Electroencephalography Study Guide

📖 Core Concepts EEG – records spontaneous electrical activity of the brain via scalp or intracranial electrodes; primary clinical tool for epilepsy and a research window into brain function. Signal source – summed postsynaptic potentials of thousands of aligned pyramidal neurons; action‑potential currents are largely invisible. International 10‑20 system – standardized scalp electrode naming (e.g., Fp1, C3, O2) that ensures reproducible placement. Frequency bands – Delta (0.5–4 Hz), Theta (4–7 Hz), Alpha (8–13 Hz), Beta (13–30 Hz), Gamma (30–100 Hz), Mu (8–13 Hz, motor). Each band has a typical behavioral state and a set of pathological meanings. Montage & referencing – modern EEG is stored referentially; any bipolar or average montage can be recomputed mathematically after acquisition. Activation procedures – hyperventilation, photic stimulation, sleep deprivation, and medication withdrawal increase the chance of capturing epileptiform activity. 📌 Must Remember Normal adult EEG: 20–100 µV amplitude, 1–30 Hz dominant frequencies. Routine EEG sensitivity for epilepsy: 29 %–55 %; a normal study does not rule out epilepsy. Interictal vs ictal: interictal = between seizures (sharp waves, spikes); ictal = during a seizure. Alpha rhythm: posterior dominant, 8–13 Hz, eyes closed → attenuates (alpha blocking) when eyes open or during mental effort. Spike‑and‑wave complex: hallmark of generalized absence seizures. Common artifacts: eye movements (frontal slow potentials), muscle (broadband 2–300 Hz), ECG (periodic spikes), line noise (50/60 Hz). Temporal resolution: EEG ≈ 1 ms; fMRI/PET ≈ seconds‑minutes. 🔄 Key Processes Preparing a clinical EEG Verify scalp impedance < 5 kΩ (gel, skin prep). Attach electrodes according to 10‑20 layout. Record baseline (eyes closed, eyes open, hyperventilation, photic stimulation). Interpreting a routine EEG Scan for normal posterior dominant rhythm → note frequency & symmetry. Look for slowing (delta/theta) → focal vs diffuse. Identify epileptiform discharges: spikes (< 70 ms), sharp waves, spike‑and‑wave complexes. Correlate with clinical events and activation maneuvers. Artifact removal workflow Visual inspection → reject severely contaminated epochs. Apply notch filter (50/60 Hz) for line noise. Use EOG/ECG reference channels → regression or ICA to subtract ocular/cardiac artifacts. Verify cleaned signal retains physiologic patterns. 🔍 Key Comparisons Scalp EEG vs Intracranial EEG Scalp: non‑invasive, low spatial resolution, poor deep‑source sensitivity. Intracranial (ECoG, SEEG): invasive, high spatial & high‑frequency resolution, detects low‑voltage fast activity. Epileptiform vs Non‑epileptiform abnormalities Epileptiform: brief (≤ 70 ms), high‑amplitude spikes/sharp waves, often repetitive. Non‑epileptiform: diffuse slowing, loss of normal rhythms, focal attenuation. Delta vs Theta in adults Delta (0.5–4 Hz): pathologic when present in wakefulness; normal in deep sleep. Theta (4–7 Hz): normal in drowsiness/young children; excess in wakefulness suggests subcortical lesion or metabolic encephalopathy. ⚠️ Common Misunderstandings “EEG shows deep brain activity.” – Deep structures are heavily attenuated; scalp EEG mostly reflects superficial cortical pyramidal cells. “A normal routine EEG excludes epilepsy.” – Sensitivity is low; seizures may be missed without prolonged or activated recording. “High‑frequency gamma always means cognition.” – Muscle activity contaminates the gamma band; must verify artifact removal. “Alpha always means relaxed wakefulness.” – In infants the dominant posterior rhythm is slower (theta range) and still considered “alpha‑like.” 🧠 Mental Models / Intuition “Dipole alignment model” – Picture thousands of pyramidal neurons as tiny arrows pointing outward; when many point the same way, their fields add → a visible scalp potential. “Signal‑to‑noise seesaw” – The larger the synchronized neuronal population, the higher the EEG amplitude; small, unsynchronized activity stays hidden. “Eye‑blink = frontal slow wave” – Visualize the corneal‑retinal dipole shifting; the resulting frontal potential is a predictable “blink” artifact. 🚩 Exceptions & Edge Cases Infant/child dominant rhythm – May be < 8 Hz yet still physiologic; classify as “alpha” only after age‑adjusted norms. Focal delta in adults – Can arise from subcortical lesions (e.g., thalamic stroke) despite delta’s usual association with diffuse metabolic encephalopathy. Medication‑induced EEG changes – Certain drugs (e.g., benzodiazepines) can increase beta activity; not always pathologic. 📍 When to Use Which Routine 20–30 min EEG – First‑line for suspected epilepsy, encephalopathy, or sleep disorders. Prolonged video‑EEG/EMU – When interictal spikes are absent, to capture ictal events, or to differentiate epileptic vs non‑epileptic seizures. Intracranial EEG – Pre‑surgical localization when scalp recordings are inconclusive or when high‑frequency activity is needed. Activation maneuvers – Hyperventilation → primarily brings out 3‑Hz spike‑and‑wave; photic stimulation → photosensitive epilepsy; sleep deprivation → increases interictal spikes. 👀 Patterns to Recognize Alpha blocking – Sudden attenuation of 8–13 Hz posterior rhythm at eye opening. Spike‑and‑wave “train” – Regular 3 Hz spikes with slow wave envelope → typical absence seizure. Generalized 4‑Hz spike‑and‑wave → idiopathic generalized epilepsy. Focal sharp‑wave cluster → localized irritative zone, possible surgical target. Diffuse delta in a comatose patient → metabolic or anoxic encephalopathy. 🗂️ Exam Traps Amplitude range – Choosing > 100 µV as “normal” ignores that pathological spikes can exceed this; the normal range is 20–100 µV for background activity. Sensitivity vs specificity – Mistaking the 29–55 % sensitivity of routine EEG for low specificity; it actually has high specificity for epilepsy. “Gamma = cognition” – Test items may present high‑frequency activity and expect you to label it as cortical processing, but muscle artifact is a frequent distractor. “Delta always abnormal” – In sleep stage III/IV or in infants, delta is physiologic; exam questions may describe delta in a waking adult to signal pathology. “All eye‑movement artifacts are frontal” – Blink artifacts can also appear as “K‑complex‑like” slow waves; be careful to check EOG channels. --- Use this guide for a rapid, high‑yield review before your EEG exam. Focus on the core concepts, memorize the must‑remember facts, and practice recognizing the patterns and traps listed above.