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.

An electroencephalogram (EEG) is a critical tool in epilepsy diagnosis. The three common EEG durations are 25 minutes, 1 hour, and 24 hours. One-hour EEGs are superior in showing epileptiform abnormalities, while 24-hour EEGs are used to characterize seizure and nonepileptic event semiology and guide treatment of status epilepticus. The term EEG montage refers to the way EEG electrodes are ordered for interpretation. Odd numbered electrodes are on the left, with even numbered on the right. Smaller numbers are closer to the midline, while z means the electrode is on the midline.This chapter will explore the numerous normal and variant findings like posterior dominant rhythm (PDR) and wicket spikes. Epileptiform findings like sharp waves or seizure patterns are indicative of epilepsy. Slowing or increased amplitude can indicate cerebral changes that are not epileptiform.Electroencephalogram reports should concisely accurately convey both the electrical findings and their clinical relevance to patient care. Electroencephalogram reports should indicate an epilepsy diagnosis only when clear electrical evidence exists.

Ictal patterns represent ongoing electrographic seizures. They are recognized by their clinical accompaniments (may be subtle) and electrographic features. Evolution is the electrographic hallmark of an ictal pattern, plus features (such as overriding fast activity) also render a pattern more ictal in appearance. Electrographic seizures will typically have a clear onset, evolution and offset. Conventionally, they should last for greater than 10 seconds. Electrographic seizures are common in after convulsive seizures, acute brain injury and in critically ill patients with altered mentation. Continuous EEG monitoring is the preferred method of diagnosing electrographic seizures. Typically, a duration of 24 hours is sufficient, but this should be extended in certain high-risk populations.

Confirm the patient’s identity, age, state(s) of recording, and the presence of any skull defects. Confirm the technical parameters of including the filter settings, sensitivity, paper speed, and time base. Note the montage you are reading in and the calibration signal. Identify the background from the foreground. Describe the background based on symmetry, continuity, voltage, organization, variability-reactivity, and sleep architecture. Categorize the foreground components as cerebral activity or artifact. Describe cerebral activity based on its location (general or lateral), occurrence (sporadic or repetitive), and morphology (slow or sharp). Then categorize the activity as normal (normal variant) or abnormal. Decide if the abnormality is epileptogenic (associated with seizures) or ictal (ongoing seizure). Evolution is the hallmark of electrographic seizure activity. Remember that isolated changes in amplitude are not evolution. Look for the use of any provocation methods, such as hyperventilation and photic stimulation, and their effect on the EEG. Before you finish up, make sure you’ve looked at the single-channel EKG and the technologist’s log.