NeurologyTier 2Topic

Electrical activity of the brain and nerve conduction

EEG and normal rhythms

EEG records summated postsynaptic potentials from cortical pyramidal neurons via scalp electrodes - reflects cortical, not deep/subcortical, activity
Normal rhythms are frequency-defined: delta (<4Hz, deep sleep/pathological if awake), theta (4-8Hz, drowsiness/pathological if awake), alpha (8-13Hz, relaxed wakefulness, eyes closed, posterior-dominant), beta (>13Hz, active concentration/anxiety)

Uses

Main uses: epilepsy diagnosis/classification, encephalopathy characterisation, and confirming electrographic seizures/status epilepticus (including non-convulsive status)

Findings

Finding	Association
Interictal epileptiform discharges (spikes/sharp waves)	Supports an epilepsy diagnosis, localises the focus
Generalised slowing	Non-specific encephalopathy
Triphasic waves	Classically hepatic/metabolic encephalopathy
Periodic lateralised epileptiform discharges (PLEDs)	Focal destructive lesion (stroke, herpes encephalitis, tumour)
Burst-suppression	Severe global cortical dysfunction (anoxic injury, deep anaesthesia)

Interpretation limits

A single normal EEG does not exclude epilepsy (interictal sensitivity only ~50%) - sleep-deprived or prolonged/video EEG increases yield
Continuous EEG monitoring is required to diagnose non-convulsive status epilepticus - clinical suspicion alone is insufficient

Clinical application

Guides antiepileptic choice by generalised vs focal epileptiform pattern
Distinguishes true seizure from psychogenic non-epileptic seizure (PNES) - a normal ictal EEG during a clinical "seizure" supports PNES

Associations

Epilepsy syndromes, hepatic/uraemic encephalopathy, herpes simplex encephalitis (PLEDs, temporal), anoxic brain injury, psychogenic non-epileptic seizures

Prognosis after cardiac arrest

EEG background reactivity and continuity after cardiac arrest carry prognostic weight for neurological recovery

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