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Electroencephalography (EEG) is a test done to assess the functioning of the brain by evaluating electrical activity generated by the brain cells. The brain cells called neurons, communicate with each other, and work together by means of electrical impulses. EEG picks up these electrical impulses using multiple metal discs placed on the head and displays it on the screen after being analyzed and amplified by the computer. The electrical impulses are displayed on the screen as wavy lines with sharp ups and downs. There are typical patterns formed by normal functioning brain cells. Any unusual pattern indicates abnormal electrical activity and thus helps in suggesting a diagnosis.
EEG is most commonly used for diagnosing and evaluating epilepsy or seizure disorders. However, it is also helpful in various other brain conditions.
EEG helps in assessing the functioning of the brain by monitoring electrical activity seen passing across the brain cells. EEG complements the brain imaging techniques such as CT or MRI, and sometimes even proves to be more useful in diagnosing brain conditions, where there are no changes in the structure of the brain.
Moreover, it also provides a continuous assessment of the cerebral function over a period of time.
Thus, EEG is especially useful in detecting and assessing seizures disorders or epilepsy where the brain cells show excessive unsynchronised electrical activity over a period of time.
Other conditions where EEG could help are as follows:
• Encephalopathy: Brain dysfunction from a variety of causes
• Encephalitis: Brain inflammation
• Brain tumor
• Head injury leading to brain damage
• Stroke: damage to the brain cells due to vascular problems
• Sleep disorders
• Brain death: it can be used to confirm brain death in a person with persistent coma.
• During surgery: continuous EEG monitoring can be done to maintain a person under the right level of anesthesia.
• For prognosis: it can also help in understanding the likely course of the condition after the episode of the first seizure.
An abnormal EEG, suggests an increased risk of seizure recurrence in both children and adults, especially if the abnormality is epileptiform. In untreated patients, The risk of seizure recurrence is 1.54 higher in untreated patients if the EEG was abnormal.
EEG is typically done in a hospital setup or neurology clinic by an EEG technologist. The EEG would be read and interpreted by a neurologist, a doctor who is specialized in diagnosing and treating nervous system diseases.
EEG test is well accepted by patients. There is no pain or any significant discomfort felt during the examination. The test generally takes about 45 to 60 minutes to complete. Sometimes it might take longer as the person would be required to sleep during the test. The test can be summarized in the following steps:
1. Placement of the electrodes on the head: at the start of the test the technician will stick the electrodes onto the head using a special paste. Sometimes a person is made to wear a special cap with the electrodes on it instead of sticking electrodes on the head. These electrodes are attached to a computer by means of multiple wires.
2. Positioning of the patient: after placing the electrodes the person is made to lie on a bed or lean back on a chair and asked to relax staying still, while the test is started.
3. Performing certain maneuvers: during the test the person is asked to do certain actions at different times like opening and closing eyes, performing few simple calculations, read a paragraph, look at a flashing light, or breathe deeply and quickly.
4. Sleep induced EEG: in some cases, the doctor might require to give medicine to induce sleeping and note the activity of the brain during sleep. This makes the test longer. Some of the tests may require a few hours to an overnight recording.
5. Video EEG: in this type of EEG the doctor would record the body movements using a camera and the brain activity using EEG. He would assess both the things to understand brain activity, its manifestations and the condition better.
6. Ambulatory EEG: in this type of EEG, after fixing the electrodes the person is asked to go home carrying the portable EEG recorder. The person is asked to wear it for several days to record the brain activity while he performs various tasks. This increases the chances of recording seizure activity. However, this type of EEG is inferior to conventional EEG done at the hospital, to differentiate between epileptic seizures and nonepileptic seizures.
After the test, the EEG technician would remove the electrodes or cap and the person is good to go with no side effects. He can continue with his daily routine. If the person was given a medicine to sleep during the test called sedatives, then he would be asked not to drive the rest of the day or operate any heavy machinery till the time the sedatives wear off.
Caffeine: a person should avoid anything containing caffeine on the day of the test. Caffeine intake can affect the test results. Caffeine-containing products usually are coffee, chocolate, tea, cola, or energy drinks.
Medicines: a person should take prescribed medicines unless instructed otherwise by the doctor.
Avoid hair products: the person should wash his or her head the night before or the day of the test. He or she should avoid using the conditioners, hair creams, styling gels, or sprays. Hair products can interfere with the adhesion of the sticky patches that hold electrodes on the scalp.
Sleep instructions: the doctor may need the person to sleep less or to avoid sleep a night before the EEG.
The EEG test is safe without any pitfalls. The EEG test just measures the electrical activity of the brain. It doesn’t interfere with the electrical activity of your brain or put electrical impulses into your brain. It can’t induce any seizure on its own however, some maneuvers might be asked to be done on purpose, which can induce seizures. During the EEG, an adhesive paste is applied that may stick to your hair which may require you to wash your head after the test. Sometimes, when a person is asked to breathe rapidly during the test, he or she may develop lightheadedness or numbness of fingers which may go on its own within a few minutes,
For the brain to function the brain cells called neurons need to transfer information continuously and communicate with each other. This transfer of information occurs by means of electrical impulses transmitted across the ends of neurons called synapses. These synapses can develop the positive or negative electrical potential for the transmission of electrical impulses.
This flow of electrical current is picked up by EEG, as a summation of positive or negative electrical potential being generated by thousands and millions of neurons present in the cortex of the brain.
Thus, an EEG recording depicts a continuously changing electrical voltage field in different parts of the brain cortex over a period of time.
