Association of CSF and Serum Neurofilament Light and Glial Fibrillary Acidic Protein, Injury Severity, and Outcome in Spinal Cord Injury

Abstract

Background and objectives: Traumatic spinal cord injury (SCI) is highly heterogeneous, and tools to better delineate pathophysiology and recovery are needed. Our objective was to profile the response of 2 biomarkers, neurofilament light (NF-L) and glial fibrillary acidic protein (GFAP), in the serum and CSF of patients with acute SCI to evaluate their ability to objectively characterize injury severity and predict neurologic recovery.

Methods: Blood and CSF samples were obtained from prospectively enrolled patients with acute SCI through days 1-4 postinjury, and the concentration of NF-L and GFAP was quantified using Simoa technology. Neurologic assessments defined the ASIA Impairment Scale (AIS) grade and motor score (MS) at presentation and 6 months postinjury.

Results: One hundred eighteen patients with acute SCI (78 AIS A, 20 AIS B, and 20 AIS C) were enrolled, with 113 (96%) completing 6-month follow-up. NF-L and GFAP levels were strongly associated between paired serum and CSF specimens, were both increased with injury severity, and distinguished among baseline AIS grades. Serum NF-L and GFAP were significantly (p = 0.02 to <0.0001) higher in AIS A patients who did not improve at 6 months, predicting AIS grade conversion with a sensitivity and specificity (95% CI) of 76% (61, 87) and 77% (55, 92) using NF-L and 72% (57, 84) and 77% (55, 92) using GFAP at 72 hours, respectively. Independent of clinical baseline assessment, a serum NF-L threshold of 170 pg/mL at 72 hours predicted those patients who would be classified as motor complete (AIS A/B) compared with motor incomplete (AIS C/D) at 6 months with a sensitivity of 87% (76, 94) and specificity of 84% (69, 94); a serum GFAP threshold of 13,180 pg/mL at 72 hours yielded a sensitivity of 90% (80, 96) and specificity of 84% (69, 94).

Discussion: The potential for NF-L and GFAP to classify injury severity and predict outcome after acute SCI will be useful for patient stratification and prognostication in clinical trials and inform communication of prognosis.

Classification of evidence: This study provides Class I evidence that higher serum NF-L and GFAP are associated with worse neurological outcome after acute SCI.

Trial registration information: Registered on ClinicalTrials.gov: NCT00135278 (March 2006) and NCT01279811 (January 2012).

Figure 1. Association of NF-L and GFAP in Paired Serum and CSF Specimens Taken Following SCI

The concentration of (A) NF-L and (B) GFAP was quantified in 349 paired serum and CSF samples from 113 patients with SCI collected between 4 and 107 hours following injury. Data were analyzed using a Spearman correlation. GFAP = glial fibrillary acidic protein; NF-L = neurofilament light; SCI = spinal cord injury.

Figure 2. Evaluation of Serum NF-L and GFAP as Biomarkers of Baseline Injury Severity (AIS Grade)

(A) NF-L and (B) GFAP were measured in serum samples from controls (n = 19, gray), AIS A (n = 78, orange), AIS B (n = 20, blue), and AIS C (n = 20, green) patients with SCI taken up to 4 days after injury. *Statistical results compared with control (shown once in 24-hour bin). +Statistical results compared within SCI groups. AIS = ASIA Impairment Scale; GFAP = glial fibrillary acidic protein; NF-L = neurofilament light; SCI = spinal cord injury.

Figure 3. Comparison of Serum NF-L and GFAP in AIS A Patients, Distinguished by Whether AIS Grade Conversion Occurred (Yes/No) at 6 Months Postinjury

Of the 74 AIS A patients with outcome assessed at 6 months, 49 (66%) remained an AIS A (no conversion, orange), whereas 25 (34%) improved in their AIS grade (yes conversion, blue). Serum (A) NF-L and (B) GFAP were graphed based on AIS A conversion status at 6 months, where *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001 NF-L and GFAP medians expressed as pg/ml below the graphs. ROC curves were generated comparing the concentration of (C) NF-L and (D) GFAP at each time point (24 hours orange, 48 hours blue, 72 hours gray, and 96 hours teal) based on conversion status. AUROC values and errors are listed in eTable 9, links.lww.com/WNL/C585. AIS = ASIA Impairment Scale; AUROC = area under the ROC; GFAP = glial fibrillary acidic protein; NF-L = neurofilament light; ROC = receiver operating characteristic; SCI = spinal cord injury.

Figure 4. Comparison of Serum NF-L and GFAP Concentration Based on the Observed AIS Grade at 6 Months

Six-month outcome assessments were available in 113/118 (96%) patients with SCI. Seventy patients (62%) were classified as AIS A or AIS B (motor complete, orange), whereas 43 (38%) were classified as AIS C or AIS D (motor incomplete, blue) at 6 months. Graph of 24-, 48-, 72-, and 96-hour (A) NF-L and (B) GFAP in patients with SCI dichotomized based on 6-month AIS outcome, where ****p < 0.0001. NF-L and GFAP medians expressed as pg/ml below the graphs. ROC curves comparing (C) NF-L and (D) GFAP concentration at each time point based on observed AIS grade. AUROC values and errors are listed in eTable 14, links.lww.com/WNL/C585. AIS = ASIA Impairment Scale; GFAP = glial fibrillary acidic protein; NF-L = neurofilament light; ROC = receiver operating characteristic; SCI = spinal cord injury.

Figure 5. Association of Serum NF-L and GFAP and 6-Month Total Motor Score Recovery in All Patients With Cervical SCI

The change in total motor score (ΔMS) was dichotomized into patients who gained ≤8 points (orange, n = 28) vs those who gained >8 points (blue n = 37) at 6 months. Graphs of serum (A) NF-L and (B) GFAP based on motor score recovery, where **p < 0.01, ***p < 0.001, and ****p < 0.0001. NF-L and GFAP medians expressed as pg/ml below the graphs. ROC curves comparing (C) NF-L and (D) GFAP concentration at all time points based on dichotomized motor score recovery. AUROC values and error are listed in eTable 19, links.lww.com/WNL/C585. AUROC = area under the ROC; GFAP = glial fibrillary acidic protein; NF-L = neurofilament light; ROC = receiver operating characteristic; SCI = spinal cord injury.

Figure 6. Association of Serum NF-L and GFAP and 6-Month Motor Score Recovery in Cervical AIS A Patients

Serum (A) NF-L and (B) GFAP were compared in cervical AIS A patients who gained ≤8 points (orange, n = 25) vs those who gained >8 points (blue n = 11) of motor score at 6 months, where *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001, and ns =not significant. NF-L and GFAP medians expressed as pg/ml below the graphs. ROC curves comparing (C) NF-L and (D) GFAP concentration at all time points based on dichotomized motor score recovery. AUROC values and errors are listed in eTable 21, links.lww.com/WNL/C585. AIS = ASIA Impairment Scale; AUROC = area under the ROC; GFAP = glial fibrillary acidic protein; NF-L = neurofilament light; ROC = receiver operating characteristic; SCI = spinal cord injury.