ELECTROENCEPHALOGRAM (EEG)
Normal and Abnormal
Invented in the first half of the twentieth century by a psychiatrist (Hans Berger) and improved by computerization and correlation with patient videos, the EEG remains the most specific laboratory test for seizures. In addition, it still helps in the diagnosis of several other neurologic conditions. Neurologists liberally order an EEG as a painless, harmless, relatively inexpensive, and, when positive, helpful diagnostic test. The routine EEG records cerebral electrical activity detected by “surface” or “scalp” electrodes- Four frequency bands of cerebral activity, represented by Greek letters, emanate from the brain EEG readers first ascertain the patient’s age and level of consciousness. They also determine the display of the electrodes (the montage) and note the time scale, which is determined by vertical lines on the EEG paper or displayed as 1-second horizontal bars.
Although approaches vary, most readers then determine the EEG’s background or dominant rhythm (see later), organization, and symmetry. EEG readers accord special attention to unusually pointed single discharges, called sharp waves or spikes, and abnormal patterns, especially if they occur in paroxysms. They judge all these features in relation to whether the patient is awake, asleep, unresponsive, or having observable seizure activity. The normal background rhythm in an awake adult consists of waves of activity in the alpha range of 8–13 cycles per second (Hertz [Hz]) detectable mostly over the occipital region
Neurologists refer to this pattern as the posterior dominant rhythm. It is prominent when individuals are relaxed with their eyes closed, but disappears if they open their eyes, concentrate, or are apprehensive. When people undergoing an EEG merely fix their gaze on a clock or add two single-digit numbers,
faster rhythms replace alpha activity. Preoccupations, concerns, or anxiety eliminate alpha activity. Because alpha activity reflects an anxiety-free state, it represents an important parameter in “alpha training,” biofeedback, and other behavior modification techniques. Alpha activity also disappears when people fall asleep or take psychotropic medicines.
In the elderly, the background rhythm typically slows but remains within or just below the alpha range. In the early stages of Alzheimer disease, the background activity is also slower than normal. In the more advanced stages of Alzheimer disease,as well as in many other neurologic illnesses, the background EEG activity not only slows well below the alpha range, but also loses its organization. Beta activity consists of high (> 13 Hz)-frequency, lowvoltage activity located maximally overlying the frontal region. It replaces alpha activity when people concentrate, become anxious, or take many hypnotics or sedatives, including benzodiazepines. Beta activity usually inserts itself into the background EEG activity of most adults.
Theta (4–7 Hz) and delta (< 4 Hz) activities occur normally in children and everyone during deep sleep, but are usually absent in healthy alert adults. When present over the entire brain, theta or delta activity in wakefulness often indicates a neurodegenerative illness, such as Alzheimer disease, or a metabolic derangement. Continuous focal slow activity with phase reversal in bipolar montages sometimes originates in an underlying cerebral lesion; however, the absence of theta or delta activity certainly does not exclude one. Spikes, sharp waves, and slowing – nonspecific changes – occur in about 3–15% of the general, healthy population. When they are isolated and asymptomatic, these anomalies have no clinical significance and require no further investigation. However, when spikes and sharp waves are repetitive and phase-reversed, they are an indication of an irritative cerebral focus with potential to
produce seizures. Moreover, paroxysms of them suggest the diagnosis.
Normal and Abnormal
Invented in the first half of the twentieth century by a psychiatrist (Hans Berger) and improved by computerization and correlation with patient videos, the EEG remains the most specific laboratory test for seizures. In addition, it still helps in the diagnosis of several other neurologic conditions. Neurologists liberally order an EEG as a painless, harmless, relatively inexpensive, and, when positive, helpful diagnostic test. The routine EEG records cerebral electrical activity detected by “surface” or “scalp” electrodes- Four frequency bands of cerebral activity, represented by Greek letters, emanate from the brain EEG readers first ascertain the patient’s age and level of consciousness. They also determine the display of the electrodes (the montage) and note the time scale, which is determined by vertical lines on the EEG paper or displayed as 1-second horizontal bars.
Although approaches vary, most readers then determine the EEG’s background or dominant rhythm (see later), organization, and symmetry. EEG readers accord special attention to unusually pointed single discharges, called sharp waves or spikes, and abnormal patterns, especially if they occur in paroxysms. They judge all these features in relation to whether the patient is awake, asleep, unresponsive, or having observable seizure activity. The normal background rhythm in an awake adult consists of waves of activity in the alpha range of 8–13 cycles per second (Hertz [Hz]) detectable mostly over the occipital region
Neurologists refer to this pattern as the posterior dominant rhythm. It is prominent when individuals are relaxed with their eyes closed, but disappears if they open their eyes, concentrate, or are apprehensive. When people undergoing an EEG merely fix their gaze on a clock or add two single-digit numbers,
faster rhythms replace alpha activity. Preoccupations, concerns, or anxiety eliminate alpha activity. Because alpha activity reflects an anxiety-free state, it represents an important parameter in “alpha training,” biofeedback, and other behavior modification techniques. Alpha activity also disappears when people fall asleep or take psychotropic medicines.
In the elderly, the background rhythm typically slows but remains within or just below the alpha range. In the early stages of Alzheimer disease, the background activity is also slower than normal. In the more advanced stages of Alzheimer disease,as well as in many other neurologic illnesses, the background EEG activity not only slows well below the alpha range, but also loses its organization. Beta activity consists of high (> 13 Hz)-frequency, lowvoltage activity located maximally overlying the frontal region. It replaces alpha activity when people concentrate, become anxious, or take many hypnotics or sedatives, including benzodiazepines. Beta activity usually inserts itself into the background EEG activity of most adults.
Theta (4–7 Hz) and delta (< 4 Hz) activities occur normally in children and everyone during deep sleep, but are usually absent in healthy alert adults. When present over the entire brain, theta or delta activity in wakefulness often indicates a neurodegenerative illness, such as Alzheimer disease, or a metabolic derangement. Continuous focal slow activity with phase reversal in bipolar montages sometimes originates in an underlying cerebral lesion; however, the absence of theta or delta activity certainly does not exclude one. Spikes, sharp waves, and slowing – nonspecific changes – occur in about 3–15% of the general, healthy population. When they are isolated and asymptomatic, these anomalies have no clinical significance and require no further investigation. However, when spikes and sharp waves are repetitive and phase-reversed, they are an indication of an irritative cerebral focus with potential to
produce seizures. Moreover, paroxysms of them suggest the diagnosis.
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