The electroencephalogram (EEG) is a test used to record and evaluate the bioelectrical activity of the brain. The electrical potentials are obtained through electrodes located on the patient's scalp.
The records can be printed on moving paper via an EEG or can be viewed on a monitor. The electrical activity of the brain can be measured in basal conditions of rest, wakefulness or sleep.
The EEG is used for the diagnosis of epilepsy, sleep disorders, encephalopathies, coma, and brain death, among many other uses. Can also be used in research.
It was previously used to detect focal brain disorders such as tumors or stroke. Today, magnetic resonance imaging (MRI) and computed tomography (CT) are used..
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The history of the EEG begins in 1870, when Fristsch and Hitzig, doctors in the Prussian army, investigated with the brains of soldiers. These were discovered in the Battle of Sedan. They soon realized that by stimulating some brain areas by galvanic current, movements were generated in the body.
However, it was in 1875 that physician Richard Birmick Caton confirmed that the brain produced electrical currents. Subsequently, this allowed the neurologist Ferrier to experiment with the “faradic current”, locating motor functions in the brain.
In 1913, Vladimir Pravdich-Neminsky was the first to perform what he called an "electrocerebrogram," examining a dog's nervous system. Until that moment, all observations were made on discovered brains, since there were no enlargement procedures that reached the interior of the skull..
In 1920, Hans Berger began experimenting with humans and 9 years later he created a method to measure the electrical activity of the brain. He coined the term "electroencephalogram" to characterize the recording of electrical fluctuations in the brain.
This German neurologist was the one who discovered the “Berger rhythm”. That is to say, the current “alpha waves”, which consist of electromagnetic oscillations that come from the synchronous electrical activity of the thalamus..
Berger, despite his great discovery, I cannot advance this method due to his limited technical knowledge.
In 1934, Adrian and Matthews, in a demonstration at the Physiological Society (Cambridge), were able to verify the “Berger rhythm”. These authors advanced with better techniques and demonstrated that the regular and wide rhythm of 10 points per second did not arise from the whole brain, but from the visual areas of association.
Later, Frederic Golla confirmed that in certain diseases there were alterations in the rhythmic oscillations of brain activity. This allowed great advances in the study of epilepsy, becoming aware of the difficulty of this issue and the need to study the brain in a comprehensive way. Fisher and Lowenback, in 1934, were able to determine the epileptiform peaks.
Finally, William Gray Walter, an American neurologist skilled in robotics, developed his own versions of the EEG and added improvements. Thanks to it, it is now possible to detect the various types of brain waves, from alpha waves to delta waves..
A standard EEG is a painless, non-invasive scan performed by attaching electrodes to the scalp with a conductive gel. It has a recording channel, which measures the voltage difference between two electrodes. Typically 16 to 24 leads are used.
The pairs of electrodes are combined creating what is called a "mount", which can be bipolar (transverse and longitudinal) and monopolar (referential). The bipolar montage is used to record the difference in voltage in areas of brain activity, while the monopolar compares an active brain zone and another with no activity or neutral activity.
The difference between an active zone and the average of all or some active electrodes can also be measured..
Invasive electrons (within the brain) can be used to study hard-to-reach areas such as the mesial surface of the temporal lobe in detail..
Sometimes it may be necessary to insert electrodes near the surface of the brain to detect electrical activity in the cerebral cortex. The electrodes are usually placed under the dura (one of the layers of the meninges) through an incision in the skull..
This procedure is called electrocorticography, and it is used to treat resistant epilepsy and for research.
There is a standardized system for electrode placement known as the "10-20 system." This implies that the distance between the electrodes must be 10% or 20% with respect to the frontal axes (from front to back) or transverse (from one side of the brain to the other)..
21 electrodes must be placed, and each electrode will be connected to one input of a differential amplifier. Amplifiers spread the voltage between the active and reference electrodes 1,000 to 100,000 times.
At present, the analog signal is in disuse and digital amplifiers are used. Digital EEG has great advantages. For example, it facilitates the analysis and storage of the signal. In addition, it allows modifying parameters such as filters, sensitivity, recording time and montages..
EEG signals can be recorded with open source hardware such as OpenBCI. On the other hand, the signal can be processed by free software such as EEGLAB or the Neurophysiological Biomarker Toolbox..
The electroencephalographic signal is represented from the difference in electrical potential (ddp) that exists between two points on the cranial surface. Each point is an electrode.
Our brain works through electrical impulses that travel through our neurons. These impulses can be rhythmic or not, and are known as brain waves. The rhythm consists of a regular wave, which has the same morphology and duration, and which maintains its own frequency..
Waves are classified according to their frequency, that is, according to the number of times the wave repeats per second, and they are expressed in hertz (Hz). The frequencies have a certain topographic distribution and reactivity. Most of the brain signal observed on the scalp is in a range between 1 and 30 Hz.
On the other hand, the amplitude is also measured. This is determined from the comparison of the distance between the baseline and the peak of the wave. Wave morphology can be sharp, pointed, in tip-wave complexes and / or sharp wave-slow wave..
