Why monitor
Most seizures in critically ill patients have no visible movement. In the landmark continuous-EEG series, 19% of monitored patients had seizures and 92% of those were purely non-convulsive; the first seizure appeared within 24 hours in 95% of non-comatose patients, and in 80% of comatose patients, who needed 48 hours to reach 87%. 9 Unexplained impaired consciousness, especially after convulsive seizures have stopped, is the main indication. 26
Describe the background first
| Feature | ACNS 2021 categories |
|---|---|
| Continuity | Continuous; nearly continuous (up to 10% attenuated or suppressed); discontinuous (10–49%); burst suppression or burst attenuation (50–99%); suppression (more than 99%) |
| Voltage | Normal (20 µV or more); low (10–20 µV); suppressed (below 10 µV) |
| Predominant frequency | Delta, theta, alpha or faster |
| Posterior dominant rhythm | Present or absent, with frequency |
| Reactivity | Reactive, unreactive, unclear, or SIRPIDs only |
| Symmetry and state changes | Present or absent |
Naming a periodic or rhythmic pattern
Main term 1, then main term 2:
Generalized, Lateralized, Bilateral Independent, Unilateral Independent, Multifocal; Periodic Discharges, Rhythmic Delta Activity, Spike-and-Wave. So LPDs, GPDs, BIPDs, LRDA, GRDA. Then modifiers: prevalence, frequency, duration, sharpness, amplitude, stimulus-induced (SI), evolving or fluctuating, and "plus" for superimposed fast (+F), rhythmic (+R) or sharp (+S) activity. 1
The old names map directly: PLEDs are LPDs, BiPLEDs are BIPDs, FIRDA is frontally predominant GRDA, and "triphasic waves" are GPDs with triphasic morphology.
Which patterns predict seizures
In almost 5,000 critically ill patients, LPDs, LRDA and GPDs were associated with seizures; GRDA was not. Risk rose with frequency (clearly so from about 1.5–2 Hz), with higher prevalence and with a "plus" modifier. 25
The ictal–interictal continuum
Patterns that are not definite seizures but could be contributing to the patient's state. ACNS 2021: periodic discharges or spike-wave averaging more than 1 and up to 2.5 Hz for at least 10 seconds; or 0.5–1 Hz with a plus modifier or fluctuation; or lateralized rhythmic delta above 1 Hz with a plus modifier or fluctuation. 1
The continuum is a description, not a diagnosis. The usual next step is a monitored trial of an intravenous anti-seizure medication, judged on the patient as well as the trace. Improvement of the EEG alone does not prove the pattern was ictal.
Electrographic status epilepticus
An electrographic seizure lasting 10 continuous minutes or more, or occupying 20% or more of any 60-minute period. 1
Burst suppression
Suppression means below 10 µV. Bursts with at least two epileptiform discharges at 1 Hz or faster within most bursts are "highly epileptiform bursts"; bursts whose first half-second looks the same from one to the next are "identical bursts", described after cardiac arrest. 1
Reactivity
A change in the amplitude or frequency of the background on stimulation, attenuation included. Test it properly and document the stimulus: voice, then touch, then a noxious stimulus, eyes opened passively. The appearance of stimulus-induced rhythmic or periodic discharges alone does not count as reactivity; it is recorded as "SIRPIDs only". 1
After cardiac arrest
European guidelines use EEG as one element of multimodal prognostication, never alone. In a patient still comatose at 72 hours or later, with confounders excluded, a poor outcome is likely when at least two of these are present: absent pupillary and corneal reflexes; bilaterally absent N20 on somatosensory evoked potentials; highly malignant EEG after 24 hours (suppressed background, with or without periodic discharges, or burst suppression); neuron-specific enolase above 60 µg/L at 48 or 72 hours; status myoclonus within 72 hours; diffuse anoxic injury on CT or MRI. 7,8
Sedation and hypothermia produce suppression and burst suppression on their own. Record what the patient was receiving, and the temperature, on every report.