Electroencephalogram (EEG): Principle and Diagnostic Importance

Introduction

The human brain is a complex organ that operates through electrical impulses generated by neurons. Monitoring and analyzing this electrical activity can provide valuable insights into brain function and dysfunction. An electroencephalogram (EEG) is a non-invasive diagnostic tool that records the brain’s electrical activity through electrodes placed on the scalp. It is widely used in neurology and clinical medicine to diagnose and monitor neurological disorders, particularly those that affect the brain’s electrical behavior.

Principle of Electroencephalogram

Electrical Activity of the Brain

The brain consists of billions of neurons that communicate via electrical impulses. These impulses are generated by the movement of ions across neuronal membranes, resulting in tiny voltage fluctuations.

EEG Recording

An EEG captures these voltage changes through electrodes placed on the scalp, which detect the summated post-synaptic potentials from thousands or millions of neurons, especially cortical pyramidal cells. These signals are then amplified and recorded as waveforms on a computer or paper.

Brain Waves

EEG waveforms are classified based on their frequency (Hz) and amplitude:

Wave Type	Frequency (Hz)	State of Brain
Delta	0.5 – 4	Deep sleep
Theta	4 – 8	Drowsiness, early sleep
Alpha	8 – 13	Relaxed wakefulness
Beta	13 – 30	Active thinking, alertness
Gamma	>30	High-level information processing

Each of these waves provides clues about brain function in different states or pathologies.

EEG Procedure Overview

Preparation:
- The scalp is cleaned.
- Electrodes (usually 16–25 or more) are attached using conductive gel or paste.
- Electrode placement follows the 10–20 International System, a standardized layout based on head measurements.
Recording:
- The subject is asked to rest with eyes closed and then perform certain activities (like deep breathing or looking at flashing lights) to provoke brain activity.
- Recordings may last 20–40 minutes (routine EEG) or extend to several hours or days (ambulatory or video EEG monitoring).
Analysis:
- Neurologists analyze the patterns, rhythms, and abnormalities in the waveforms to interpret brain function.

Importance of EEG in Diagnostic Medicine

1. Epilepsy and Seizure Disorders

EEG is the gold standard for diagnosing epilepsy.
It can detect interictal spikes, sharp waves, and seizure patterns.
Helps in seizure classification (e.g., focal vs. generalized) and localizing the seizure focus.

2. Sleep Disorders

EEG is integral to polysomnography used in diagnosing sleep apnea, narcolepsy, and parasomnias.
It differentiates sleep stages (NREM, REM) based on characteristic wave patterns.

3. Encephalopathies

EEG helps identify metabolic or toxic encephalopathies, which show diffuse slowing of brain activity.
Useful in hepatic encephalopathy, uremic encephalopathy, and hypoxic brain injury.

4. Coma and Brain Death Evaluation

In comatose patients, EEG helps assess the degree of brain activity.
Electrocerebral silence (flat EEG) can support the diagnosis of brain death.

5. Monitoring in Critical Care

Continuous EEG (cEEG) is used in ICUs to detect non-convulsive seizures or status epilepticus.
It is also used during neurosurgical procedures and for depth of anesthesia monitoring.

6. Psychiatric and Cognitive Disorders (Experimental/Research)

EEG is used in studying schizophrenia, depression, ADHD, and dementia, although its routine clinical application in psychiatry is limited.

Advantages of EEG

Non-invasive and painless
Relatively low cost
Real-time brain activity monitoring
Portable versions available for bedside and ambulatory use

Limitations of EEG

Poor spatial resolution – cannot localize deep brain activity accurately.
Sensitive to artifacts – movements, muscle activity, and electrical noise can affect recordings.
May require prolonged or repeated recordings to capture episodic abnormalities.

Advanced EEG Techniques

Video EEG Monitoring: Combines video recording with EEG to correlate behavioral events with electrical activity, crucial for epilepsy surgery evaluation.
Quantitative EEG (qEEG): Uses mathematical algorithms to analyze brain wave patterns for research or neurofeedback.
Evoked Potentials (EPs): Measures brain response to sensory stimuli (visual, auditory, somatosensory) using EEG principles.

Conclusion

The electroencephalogram (EEG) remains a cornerstone in the field of clinical neurophysiology. Its ability to provide a window into the electrical workings of the brain makes it indispensable for diagnosing epilepsy, sleep disorders, encephalopathies, and other neurological conditions. While it has certain limitations, ongoing advancements in EEG technology and signal analysis are expanding its applications in both clinical and research settings. A solid understanding of its principle and interpretation is vital for healthcare professionals involved in neurological care.

Frequently asked questions

1. What is an electroencephalogram (EEG) used for?

An electroencephalogram (EEG) is primarily used to detect abnormal brain activity. It helps diagnose conditions like epilepsy, seizures, sleep disorders, encephalopathy, brain tumors, and coma. EEG is also used to monitor brain function during surgery or in intensive care settings.

2. How does an EEG test work?

An EEG test works by placing small electrodes on the scalp to record the electrical signals produced by brain cells (neurons). These signals are amplified and displayed as brain wave patterns, which neurologists analyze to detect abnormalities in brain function.

3. Is an EEG test painful or dangerous?

No, an EEG is a safe, non-invasive, and painless test. It only records brain activity and does not send any electricity into the brain. Most people experience no side effects, though in rare cases, flashing lights used during the test may trigger a seizure in photosensitive individuals.

4. How long does an EEG test take?

A routine EEG typically takes 20 to 40 minutes, while sleep-deprived EEGs or ambulatory EEGs may take several hours to days. In some cases, video EEG monitoring may be conducted for 24 to 72 hours to capture infrequent abnormal activity.

5. Can an EEG detect brain damage or tumors?

An EEG can indicate abnormal brain function caused by damage, such as from head injuries, strokes, or infections, but it does not show structural images of the brain like a CT scan or MRI. However, it may detect electrical abnormalities associated with brain tumors or lesions.