What does eeg measure

What is an EEG?

An electroencephalogram (EEG) is a diagnostic test that records the electrical activity of the brain. The brain is composed of billions of nerve cells called neurons, which communicate with each other through electrical impulses. These impulses generate tiny electrical fields that can be detected on the surface of the scalp. An EEG system uses a set of small metal discs, called electrodes, which are attached to the scalp. These electrodes are connected by wires to a computer that amplifies the electrical signals and records them as wavy lines on a screen or paper. These patterns represent the brain's electrical activity over a period of time.

How Does an EEG Work?

The fundamental principle behind EEG is that neurons in the brain communicate through electrical signals. When a large group of neurons fires simultaneously, it creates a measurable electrical potential. The electrodes placed on the scalp pick up these voltage fluctuations. These signals are very small, typically in the range of microvolts (millionths of a volt), so they are amplified significantly by the EEG machine. The amplified signals are then displayed as a series of waveforms, each representing the electrical activity from a specific area of the brain. Neurologists and trained technicians interpret these waveforms, looking for abnormal patterns that might suggest a neurological issue.

What Does an EEG Measure Specifically?

An EEG measures the collective electrical activity of populations of neurons in the cerebral cortex. It doesn't measure the activity of a single neuron, but rather the summed electrical potentials generated by thousands or millions of neurons firing together. The rhythmic patterns observed in an EEG are often referred to as brainwaves, and they are classified by their frequency (how fast they oscillate) and amplitude (how large they are). Common brainwave frequencies include:

• Delta waves: Slowest frequencies (0.5-4 Hz), typically seen during deep sleep.
• Theta waves: (4-8 Hz), often associated with drowsiness, light sleep, and certain cognitive processes.
• Alpha waves: (8-13 Hz), usually present when a person is awake and relaxed, especially with eyes closed.
• Beta waves: (13-30 Hz), associated with active thinking, concentration, and alertness.
• Gamma waves: (30-100 Hz), linked to higher cognitive functions like learning and memory.

The EEG records these brainwave patterns and can reveal abnormalities such as excessive slow waves (indicating brain dysfunction), sharp waves or spikes (characteristic of epilepsy), or periods of no electrical activity (which can signify brain death).

Why is an EEG Performed?

EEGs are a valuable tool for diagnosing and monitoring a variety of neurological conditions. Some common reasons for performing an EEG include:

• Diagnosing epilepsy and seizure disorders: EEG is the primary tool for detecting abnormal electrical discharges that cause seizures. It can help differentiate between seizure types and locate the origin of seizures in the brain.
• Investigating unexplained seizures or fainting spells: If someone experiences events that might be seizures but are not witnessed, an EEG can help determine if there is underlying abnormal electrical activity.
• Diagnosing other brain disorders: EEG can help identify issues such as brain tumors, strokes, encephalitis (brain inflammation), and meningitis.
• Evaluating sleep disorders: EEGs are a key component of polysomnography (sleep studies) to diagnose conditions like narcolepsy and sleep apnea.
• Assessing confusion or altered consciousness: It can help determine the cause of sudden changes in mental status, such as in cases of encephalopathy.
• Monitoring brain activity during surgery: In certain types of surgery, particularly brain surgery or carotid endarterectomy, EEG can be used to monitor the brain's function and detect any signs of distress.
• Determining brain death: An EEG can be used as part of the criteria to declare brain death, showing a lack of electrical activity in the brain.

What to Expect During an EEG?

An EEG is a non-invasive and generally painless procedure. Before the test, a technician will clean small areas on your scalp. Then, they will attach small, flat metal discs (electrodes) to your scalp using a special paste or adhesive. Usually, 16 to 25 electrodes are placed to cover different areas of the brain. These electrodes are connected by wires to a recording machine. The actual recording typically lasts between 20 to 60 minutes, but sometimes longer recordings, including overnight studies, may be necessary. During the test, you will be asked to lie still, relax, and keep your eyes closed. You might also be asked to perform certain actions, such as opening and closing your eyes, looking at a flashing light (photic stimulation), or taking deep breaths (hyperventilation). These activations are used to provoke abnormal brain activity that might not be present during a resting state.

Limitations of EEG

While EEG is a powerful diagnostic tool, it has limitations. It primarily measures activity from the cerebral cortex, the outer layer of the brain, and may not detect abnormalities deep within the brain. Also, the electrical signals can be affected by factors such as scalp hair, skull thickness, and movement. A normal EEG does not completely rule out epilepsy, as abnormal activity might not occur during the recording period. Therefore, EEG results are always interpreted in conjunction with a patient's medical history, symptoms, and other diagnostic tests.