Ictal-Interictal Continuum

Seizures represent a common neurological problem among critically ill patients that requires evaluation and management in the hospital setting. These patients may develop seizures due to multiorgan failure, severe metabolic disturbances, or primary central nervous system pathologies. In parallel, such patients may present with seizures as the primary medical issue that gives rise to these complications. Untreated isolated seizures can quickly give rise to nonconvulsive or convulsive status epilepticus, which is associated with elevated morbidity and mortality. Treating seizures can be challenging due to pathophysiological changes that increase resistance to intervention and comorbid illnesses that impact antiseizure medication strategies.  Early identification and treatment of seizures in critically ill patients is considered a pillar of neurocritical care. However, obtaining clarity about whether a patient’s clinical disposition is reflective of an ongoing ictal process is not always feasible, owing to limitations of the diagnostic modalities used to identify these particular abnormalities in the clinical setting. The conceptualization of this diagnostic uncertainty has given rise to the expanded use of the term “ictal-interictal continuum” (IIC). The earliest use of the term IIC can be traced to Pohlmann-Eden et al, who hypothesized, in the context of reviewing the clinical implications of lateralized periodic discharges (LPDs), that these regularly appearing focal transients reflect a dynamic pathophysiological state in which a combination of clinical factors and patient-specific susceptibilities contributes to whether definitive seizures ultimately emerge from this pattern. The rising utilization of continuous video EEG in the acute care setting has revealed that LPDs and other periodic and rhythmic patterns are extremely common. Significant attention has subsequently been devoted to understanding the neurophysiological substrates that underlie these electroencephalographic (EEG) patterns, and in turn, the clinical implications for treatment and outcomes.  Since its initial description, IIC patterns have expanded to include other rhythmic and periodic patterns known to be associated with an increased risk of seizures, eg, generalized periodic discharges (GPDs), bilateral independent periodic discharges (BIPDs), and lateralized rhythmic delta activity (LRDA). To mitigate confusion, the American Clinical Neurophysiology (ACNS) convened a group of experts to standardize critical care EEG terminology. In the most recent iteration of this expert opinion, published in 2021, patterns lying on the IIC are now based upon well-defined electrographic and clinical criteria to increase uniformity among both clinicians and investigators.