This chapter focuses on rhythmic and periodic patterns (RPPs). These are common EEG patterns found in critically ill patients. The variety of different types of patterns and their standardized naming conventions are described here. These patterns span from serving as markers of encephalopathy, to markers of seizure risk (interictal), to status epilepticus itself (ictal), and everything in between. This broad potential of diagnostic significance is known as the ictal–interictal injury continuum (IIIC). This chapter describes strategies for differentiating between more benign and more malignant RPPs. Basic management strategies for these IIIC patterns are also described in this chapter.

Repetitive waveforms may repeat in quick succession (rhythmic) , after nearly regular intervals (periodic), or spike and wave (SW). Periodic discharges are considered markers for neuronal injury. Most forms of repetitive abnormalities are epileptogenic; in specific situations they may also represent ictal patterns. GSW discharges of more than 3 Hz or other evolving discharges of more than 4 Hz are unequivocally ictal patterns, whereas GSW of less than 3 Hz or other evolving discharges of less than 4 Hz lie on an ictal-interictal spectrum. This means that these patterns may or may not be ictal depending on their clinical and electrographic accompaniments. A repetitive pattern is described as a combination of a main term 1 with a main term 2 based on the American Clinical Neurophysiology Society (ACNS) standardized critical care EEG terminology. Main term 1 could be either generalized (G), lateralized (L), bilateral independent (BI), or multifocal (Mf). Main term 2 could be either periodic discharges (PD), rhythmic delta activity (RDA), or generalized spike-wave (GSW). Common rhythmic abnormalities include GRDA and LRDA. Common periodic abnormalities include GPDs, LPDs, and BIPDs.The term “SI” is added to the combination of main terms to denote a stimulus-induced pattern (e.g., SI-GPDs for stimulus-induced GPDs).