The ictal–interictal continuum (IIC) describes certain rhythmic or periodic EEG patterns that are more concerning than ordinary interictal abnormalities but do not meet formal criteria for definite electrographic seizure or status epilepticus. It is one of the major grey zones in critical-care EEG.
Why the old binary model is not enough
EEG patterns were traditionally discussed as either ictal—meaning seizure activity—or interictal—meaning activity between seizures.
Critical-care monitoring revealed many patterns that do not fit cleanly into either category.
What kinds of patterns are involved
Examples include some lateralised or generalised periodic discharges and rhythmic delta patterns with particular frequencies or additional features.
Formal ACNS terminology defines which EEG patterns qualify for the IIC, but the label itself does not prove that a pattern is causing clinical symptoms or neuronal injury.
Why clinicians care about these patterns
Some IIC patterns are associated with a higher probability of electrographic seizures and may increase metabolic demand in injured brain tissue.
The concern is that an apparently ‘borderline’ pattern may represent harmful ongoing ictal activity in some patients.
Why aggressive treatment can also cause harm
Escalating antiseizure or anaesthetic treatment can cause hypotension, respiratory suppression, prolonged ventilation, infection and other intensive-care complications.
That means the decision is not simply whether the EEG looks abnormal. Clinicians must balance the possible harm of undertreating ictal activity against the harm of overtreatment.
Clues that may support ictality
Features that can increase concern include evolution in frequency or morphology, association with a clinical sign, high seizure risk, metabolic or imaging correlates and improvement after a diagnostic antiseizure medication trial.
None of these should be interpreted in isolation.
The role of imaging and metabolism
Peri-ictal MRI changes, PET hypermetabolism and other physiological markers can sometimes support the interpretation that an EEG pattern is biologically active.
These tools are adjuncts, not universal tests.
An active research area
Recent reviews continue to emphasise controversy over where status epilepticus ends and the IIC begins, which patterns require treatment and how treatment intensity should be chosen.
The IIC is therefore best understood as a structured description of uncertainty rather than a diagnosis by itself.
Related reading
Sources and further reading
- Rethinking the electrographical boundaries of status epilepticus — 2026 review
- Ictal-Interictal Continuum in the pediatric ICU — 2024
- Electrographic seizures and IIC patterns in critically ill patients
Evidence checked: September 2026.