Influence of refractive error on pupillary dynamics in the normal and mild traumatic brain injury (mTBI) populations

Abstract

Purpose: There have been several studies investigating static, baseline pupil diameter in visually-normal individuals across refractive error. However, none have assessed the dynamic pupillary light reflex (PLR). In the present study, both static and dynamic pupillary parameters of the PLR were assessed in both the visually-normal (VN) and the mild traumatic brain injury (mTBI) populations and compared as a function of refractive error.

Methods: The VN population comprised 40 adults (22-56 years of age), while the mTBI population comprised 32 adults (21-60 years of age) over a range of refractive errors (-9.00D to +1.25D). Seven pupillary parameters (baseline static diameter, latency, amplitude, and peak and average constriction and dilation velocities) were assessed and compared under four white-light stimulus conditions (dim pulse, dim step, bright pulse, and bright step). The Neuroptics, infrared, DP-2000 binocular pupillometer (30Hz sampling rate; 0.05mm resolution) was used in the monocular (right eye) stimulation mode.

Results: For the majority of pupillary parameters and stimulus conditions, a Gaussian distribution best fit the data, with the apex centered in the low myopic range (-2.3 to -4.9D). Responsivity was reduced to either side of the apex.

Conclusions: Over a range of dynamic and static pupillary parameters, the PLR was influenced by refractive error in both populations. In cases of high refractive error, the PLR parameters may need to be compensated for this factor for proper categorization and diagnosis.

Keywords: Error refractivo; Infrared pupillometry; Lesión cerebral traumática leve (mTBI); Mild traumatic brain injury (mTBI); Miopía; Myopia; Pupil light reflex (PLR); Pupilometría por infrarrojos; Reflejo pupilar a la luz (RPL); Refractive error.

Figure 1

The Neuroptics DP-2000 laboratory-based, binocular pupillometer with subject being tested (used with permission, Neuroptics Inc.).

Figure 2

Top: schematic representation of a pupil response profile and the associated pupil parameters assessed as indicated by the open circles. The prestimulus time is 0.5 s, and the post-stimulus time is 6.0 s. Bottom: schematic representation of the four experimental test stimulus conditions. The x-axis represents the relative time, and the y-axis represents the relative stimulus intensity. Dim = 4 lx, bright = 251 lx, pulse = 100 ms, and step = 1000 ms.

Figure 3

Selected representative pupillary parameter profiles as a function of refractive error (RE) in diopters, showing the three general response categories. Data points represent the mean value for each subject. Closed circles represent the normals, and open circles represent those with mTBI. Solid lines represent the best fit for the normals, and dashed lines represent the best fit for those with mTBI. Avg DV = average dilation velocity in mm/s. Max DV = maximum dilation velocity in mm/s. Time is in s.

Figure 4

Maximum and minimum pupillary diameter in mm (y-axis) as a function of increasing age in years (x-axis) and diagnostic group.

Figure A1

Pupillary parameter profiles as a function of refractive error (diopters) showing the range of fit for both diagnostic categories under the four different stimulus conditions. Data points represent the mean value for each subject. Closed circles represent the normals, and open circles represent the mTBI subjects. Solid lines represent best fit for the normals, and dashed lines represent the best fit for those with mTBI. The asterisk signifies that the mean for that parameter is significantly different between the normal group and mTBI group. 15 out of 28 parameters showed a significant difference between the two groups.