Background and objectives:Electroencephalography (EEG) is widely used for prediction of neurological outcome after cardiac arrest. To better understand the relationship between EEG and neuronal injury, we explore the association between EEG and neurofilament light (NFL) as a marker of neuroaxonal injury. We evaluate whether highly malignant EEG patterns are reflected by high NFL levels. Additionally, we explore the association of EEG backgrounds and EEG discharges with NFL.Methods:Post-hoc analysis of the Target Temperature Management after out-of-hospital cardiac arrest (TTM)-trial. Routine EEGs were prospectively performed after the temperature intervention ≥36 hours post-arrest. Patients who awoke or died prior to 36 hours post-arrest were excluded. EEG-experts blinded to clinical information classified EEG background, amount of discharges and highly malignant EEG patterns according to the standardized American Clinical Neurophysiology Society terminology. Prospectively collected serum samples were analyzed for NFL after trial completion. The highest available concentration at 48 or 72-hours post-arrest was used.Results:262/939 patients with EEG and NFL data were included. Patients with highly malignant EEG patterns had 2.9 times higher NFL levels than patients with malignant patterns and NFL levels were 13 times higher in patients with malignant patterns than those with benign patterns (95% CI: 1.4-6.1 and 6.5-26.2 respectively, effect size 0.47, p<0.001). Both background and the amount of discharges were independently strongly associated with NFL levels (p<0.001). The EEG background had a stronger association with NFL levels than EEG discharges (R2=0.30 and R2=0.10, respectively). NFL levels in patients with a continuous background were lower than for any other background (95% CI for discontinuous, burst-suppression and suppression, respectively: 2.26-18.06, 3.91-41.71 and 5.74-41.74, effect size 0.30 and p<0.001 for all). NFL levels did not differ between suppression and burst-suppression. Superimposed discharges were only associated with higher NFL levels if the EEG background was continuous.Discussion:Benign, malignant, and highly malignant EEG patterns reflect the extent of brain injury as measured by NFL in serum. The extent of brain injury is more strongly related to the EEG background than superimposed discharges. Combining EEG and NFL may be useful to better identify patients misclassified by single methods.Clinical Trials Registration Number:clinicaltrials.gov, NCT01020916