Non-invasive electroencephalogram (EEG)-based methods for detecting a spreading depolarization secondary to a brain injury in a patient who exhibits high-amplitude delta activity in at least one channel of a scalp EEG of an injured brain hemisphere of the patient include (a) recording a baseline scalp EEG pattern in the patient at a channel exhibiting high amplitude delta activity; (b) recording a continuous scalp EEG pattern in the patient across a time frame at the at least one channel; and (c) detecting a spreading depolarization during the time frame by observing at least one feature indicative of a spreading depolarization in the continuous scalp EEG recording pattern relative to the baseline scalp EEG pattern at the at least one channel. Scalp EEG recordings are time-compressed prior to analysis. Methods of treating brain-injured patients and triaging brain-injured patients apply the non-invasive EEG methods.