Luminance gain is altered in the retina and visual cortex of patients with mild Traumatic Brain Injury (mTBI)

Abstract

Purpose : To evaluate the simultaneous recorded light induced electrical activity of the retina and the visual cortex of patients with mild Traumatic Brain Injury (mTBI).

Methods : Flash electroretinogram (ERG) and visually evoked potentials (VEP) were recorded from mTBI patients (n=8) and controls(n=8) using the Espion ColorDome (Diagnosys LLC). Visual stimuli consisted of <5ms full-field red (627 nm) flashes (0.006-6.4 phot cd.s/m²) delivered on a constant (448 nm) background (7 phot.cd/m²). Amplitudes of the Photopic Negative Response (PhNR) of the ERG that reflects retinal ganglion cell activity and the N₂P₂ component of the VEP were plotted as a function of flash strength and fitted with a Naka-Rushton equation V(I) = (V_max*Iⁿ)/(Iⁿ+Kⁿ). Fit parameters of saturated amplitude (V_max), slope (n) and semisaturation constant (K) were compared between mTBI patients and controls.

Results : n of the PhNR for mTBI patients (1.6±0.37) was steeper (p=0.004) than that of controls (1.1±0.18). There was no significant difference between the two groups for PhNR V_max (37μv±11μv vs 32μv±11μv) and K (0.19±0.09 vs 0.27±0.12). The VEP V_max of mTBI patients was significantly lower than that of the controls (11μv±4μv vs 19μv±4μv, p=0.005) and VEP n was significantly steeper for mTBI patients compared to controls (1.87±0.43 vs 0.72±0.25, p=0.00006). VEP K was not significantly different between the two groups (0.04±0.03 vs 0.05±0.03). While the average value of n indicated shallower slopes for the VEP than ERG in control subjects the average slope of the VEP was steeper than that of the ERG for mTBI. r² was 0.78 for regression analysis of the slope differences between ERG and VEP measures and VEP V_max and the slope was significantly different from zero (p=0.046) with VEP amplitudes within or closer to the normal range when n of ERG was greater than that of VEP.

Conclusions : The luminance gain (n) of the PhNR and VEP is elevated in mTBI patients and is likely an adaptive mechanism to increase neuronal response. The luminance gain of the VEP on average was greater than that of the PhNR in mTBI subjects. Since the full-field ERG is the summed response from the whole retina and the VEP recorded with the active electrode above the inion is dominated by input from the macula, further studies are warranted to explore the contributions of the macula and retinal periphery to luminance gain changes in mTBI.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.