to a series of Ganzfeld light stimuli with a stepwise increase in stimulus intensity (Diagnosys, Lowell, MA). One eye of eight normal subjects was treated with 0.2% brimonidine tartrate ophthalmic solution to achieve a reduction in pupil size by inhibiting sympathetic tone to the dilator muscle without affecting the iris sphincter muscle. The pupil responses from both eyes were recorded simultaneously. The amount of pupil contraction as a function of stimulus intensity was compared between the brimonidine treated, miotic eye and the untreated eye.
Results:
Brimonidine treatment produced significant reduction in pupil size in normal subjects (mean reduction in pupil size: 1.8±0.35 mm, p<0.05). For increasing light intensity, the treated pupil started to show reduced pupil contractions compared to the contralateral untreated pupil when the peak of pupil contraction reached an average pupil size of 3.3±0.60 mm (range 2.4 to 4.4 mm) among the eight subjects. When measured by percent pupil contraction (contraction amplitude/baseline pupil diameter), the pupil response as a function of stimulus intensity in the treated, miotic eye did not differ from that in untreated eye.
Conclusions:
Iris mechanics limit the amount of pupil contraction and can act to reduce the assessed neuronal integration of the pupil light reflex. The pupil size where this starts to occur was found to be on the average, 3.3 mm, but could vary among individuals. Pupil response assessed by using percent contraction amplitude is least affected by mechanical effects and might be a more reasonable approximation of afferent input.
Keywords: pupillary reflex • iris • drug toxicity/drug effects