By Hai Chen, MD, PhD
Synopsis: Abnormal background continuity, ictal-interictal continuum features, and an unreactive electroencephalogram (EEG) background were independently associated with unfavorable outcomes in the overall cohort of patients with acute brain injuries. In addition, an unreactive EEG predicted unfavorable outcomes at six months with high specificity across multiple patient subgroups. However, the sensitivity of unreactive EEG for predicting unfavorable outcomes was low.
Source: Benghanem S, Novy J, Schindler KA, et al. Unreactive EEG background is associated with unfavorable outcome in patients with disorders of consciousness of various etiologies: An adult cohort study. Eur J Neurol. 2025;32(8):e70312.
Assessing neurological outcomes in patients following acute brain injury remains a clinical challenge. Electroencephalography (EEG) has been used widely as an indicator of cerebral function and serves as a prognostic marker in cases of hypoxic-ischemic brain injury (HIBI). In this study, the authors evaluated the prognostic performance of EEG in predicting neurological outcomes in critically ill patients with various forms of acute brain injury.
A total of 364 adult patients with acute brain injuries and impaired awareness were included in the study. Patients with invasive procedures, recent seizures, or status epilepticus were excluded. Clinical management was determined by the treating teams at participating centers and was not standardized; caregivers were not blinded to the EEG results. Patients were randomized to receive either two 20- to 30-minute EEG recordings over a 48-hour period or a single continuous EEG recording lasting 30-48 hours. EEG reactivity was assessed at least once daily. Background EEG reactivity was defined as a reproducible change in amplitude or frequency of the background cerebral activity in response to stimulation. The primary outcome measure was the modified Rankin Scale (mRS) score at six months.
In the cohort, 112 patients had HIBI, 85 had intracranial hemorrhage (ICH), 28 had ischemic stroke, 48 had traumatic brain injury (TBI), 23 had toxic-metabolic encephalopathy, seven had encephalitis, and 114 had unknown or other etiologies. A majority of patients (84.3%) had a reactive EEG background. At six months, 111/364 (30.5%) reached a good neurological outcome (defined as an mRS score < 3), while mortality was reported in 177 out of 364 patients (48.6%). At six months, patients with unfavorable outcomes (mRS score 3-6) were more likely to be female, older, have a higher Simplified Acute Physiology Score, and a higher baseline mRS score before admission. ICH patients also most frequently had unfavorable outcomes among diagnostic subgroups. In addition, certain EEG features, including a discontinuous or suppressed background, low frequency, lack of reactivity, presence of seizures or ictal-interictal continuum (IIC), also were associated with unfavorable neurological outcome.
Multivariate logistic regression analysis identified several independent predictors of unfavorable neurological outcomes at six months. These included older age (odds ratio [OR], 1.04), female sex (OR, 2.56), higher pre-admission mRS score (OR, 1.35), diagnosis of ICH (OR, 4.00) or ischemic stroke (OR, 4.35), presence of IIC patterns on EEG (OR, 2.78), abnormal EEG background continuity (OR, 2.33), and unreactive EEG background (OR, 10.9).
The authors then investigated the prognostic value of EEG reactivity based on the etiology of acute brain injury. Patients with a favorable outcome more frequently exhibited a reactive background in both the HIBI (96.9% vs. 53.2%), and TBI (94.1% vs. 77.4%) subgroups, but not in ICH patients (94.1% vs. 91.2%). Furthermore, the authors examined the prognostic value of unreactive EEG in predicting an unfavorable outcome in both the overall cohort and specific subgroups. An unreactive background was associated with unfavorable outcome, with a high specificity across the board: 97.3% in the whole group, 97% in HIBI subgroup, 94.1% in both the TBI and ICH groups. However, the sensitivity is lower in the overall cohort (22.1%), as well as the TBI and ICH groups (22.6% and 8.8%, respectively). The sensitivities were higher in the HIBI cohort compared to other etiologies, reaching 46.8%.
Commentary
Current tools remain limited in predicting neurological outcome in patients with acute brain injuries. EEG characteristics are correlated with the severity of brain injury, and EEG reactivity reflects dynamic brain responses to external stimulations, requiring functional integrity of different neuroanatomical structures from the brainstem to subcortical and cortical regions. This study investigated the prognostic utility of EEG in predicting neurological outcomes at six months. Strengths of the study include its prospective, randomized design and the use of standardized EEG terminology, which enhances data consistency and minimizes subjectivity in interpretation.
In this study, an unreactive EEG was highly specific (sensitivity 94% to 97%) for predicting poor neurological outcome at six months in patients with disorders of consciousness after acute brain insult. Thus, EEG reactivity could help to reduce prognostic uncertainty in patients with acute brain injury and mitigate the risk of false pessimistic predictions. However, the sensitivity was low to medium (8.8% to 46.8%) across three major subgroups, indicating this marker primarily identifies the most severely affected patients. Therefore, EEG reactivity should be integrated into a multimodal prognostic approach that includes clinical examination, neuroimaging, and other physiological markers in the prognostic evaluation.
Hai Chen, MD, PhD, is Assistant Professor of Clinical Neurology, Weill Cornell Medical College.
