Slowing
Focal polymorphic delta marks a structural or functional lesion beneath it; continuous delta with loss of faster activity on the same side implies more damage than intermittent slowing. Generalized slowing is the signature of diffuse encephalopathy. Neither names the cause. 11
Acute stroke and subarachnoid haemorrhage
Ischaemia first removes fast activity, then adds delta; severe ischaemia attenuates everything. Continuous EEG is recommended to detect seizures in acute brain injury with unexplained impaired consciousness, and is used in some centres to detect delayed cerebral ischaemia after subarachnoid haemorrhage through a falling alpha-to-delta ratio or loss of alpha variability. 26
Coma patterns
| Pattern | Usual meaning |
|---|---|
| Reactive theta–delta | Mild to moderate encephalopathy, potentially reversible |
| GPDs with triphasic morphology | Often toxic-metabolic; not specific |
| Alpha or theta coma, unreactive | Depends on cause: poor after anoxia, often good after intoxication |
| Spindle coma | Brainstem or thalamic dysfunction; relatively favourable after trauma |
| Burst suppression | Anaesthesia, hypothermia or severe diffuse injury |
| Suppression | Severe injury, deep sedation or hypothermia |
Electrocerebral inactivity
ECI means no EEG activity above 2 µV. The ACNS minimum technical standards: 5
- At least eight scalp electrodes, with inter-electrode distances of at least 10 cm
- Impedances between 100 Ω and 10 kΩ, and the integrity of the whole system tested
- Sensitivity of at least 2 µV/mm for at least 30 minutes
- Low-frequency filter no higher than 1 Hz, high-frequency filter no lower than 30 Hz
- No reactivity to intense somatosensory, auditory and visual stimulation
- Additional monitoring (ECG, respiration) to identify artefact; a qualified technologist; repeat if in doubt
Death by neurological criteria is a clinical determination. ECI occurs reversibly with sedative intoxication and hypothermia. The 2023 AAN/AAP/CNS/SCCM guideline does not accept EEG as an ancillary test, while some national protocols still require or allow it. Know yours.6
Functional (dissociative) seizures
The diagnosis is documented when a typical event is recorded on video-EEG and shows no ictal change, with normal awake rhythms before, during and after, in a semiology consistent with a functional seizure. 19 Two cautions: muscle and movement artefact can hide the EEG, and focal aware seizures and some frontal lobe seizures may show nothing on the scalp. Epilepsy and functional seizures also coexist.
The older terms are psychogenic non-epileptic seizures and pseudoseizures; the last should be retired. In the CODES trial, adding seizure-specific cognitive behavioural therapy to standardized care did not reduce seizure frequency at 12 months, though several secondary outcomes improved. 28
Benign variants worth knowing by name
- Wicket spikes: temporal, arciform, in drowsiness, no slow wave
- Rhythmic mid-temporal theta of drowsiness
- Small sharp spikes (benign sporadic sleep spikes)
- 14 and 6 Hz positive bursts, in adolescents
- 6 Hz "phantom" spike-and-wave
- SREDA, the one that mimics a seizure