There are some shortcomings of routinely done EEG in assessing seizure disorders or epilepsy.
1. Non-ictal monitoring: The typical EEG usually doesn’t record the ictal events (the period when the seizure is happening) for which the patient is seeking medical help. Thus, it is an indirect assessment where some cases show overlapping features or different findings in the non-ictal period when there is relatively less or no abnormal electrical activity in the brain.
Prolonged EEG-video: Because of this, prolonged EEG-video monitoring may prove to be more helpful in such cases. However, prolonged EEG-video monitoring is expensive and not available everywhere and hence is used in the following scenarios:
• Patients having recurrent attacks with atypical features.
• Cases where routine or sleep-deprived EEG couldn’t suggest any diagnosis.
• Patients having recurrent seizures despite adequate treatment, raising suspicion of incorrect seizure diagnosis or classification.
2. Lower yield of first interictal EEG: The first interictal EEG has been found to be normal in about 50% of patients with epilepsy.
This shortcoming can be improved by performing multiple EEGs which may show abnormality in more than 90% cases. However, performing EEGs more than 4 times is not much helpful.
3. Timing: the duration of the day when EEG is performed is also important. By performing an EEG in the morning time than in the afternoon, in cases of generalized epilepsy, the diagnostic ability of EEG can be improved.
4. Sleep-deprived EEG: it has been found that a sleep-deprived EEG performed after a nap is most useful in patients of Juvenile myoclonic epilepsy (JME). This is because both seizures and epileptiform discharges tend to occur more likely after arousal from sleep.
The EEG is seen as a two-axis graph (X and Y axis graph) where continuous waves of electrical activity (on X-axis) are seen as sharp ups and downs recorded against the time (Y-axis).
This wave pattern over time is represented in terms of hertz (Hz).
There are several patterns of electrical activity that are noted during various sates brain functioning, some of them are given below:
1. Alpha rhythm: this rhythm usually oscillates between 8 to 12 Hz and is the predominant wave pattern seen in a normal person with eyes closed. This is seen in people above 3 years of age and is predominantly noted on EEG to be arising from the occipital region or posterior aspect of the head. Alpha rhythm is reduced or completely blocked by acts of eye-opening, mental activity and drowsiness. Normally, the alpha rhythm is symmetrical on both sides of the head and when it is more on one side above the range of 50% then it is considered abnormal. There are several variants of normal alpha rhythm such as temporal alpha, anterior alpha, and paradoxical alpha.
2. Beta frequencies: comprises of the remainder of the normal waking EEG which is seen in the fronto-central aspect of temporal regions of the head. These frequencies oscillate above 13 Hz with low spikes. When beta frequencies have prominent spikes and seen over the frontal region or with generalized distribution, they likely suggest the use of sedative drugs such as benzodiazepines or barbiturates.
3. Theta and delta frequencies: these frequencies oscillate at a lower range than alpha frequency ie below 8 Hz. Theta frequency has a range of 4 to 7 Hz whereas delta frequency has a range of 1 to 3 Hz. The normal adults may show occasional theta frequencies over the temporal region which becomes more prominent after the age of 60 years. Delta activities are infrequently seen or not seen at all in normal adults during wakefulness. The amount and strength of these frequencies depend on the depth of the sleep. These frequencies are more prominent in infants and young children which progressively decrease with maturation.
4. Mu rhythm: this rhythm has frequency range of alpha waves oscillating between 8 to 12 Hz. However, Mu rhythm is seen in central head regions and is unrelated to eye-opening or closure, unlike alpha rhythm. Mu rhythm is seen in 20 to 40% normal adults and seen to react to movement, thought of movement, or the somatosensory stimulus.
5. Lambda waves: appear similar to the Greek letter “lambda (Λ)”. These waves can be elicited by pattern viewing, predominantly in the occipital region of the head.
Epileptiform Discharges: These are sharp up waves or spikes that are seen in the interictal period (the period between seizure episodes) in patients with epilepsy. Sometimes these are also seen in people who have not experienced a seizure but have a genetic predisposition to epilepsy. These discharges may be focal or generalized, depending on the type of seizure experienced.
Focal Polymorphic Slow Activity: When there is a continuous irregular activity seen in the range of the delta (1 to 4 Hz) or theta (4 to 7 Hz) frequencies, from a region of the brain suggest strong suspicion of a localized brain lesion such as hemorrhage, infarction, tumor, or abscess. Intermittent focal slow activity is less suggestive of such lesions.
Generalized Polymorphic Slow Activity
This pattern is obtained on EEG in cases of encephalopathies due to toxic, metabolic, or infectious causes and in cases of static encephalopathy leading to brain damage. Here EEG shows diffuse disturbances in background rhythms along with the excessive slow activity.
Intermittent Monomorphic Slow Activity
This pattern shows the sudden development of generalized bisynchronous rhythmic theta or delta waves. This is usually seen in thalamocortical dysfunction, obstructive hydrocephalus, deep midline or posterior fossa lesions, metabolic or toxic disorders, and also as a nonspecific functional disturbance in patients with generalized epilepsy.
This is usually seen in cortical diseases of the brain.
Generalized voltage attenuation is usually seen in the diffuse decrease in functioning such as seen in cases of anoxia or in some degenerative diseases.
When there is complete loss of electrical voltage seen on EEG it indicates brain death in an appropriate clinical situation.
When voltage attenuation is seen in a localized area it suggests localized cortical abnormality such as atrophy, contusion, cyst, or an extra-axial lesion such as a meningioma or extra-axial hematoma.
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