SREDA is a rare pattern consisting of an evolving rhythm that is broadly distributed across the parietal and posterior temporal regions with frequency progression from the delta to the theta range (Westmoreland and Klass, 1997). Occasionally, the vertex is included in the field. It replaces the co-localized background activity throughout its occurrence, but it does not alter neighboring activity. In particular, the alpha rhythm is unchanged when SREDA’s distribution does not include the posterior regions. SREDA’s onset may be the abrupt development of a delta frequency range rhythm or the gradual coalescing of individual high-amplitude, monophasic or diphasic slow waves into a delta rhythm. Once the delta frequency rhythm is present, the frequency evolves into the theta range and the waveform becomes sharply contoured 5 to 7 Hz activity. This evolution occurs over what is commonly 40 to 80 seconds, but the evolution may be as short as 10 seconds or as long as 5 minutes (Westmoreland and Klass, 1981). The amplitude throughout the evolution usually is between 40 and 100 μV but it may be as high as 500 μV. The lower voltages are more likely to be present on bipolar montages because of the broad field. Other aspects of the rhythm, including the waveform and distribution, do not evolve or even have any significant change. The rhythm is typically bilateral and synchronous and it is symmetric in about 65% of instances, but it is predominantly unilateral in about 20%. Resolution may be either abrupt or gradual, and, upon resolution, the background activity immediately returns. Specifically, the region in which SREDA occurs does not demonstrate focal slowing or attenuation once the SREDA resolves.

SREDA occurs most commonly in the elderly and usually in individuals older than 50 years. However, younger individuals, including children, also may have SREDA (Nagarajan et al., 2001

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