Electroencephalography

EEG Fundamentals - Brain's Electric Chat

• EEG: Non-invasive recording of brain's spontaneous electrical activity using scalp electrodes; measures voltage fluctuations.
• Signal Origin: Primarily summated postsynaptic potentials (PSPs) from synchronously active cortical pyramidal neurons.
  • Excitatory Postsynaptic Potentials (EPSPs)
  • Inhibitory Postsynaptic Potentials (IPSPs)
• Volume Conduction: Signals travel through brain tissue, CSF, skull, and scalp to reach electrodes.
• Electrode Placement: Standardized International 10-20 system (e.g., Fp1, Fz, C3, Pz, Oz) ensures reproducibility.
• Montages: Define how electrode pairs are connected for recording.
  • Bipolar: Between two active scalp electrodes.
  • Referential: Between an active electrode and a designated, relatively inactive reference point (e.g., earlobe, mastoid).
• Amplitude: Typically 10-100 $\mu V$.

⭐ EEG primarily reflects the summated excitatory and inhibitory postsynaptic potentials (EPSPs & IPSPs) of vertically oriented pyramidal neurons in the superficial layers of the cerebral cortex, not action potentials directly.

Normal EEG Rhythms - Wave Watching 101

The primary EEG rhythms are categorized by their frequency, amplitude, typical scalp location, and associated physiological or behavioral states:

📌 Mnemonic (frequency ↑): Deep Thoughts Are Best (Delta, Theta, Alpha, Beta).

Provoking Patterns - Stirring Up Waves

• Sleep Stages & Associated EEG Waveforms:

  • N1 (Drowsiness): ↓ Alpha, ↑ Theta; Vertex sharp waves (V waves).
  • N2 (Light Sleep): Sleep spindles (12-14 Hz bursts); K-complexes (large biphasic waves).
  • N3 (Deep/SWS): Delta waves (0.5-2 Hz, >20% epoch); high amplitude, slow waves.
  • REM Sleep: Low voltage, mixed frequency (theta, beta); characteristic sawtooth waves. EEG resembles awake state.

• Provocative Maneuvers (Activation Procedures):

  • Hyperventilation (HV): Deep breathing for 3-5 minutes.
    • Mechanism: ↓PaCO₂ (hypocapnia) → cerebral vasoconstriction → relative hypoxia → diffuse EEG slowing (delta).
    • Utility: Effectively provokes typical absence seizures (generalized 3 Hz spike-wave).
  • Intermittent Photic Stimulation (IPS): Stroboscopic light flashes at varying frequencies (e.g., 1-30 Hz).
    • Responses: Photic driving (occipital rhythm entrainment); photoparoxysmal response (PPR - generalized epileptiform discharges).
  • Sleep Deprivation: Significantly lowers seizure threshold, unmasking abnormalities.
    • Utility: Increases yield of Interictal Epileptiform Discharges (IEDs), aids epilepsy diagnosis.

⭐ Hyperventilation is a key activator for typical absence seizures, often eliciting the characteristic 3 Hz spike-and-wave discharges.

Abnormal EEG - Decoding Distress Calls

• Epileptiform Discharges: Hallmark of seizure predisposition.
  • Spikes: Brief (<70 ms), sharply contoured.
  • Sharp waves: Longer duration (70-200 ms), less sharp.
  • Spike-and-wave complexes:
    • Generalized 3 Hz: Typical absence seizures.
    • Slow spike-wave (<2.5 Hz): Lennox-Gastaut syndrome.
• Non-Epileptiform Abnormalities:
  • Slowing (focal/generalized): Indicates cerebral dysfunction (ischemia, tumor, metabolic).
  • Periodic Patterns:
    • PLEDs (Periodic Lateralized Epileptiform Discharges): Acute destructive lesions (stroke, HSV encephalitis).
    • Triphasic Waves: Characteristic of metabolic encephalopathy, especially hepatic.
    • Burst Suppression: Severe encephalopathy, anoxia, deep anesthesia; ominous sign.
  • Specific Patterns:
    • Hypsarrhythmia: Disorganized, high-voltage slow waves and spikes; seen in Infantile spasms (West syndrome).
    • GPEDs/PSIDDs (Generalized Periodic Epileptiform Discharges): Often in Creutzfeldt-Jakob Disease (CJD).
    • PLIDDs (Periodic Long-Interval Diffuse Discharges): Associated with Subacute Sclerosing Panencephalitis (SSPE).

⭐ Triphasic waves, classically seen in hepatic encephalopathy, are high-amplitude (>70 μV) positive deflections, preceded and followed by negative waves, repeating periodically at 1-2 Hz.

High‑Yield Points - ⚡ Biggest Takeaways

• Alpha waves (8-13 Hz): Characteristic of awake, relaxed adults with eyes closed; primarily occipital.
• Beta waves (>13 Hz): Associated with alertness, active thinking, and REM sleep; mainly frontal.
• Theta waves (4-7 Hz): Seen in drowsiness, light sleep, and emotional stress in adults.
• Delta waves (<4 Hz): Indicate deep sleep (NREM 3 & 4), infancy, or significant brain pathology.
• Sleep spindles & K-complexes: Definitive hallmarks of Stage N2 sleep.
• Hypsarrhythmia: A chaotic EEG pattern pathognomonic for infantile spasms (West syndrome).
• Triphasic waves: Often indicative of metabolic encephalopathy (e.g., hepatic encephalopathy).