In the electroencephalogram, 4 main bandwidths known as alpha, beta, theta and delta can be observed..
They consist of broad waves, whose frequency is between 14 and 35 Hz. They appear when we are awake doing activities that require intense mental effort, such as taking an exam or studying.
They are of greater amplitude than the previous ones, and their frequency oscillates between 8 and 13 Hz. They arise when the person is relaxed, without making significant mental efforts. They also appear when we close our eyes, daydream, or perform activities that we have highly automated..
They have a greater amplitude but a lower frequency (between 4 and 8 Hz). They reflect a state of great relaxation, prior to the beginning of sleep. Specifically, it is linked to the early stages of sleep.
These waves are the ones with the lowest frequency of all (between 1 and 3 Hz). They are associated with deeper stages of sleep (stages 3 and 4, where you don't usually dream).
To perform the EEG, the patient needs to be relaxed, in a dark environment and with his eyes closed. It usually lasts about 30 minutes.
Initially, activation tests such as intermittent photostimulation (applying light stimuli with different frequencies) or hyperventilation (breathing through the mouth regularly and deeply for 3 minutes) are performed..
It can also induce sleep or, conversely, keep the patient awake. This depends on what the researcher intends to observe or verify. This video shows the application in an adult:
To interpret an EEG it is necessary to know the normal activity of the brain according to the age and condition of the patient. It is also necessary to examine artifacts and possible technical problems to minimize misinterpretations..
An EEG may be abnormal if epileptiform activity is present (suggesting an epileptic process). This can be localized, generalized or with a particular and unusual pattern.
It can also be abnormal when slow waves are visualized in a specific area, or generalized asynchrony is found. There may also be abnormalities in amplitude or when there is a line that deviates from normal..
Currently other more advanced techniques have been developed such as video-EEG monitoring, ambulatory EEG, telemetry, brain mapping, in addition to electrocorticography..
There are different types of EEGs listed below:
It is the one performed when the patient is in a waking state, so no preparation is required. To avoid using products that may affect the examination, a good cleaning of the scalp is carried out.
Previous preparation is necessary. The patient must be awake for 24 hours before its performance. This is done in order to be able to make physiological tracings of the sleep phases in order to detect abnormalities that cannot be obtained through the baseline EEG..
It is a normal EEG, but its distinctive feature is that the patient is videotaped during the process. Its purpose is to obtain a visual and electrical record to observe if seizures or pseudo-crises appear.
It is a necessary technique to observe cerebral cortical activity or its absence. It is the first step of the so-called “brain death protocol”. It is essential to start the device for the extraction and / or transplantation of organs.
The EEG is used in a wide variety of clinical and neuropsychological conditions. Here are some of its uses:
The EEG in epilepsies is essential for the diagnosis, since it allows to differentiate it from other pathologies such as psychogenic crises, syncope, movement disorders or migraines.
It is also used to classify epileptic syndrome, as well as to control its evolution and the effectiveness of treatment..
Encephalopathies involve damage or malfunction of the brain. Thanks to the electroencephalogram, it is possible to know if certain symptoms are due to an "organic" brain problem, or are the product of other psychiatric disorders.
The electroencephalogram is useful to control the depth of anesthesia, preventing the patient from going into a coma or waking up.
The EEG is essential in intensive care units to monitor brain function. Especially seizures, the effect of sedatives and anesthesia in patients in an induced coma, as well as to check for secondary brain damage. For example, the one that can occur in a subarachnoid hemorrhage.
It is used to diagnose abnormal changes in the body that can affect the brain. It is usually a necessary procedure to diagnose or monitor brain diseases such as Alzheimer's, head injuries, infections or tumors.
Certain electroencephalographic patterns may be of interest for the diagnosis of some pathologies. For example, herpetic encephalitis, cerebral anoxia, barbiturate poisoning, hepatic encephalopathy, or Creutzfeldt-Jakob disease.
In newborns, the EEG can provide information about the brain to identify possible abnormalities based on their life span.
The electroencephalogram is necessary to assess the patient's state of consciousness. It provides data on both the prognosis and the degree of slowing down of brain activity, so that a lower frequency would indicate a reduction in the level of consciousness.
It also allows us to observe whether brain activity is continuous or discontinuous, the presence of epileptiform activity (which indicates a worse prognosis) and reactivity to stimuli (which shows the depth of the coma).
In addition, through it, the presence of sleep patterns can be verified (which are infrequent when the coma is deeper).
The EEG is very important for the diagnosis and treatment of multiple sleep disorders. The patient can be examined while they sleep and the characteristics of their brain waves can be observed.
The most widely used test for soil studies is polysomnography. This, in addition to including an EEG, simultaneously records the patient on video. In addition, it allows you to analyze your muscle activity, respiratory movements, air flow, oxygen saturation, etc..
The EEG is used in research, especially in neuroscience, cognitive psychology, neurolinguistics, and psychophysiology. In fact, many of the things we currently know about our brain are due to research conducted with EEGs..
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