Introduction to EEG

Summary

This video provides an introduction to Electroencephalography (EEG), covering key concepts like the international 10-20 system for electrode placement, how EEG is recorded, and different montages for displaying results. The video also explores common EEG patterns such as eye blinks, alpha rhythms, and focal or generalized epileptiform discharges. It emphasizes the role of EEG in diagnosing epilepsy and highlights its diagnostic yield, depending on factors like seizure timing and electrode placement. The content serves as a basic overview with references to deeper dives in subsequent videos for those interested in more detailed exploration.

Takeaways

• 😀 EEG (Electroencephalography) is used to record brain activity and is essential in clinical neurology for diagnosing conditions like epilepsy.
• 😀 The International 10-20 system is the standard method for placing electrodes on the scalp, with precise measurements taken from key head landmarks.
• 😀 The electrodes in the 10-20 system are labeled based on their position: odd numbers are on the left side, and even numbers are on the right.
• 😀 A differential amplifier is used to record EEG signals by measuring the difference between two inputs, which helps highlight small brain electrical signals.
• 😀 EEG signals can be displayed in various montages, including the bipolar montage and common average reference montage, for different types of analysis.
• 😀 The bipolar montage shows the difference between adjacent electrode pairs, while the common average reference montage compares individual electrodes to the average signal of the whole head.
• 😀 Eye blinks create large electrical signals due to the dipole formed by the retina and cornea, which can be detected on frontal electrodes during an EEG.
• 😀 The alpha rhythm is a normal EEG pattern that appears when a person is awake and relaxed, especially with their eyes closed, at around 10 Hz.
• 😀 Epileptiform discharges, either focal or generalized, indicate abnormal brain activity and are often associated with seizures, helping in epilepsy diagnosis.
• 😀 The likelihood of detecting epileptiform discharges in a single EEG within 24 hours of a first seizure is about 50%, and this increases with prolonged EEG recordings.
• 😀 EEG sensitivity in detecting abnormalities depends on the size and location of the abnormal brain activity. Larger and more superficial areas are easier to detect than smaller, deeper ones.

Q & A

What is the International 10-20 system in EEG recording?

The International 10-20 system is a standard method of placing EEG electrodes on the scalp. It uses specific points on the head, such as the nasion (bridge of the nose), inion (bony prominence at the back of the head), and preauricular points (near each ear), to guide electrode placement. Measurements are made based on divisions of 10% and 20% of the total head circumference.

How are the electrodes labeled in the 10-20 system?

Electrodes are labeled with letters and numbers. The letters represent the anatomical location (e.g., F for frontal, C for central, P for parietal), while the numbers indicate the side of the head. Odd numbers are on the left, even numbers on the right, with the midline electrodes represented by 'Z' (zero).

What role does the differential amplifier play in EEG recording?

The differential amplifier records EEG signals by comparing the electrical activity from two inputs (electrodes) and displaying the difference between them. This method cancels out common signals, highlighting small differences in electrical activity, which is essential for detecting the subtle brain activity measured by EEG.

What is a bipolar montage in EEG, and how is it structured?

A bipolar montage is a type of EEG display where the difference in electrical activity between adjacent electrodes is recorded as a single channel or derivation. For example, the difference between FP2 and F8 is displayed as one channel, and a series of these channels forms a montage, such as an anterior-posterior bipolar montage.

How does a common average reference montage differ from a bipolar montage?

In a common average reference montage, each EEG channel compares the electrical activity of a specific electrode to the average activity of all other electrodes on the scalp. This contrasts with the bipolar montage, where adjacent electrode pairs are compared directly.

What is the alpha rhythm in EEG, and when is it most commonly observed?

The alpha rhythm is a normal pattern of brain wave activity observed in the EEG, typically around 10 Hz. It is most prominent when a person is awake and relaxed, especially when their eyes are closed.

What is an eye blink artifact in EEG, and how does it appear on the recording?

An eye blink artifact occurs when a patient blinks their eyes, creating a large positive signal at the frontal polar electrodes (FP1 and FP2) due to the movement of the eyeball and the polarity difference between the cornea and retina. This artifact can obscure or distort the EEG signal.

What are focal epileptiform discharges, and how are they related to seizures?

Focal epileptiform discharges are abnormal electrical patterns seen in EEG that indicate localized abnormal brain activity, potentially indicating that the brain is capable of generating seizures. These discharges are often seen in patients with epilepsy and can help diagnose the condition.

What is the likelihood of finding epileptiform discharges in an EEG after a first seizure?

The likelihood of detecting epileptiform discharges in an EEG within 24 hours of a first seizure is about 50%. However, if the EEG is conducted more than 24 hours after the seizure, the likelihood decreases. Prolonged EEG monitoring increases the chance of detecting abnormal discharges, with an 80-90% detection rate.

Why might some patients with epilepsy have a normal routine EEG?

Patients with epilepsy may have a normal routine EEG due to factors such as the size and location of the abnormal brain activity. If the epileptic activity is confined to a small area or deep within the brain, it may not be detected by the surface electrodes. Additionally, abnormal activity may not occur during the time the EEG is recorded.