Granularity of EEG reactivity in response to stimulation: "Atypical" reactivity in non-sedated and sedated patients

We describe the EEG phenomenon of increased arrhythmic delta-frequencies upon stimulation without sedating medications ("atypical reactivity") and characterize patients with this pattern. This is currently not differentiated from the expected reactivity pattern of increased faster-frequencies, according to the American Clinical Neurophysiology Society's (ACNS) critical care EEG terminology.

This was a retrospective single-center study of adult patients (2015-2022) with EEG reactivity defined by arrhythmic delta-frequencies emerging with stimulation. Raw EEGs were reviewed. Patient demographics, medical conditions, and outcome data were extracted.

Ninety patients with atypical reactivity were identified: 70 without confounding sedating medications (non-sedation group), and 20 receiving sedating medications (sedation group). Atypical reactivities were marked by increased arrhythmic delta-activity compared to pre-stimulation baseline, often with decreased faster-frequencies. Nearly all patients in both groups (81%, 100%) had toxic-metabolic etiologies of encephalopathy regardless of sedation. Other encephalopathic EEG patterns were commonly present in both groups (57%, 55%). Survival at 30-days and hospital discharge were overall fair.

We propose an addition to the ACNS critical care EEG terminology, called atypical reactivity. Reversible toxic-metabolic encephalopathies are highly prevalent in patients with atypical reactivity.

Identifying atypical reactivity may help differentiate the etiologies and prognoses of patients with encephalopathy.