🔬 Electroencephalogram

An electroencephalogram (EEG) is a non-invasive test that records the electrical activity of the brain via electrodes placed on the scalp. It is a fundamental tool for the diagnosis and classification of epilepsy, as well as for evaluating encephalopathy and monitoring status epilepticus. It reflects the physiological function of the cerebral cortex in real-time.

The main indications are the investigation of suspected epilepsy or paroxysmal events where the diagnosis is unclear. It is used to classify seizure disorders (focal vs. generalised) and to monitor patients in status epilepticus. It may also be used in the evaluation of altered mental status, dementia, and sleep disorders. In specific cases, it is used for the prognosis of hypoxic-ischaemic brain injury post-cardiac arrest.

Small metal electrodes are attached to the scalp in a standardised arrangement (the 10-20 system) using a conductive paste. The electrodes detect the summation of excitatory and inhibitory postsynaptic potentials from the underlying cerebral cortex. During the test, 'provocative' maneuvers like hyperventilation and photic stimulation (flashing lights) are often used to elicit abnormal activity. The procedure is painless and usually lasts 20 to 40 minutes.

A normal adult EEG shows a well-organised 'background' rhythm. The posterior dominant Alpha rhythm (8-13 Hz) should be present when the eyes are closed and should attenuate (disappear) when the eyes open. Fast Beta activity is often seen in the frontal regions. There should be no epileptiform discharges (spikes) and no persistent focal or generalised slow-wave activity (Theta or Delta) during wakefulness.

The EEG captures the frequency and amplitude of brain waves. Normal rhythms include Alpha waves (8-13 Hz) seen in the posterior head during relaxed wakefulness with eyes closed, and Beta waves (>13 Hz) associated with active thinking. Interpretation requires identifying deviations from these rhythms, such as Delta (slow) waves in an awake adult, or the presence of paroxysmal discharges. Findings must always be correlated with the clinical history, as an abnormal EEG does not always equate to a clinical diagnosis of epilepsy.

Abnormalities include 'spikes' and 'sharp waves', which are indicative of a predisposition to seizures (epileptiform discharges). Generalised spike-and-wave patterns (3Hz) are classic for absence seizures. Focal slowing may indicate a structural lesion like a tumour or stroke, while generalised slowing of the background rhythm suggests a diffuse encephalopathy (e.g., toxic, metabolic, or degenerative). Specific patterns like periodic sharp wave complexes can be seen in Creutzfeldt-Jakob disease or herpes simplex encephalitis.

EEG is primarily used to support the diagnosis of epilepsy and to classify seizure types, which guides pharmacological management. It is also vital in the critical care setting for diagnosing non-convulsive status epilepticus. Furthermore, EEG helps differentiate between organic seizures and dissociative (non-epileptic) seizures, though video-telemetry is often required for definitive diagnosis. It also has a role in assessing brain function in metabolic encephalopathy and determining brain death.

Mistaking normal variants or artifacts for epileptiform activity is a common error that can lead to a misdiagnosis of epilepsy. For example, 'wicked spikes' or muscle tension can mimic seizure activity. Over-interpreting minor irregularities in children, whose EEG patterns are vastly different from adults, is another potential pitfall. Clinicians must avoid treating the 'EEG' rather than the patient.

A single routine EEG has low sensitivity; a normal recording does not exclude epilepsy, as interictal discharges may not occur during the 20-30 minute recording window. Conversely, many healthy individuals may show minor EEG abnormalities that have no clinical significance (low specificity). It is highly susceptible to artifacts from muscle movement, eye blinks, and external electrical interference, which can complicate interpretation.

For the MLA, remember that EEG is a supportive tool and not a 'stand-alone' diagnostic test for epilepsy; the diagnosis remains clinical. Know the 3Hz spike-and-wave association with absence seizures. Understand that a normal EEG does not rule out epilepsy. Recognize its role in status epilepticus management (especially non-convulsive).