Electroencephalography (EEG) is the "stethoscope of the cortex." While MRI and CT scans provide high-resolution structural maps (the "hardware"), only EEG captures the millisecond-by-millisecond functional dynamics (the "software") of the human brain. It remains the gold standard for diagnosing epilepsy, monitoring coma, and evaluating encephalopathy.
The Physics of the Signal
A fundamental misconception is that scalp EEG records "action potentials." It does not. Action potentials are discrete, all-or-none events lasting only 1-2 milliseconds—far too brief to summate into a recordable field through the skull.
The Dipole Generator
The EEG signal represents the summation of Excitatory and Inhibitory Postsynaptic Potentials (EPSPs and IPSPs) generated by Pyramidal Neurons in cortical layers III, V, and VI.
- Input: Neurotransmitter hits the apical dendrite.
- Sink (-): Positive ions ($Na+$, $Ca++$) flow IN.
- Source (+): Passive current flows OUT of the soma.
- Result: A charge separation (Dipole).
The 6cm² Rule: A single neuron is invisible. It takes ~6-10 cm² of synchronous cortical activity to generate a voltage large enough to penetrate the skull and be recorded.
Instrumentation & Filters
The EEG technologist acts as a "signal engineer," extracting microvolt-level brain waves from a sea of environmental noise. This relies on the Differential Amplifier.
Common Mode Rejection: If 60Hz noise hits both inputs equally, it is subtracted to zero ($Noise - Noise = 0$).
Filters: The Danger Zones
Filters are essential tools, but improper use can act as a "clinical eraser," removing pathology from the screen.
| Filter | Standard | Clinical Risk & Physics |
|---|---|---|
| LFF (High Pass) |
1 Hz TC: 0.16s |
Risk: Erasing Tumors. The LFF attenuates slow waves. If set too high (e.g., 5 Hz), you filter out the polymorphic Delta activity associated with tumors or strokes, making a pathologic EEG look "normal." |
| HFF (Low Pass) |
70 Hz |
Risk: Missing Epilepsy. The HFF attenuates fast waves. If set too low (e.g., 35 Hz), it cuts off the sharp "peak" of a spike. An epileptic discharge becomes a round, benign bump. |
Localization Logic
To pinpoint a seizure focus, we use "Montages"—logical arrangements of electrodes. There are two distinct mathematical rules for localization.
Bipolar Montage
Rule: Phase Reversal
Electrodes are linked in chains (Fp1-F3, F3-C3). Each channel represents the difference between neighbors.
Referential Montage
Rule: Amplitude
All electrodes are compared to a single reference (e.g., Cz). Localization relies on voltage height.
Activation Protocols
Hyperventilation (HV): The Biochemistry
HV is not just breathing fast; it is a metabolic stress test. The mechanism is a precise biochemical cascade:
Clinical Correlation
HV is the most potent trigger for Childhood Absence Epilepsy. The classic "3 Hz Spike-and-Wave" pattern will often appear within 60 seconds.
Contraindications: Sickle Cell Disease, Moyamoya, Recent Stroke/TIA, Subarachnoid Hemorrhage.
Epileptiform Patterns
Interictal Epileptiform Discharges (IEDs) are electrical "sparks" indicating cortical irritability. We must differentiate them from benign variants.
The Spike vs. Sharp Wave
High Seizure RiskA transient, clearly distinguishable from background, with a pointed peak. Looks like it could "prick your finger."
Blunter morphology. Clinical significance is identical to the spike. Both indicate focal epilepsy.
Specific Syndromes
Juvenile Myoclonic Epilepsy (JME)
Characterized by Polyspike-and-Wave (4-6 Hz). Often triggered by Photic Stimulation (flashing lights) and sleep deprivation.
West Syndrome (Infantile Spasms)
Characterized by Hypsarrhythmia—a chaotic, high-voltage, disorganized background. Emergency: Requires ACTH/Steroids to prevent regression.
Critical Care Terminology
In the ICU, we use the Ictal-Interictal Continuum (IIC) to describe patterns in comatose patients.
| LPDs |
Lateralized Periodic Discharges (formerly PLEDs).
Sharp waves recurring every 1-2s over one hemisphere. Highly specific for acute structural lesions: HSV Encephalitis, Acute Stroke, or Abscess. |
| SIRPIDs |
Stimulus-Induced Rhythmic Discharges.
Seizure-like patterns triggered only by stimulation (suction, sternal rub). Indicates a "hyperexcitable" cortex but may not require aggressive anesthesia if they stop when stimulation stops. |
Conclusion
EEG is a discipline of details. From the physics of the differential amplifier to the biochemistry of hyperventilation, every variable affects the diagnosis. A technically poor EEG is worse than no EEG at all—it can hide a tumor or falsely diagnose epilepsy. Precision is patient care.