Abstract
Purpose :
To explore the dynamics of pupil response recovery to increasing test flash strengths in patients with mild Traumatic Brain Injury (mTBI).
Methods :
Pupil responses were recorded with a custom pupilometer (Diagnosys LLC) to increasing strengths of 2-minute blue test stimuli from mTBI (N=5, 22-53yrs) and controls (N=5, 25-54yrs). Consensual responses were recorded after dilating the pupil of the stimulated eye. Baseline pupil diameter was recorded after 5-minutes of dark-adaptation and thereafter at fixed intervals for stimuli in the range of 10-4 to 10 Cd/m2. Subject was tested on 2 protocols, one with and the other without a 2-minute inter-stimulus interval (ISI) of darkness between successive stimuli (ISI and no-ISI). Pupil diameter averaged in the last 300ms of the test stimulus duration and normalized to the baseline was plotted as a function of the log stimulus strength and fitted with a sigmoidal function of the form y=Min+[(Max-Min)/(1+10^(logEC50-x)]. The fit parameters of maximum diameter (Max) and the stimulus strength at diameter midway between Max and Min values were compared between mTBI and controls.
Results :
Average baseline diameter of mTBI was greater than that of controls (5.4±1.4mm vs 6.9±0.7mm) but not statistically significant (p=0.057). The pupil diameter reduced following stimulus onset and the maximum constriction was not significantly different between the two groups for both protocols. The diameter recovered to baseline at lower stimulus strengths and magnitude of recovery reduced with increasing stimulus strength with no recovery at the highest strength that was above the threshold of melanopsin activation. Normalized Max diameter was larger for the mTBI relative to controls for the no-ISI condition (94±5% vs 86±7%, p=0.04). A similar trend was observed for the ISI protocol, but was not statistically significant (96±4% vs 90±9%, p=0.2). Log EC50 value was larger for mTBI relative to controls for both no-ISI (0.57±0.12 vs 0.35±0.07, p=0.006) and ISI (0.68±0.23 vs 0.31±0.05, p=0.007) protocols.
Conclusions :
The greater recovery of pupil diameter (~8%) in the presence of test stimuli and increase in the stimulus strength (by 0.2 to 0.3 log units) for recovery to midpoint diameter likely indicates an increase in the light-adaptation of the sensory neurons driving pupillary responses in mTBI patients.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.