Post-acute blood biomarkers and disease progression in traumatic brain injury

Abstract

There is substantial interest in the potential for traumatic brain injury to result in progressive neurological deterioration. While blood biomarkers such as glial fibrillary acid protein (GFAP) and neurofilament light have been widely explored in characterizing acute traumatic brain injury (TBI), their use in the chronic phase is limited. Given increasing evidence that these proteins may be markers of ongoing neurodegeneration in a range of diseases, we examined their relationship to imaging changes and functional outcome in the months to years following TBI. Two-hundred and three patients were recruited in two separate cohorts; 6 months post-injury (n = 165); and >5 years post-injury (n = 38; 12 of whom also provided data ∼8 months post-TBI). Subjects underwent blood biomarker sampling (n = 199) and MRI (n = 172; including diffusion tensor imaging). Data from patient cohorts were compared to 59 healthy volunteers and 21 non-brain injury trauma controls. Mean diffusivity and fractional anisotropy were calculated in cortical grey matter, deep grey matter and whole brain white matter. Accelerated brain ageing was calculated at a whole brain level as the predicted age difference defined using T1-weighted images, and at a voxel-based level as the annualized Jacobian determinants in white matter and grey matter, referenced to a population of 652 healthy control subjects. Serum neurofilament light concentrations were elevated in the early chronic phase. While GFAP values were within the normal range at ∼8 months, many patients showed a secondary and temporally distinct elevations up to >5 years after injury. Biomarker elevation at 6 months was significantly related to metrics of microstructural injury on diffusion tensor imaging. Biomarker levels at ∼8 months predicted white matter volume loss at >5 years, and annualized brain volume loss between ∼8 months and 5 years. Patients who worsened functionally between ∼8 months and >5 years showed higher than predicted brain age and elevated neurofilament light levels. GFAP and neurofilament light levels can remain elevated months to years after TBI, and show distinct temporal profiles. These elevations correlate closely with microstructural injury in both grey and white matter on contemporaneous quantitative diffusion tensor imaging. Neurofilament light elevations at ∼8 months may predict ongoing white matter and brain volume loss over >5 years of follow-up. If confirmed, these findings suggest that blood biomarker levels at late time points could be used to identify TBI survivors who are at high risk of progressive neurological damage.

Keywords: glial fibrillary acid protein (GFAP); neurofilament light (NFL); neuroimaging; outcome; traumatic brain injury.

Figure 1

Comparison of healthy volunteer levels of GFAP (A) and NFL (B) (plotted on a linear scale) compared to patients approximately 8 months and >5 years after a TBI. GFAP: healthy volunteer (HV) versus TBI ∼8 months P = 0.086, HV versus TBI >5 years P = 0.11, TBI ∼8 months versus TBI >5 years P = 0.0087. NFL: HV versus TBI ∼6months P < 0.0001, HV versus TBI >5 years P = 0.55, TBI ∼8 months versus TBI >5 years P = 0.0025. ns P > 0.05, *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001, ****P ≤ 0.0001.

Figure 2

Log GFAP and log NFL levels correlate at each time point post-TBI (A: ~6 to 8 months; and B: >5 years after injury), but the strength of this correlation decreases over time. The R² are shown adjusted for age, sex, and time since injury. GFAP and NFL are shown on log scales, but figures denote actual concentrations in pg/ml.

Figure 3

Temporal changes in GFAP (A) and NFL (B) in patient subset with data at ∼8 months and >5 years post-TBI (absolute values). The solid horizontal line represents the mean value for healthy volunteers and the dotted lines the standard deviations.

Figure 4

Log GFAP and log NFL levels at ∼8 months significantly correlate with fractional anisotropy and mean diffusivity in WBWM (A, B, E and F), mean diffusivity in the cortical grey matter (C and G) and the deep grey matter (D and H). The R² values shown are adjusted for age, sex, and time since injury. GFAP and NFL are shown on log scales, but figures denote actual concentrations in pg/ml.

Figure 5

Predicted brain age, predicted brain age difference for patients imaged >5 years after injury, and the mean JD for the subset of patients and controls imaged longitudinally. Predicted brain age versus actual for grey matter (WBGM) (A: Healthy volunteers R = 0.85 P < 0.0001, Patients R = 0.83, P < 0.0001) and WBWM (B: Healthy Volunteers 0.77, P < 0.001, Patients R = 0.87 P < 0.001). Comparison of PAD and mean JD for WBGM and WBWM between healthy volunteers and patients >5 years after injury (C and D). Comparison of the mean JD for WBGM and WBWM between patients imaged from ∼8 months and >5 years after injury compared to controls imaged twice over the same period (E and F). ****P < 0.00001.

Figure 6

Grey (A) and white matter (B) VBM for patients >5 years after TBI and control subjects. Results are corrected for FWE P < 0.05. The covariates in the model were age, sex and total intracranial volume.

Figure 7

NFL levels at ∼8 months post-TBI, adjusted for age, sex, and time post-TBI, predict WBWM rate of volume loss between ∼8 months and >5 years defined using the JD. Adjusted R = 0.41, P = 0.04. Absolute values for NFL shown (pg/ml).

Figure 8

PAD and biomarker levels at 5-year MRI in patient subgroups. (A and B) PAD in WBGM and WBWM and (C and D) levels of GFAP and NFL at >5-year MRI in subgroups of patients who showed improving (Improve; increase in GOSE ≥ 1 point), stable (no change in GOSE), or worsening (Worse; reduction in GOSE ≥ 1 point) between ∼8 months and >5 years post-injury. HV = healthy volunteers. Figures above box plots show unadjusted P-values for comparisons (Mann-Whitney U).