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Electroencephalography
The electroencephalogram (EEG) is a record of the electrical activity of the nerve cells in the brain. The EEG is based on the measurement of electrical fields generated by volume conduction of ionic currents from nerve cells through the extracellular space. Recorded EEG potentials arise from extracellular current flow from summated excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). The EEG does not record activity from single neurons, but is dependent on the summation of thousands to millions of postsynaptic potentials (PSPs), and therefore represents activity from a large neuronal aggregation. Although nerve action potentials have higher voltage changes than EPSPs and IPSPs, due to the lack of summation and short duration of action potentials, they usually add little to EEG activity. During seizures, there is synchronous firing of large ensembles of action potentials from neurons, which may contribute to the EEG signal. The usual way to record an EEG is to attach small metal disc electrodes to the scalp in standardized positions. The signal from these electrodes is then amplified, digitized, and electronically stored. The EEG is then read on a computer screen.
Brain Wave Activity . Brain activity consists of waveforms that vary in polarity, shape, and frequency, and usually range in voltage from 20 to 60 microvolts. Scalp EEG activity shows oscillations at a variety of frequencies, representing synchronized activity over a network of neurons. EEG waveforms are labeled according to their frequency, measured in cycles per second or Hertz (Hz). Alpha activity ranges between 8 and 13 Hz. The alpha rhythm is predominantly over the posterior head region and is the characteristic background frequency of the normal awake person. It occurs when the eyes are closed and attenuates when the eyes are open. Beta activity is low amplitude, fast activity with a frequency of 13 to 30 Hz and is usually present over the anterior head regions. Theta activity ranges from 4 to 7 Hz, and delta activity occurs at a frequency of less than 4 Hz. There is a developmental maturation of the EEG. For example, in the newborn infant, the EEG does not show continuous mixed-waveform activity, as would be expected in an adult. Instead, an infant has continuous amorphous delta activity. The other waveform frequencies progressively emerge as the infant's brain develops. EEG patterns change during different stages of sleep and contribute to the definition of sleep stages. The EEG patterns are very different for rapid eye movement (REM) stages compared with non-REM sleep. For stage II non-REM sleep, the EEG shows spindle activity (10-14 Hz sinusoidal activity) and vertex sharp waves. During stage III to IV, non-REM sleep high-voltage delta activity predominates. The EEG during REM sleep resembles the EEG during wakefulness, with a low-amplitude background consisting of a mix of frequencies.
The main types of abnormalities that may be seen in the EEG are slowing of background frequencies, epileptiform activity, and suppression of activity. Slowing of background activities can either be diffuse or focal. Diffuse slowing suggests widespread brain dysfunction, which can be caused by a variety of insults, such as global brain injury, toxins, inflammation, or degenerative processes. Focal slowing is often indicative of a structural lesion, such as a tumor or a stroke. Epileptiform activity indicates the patient is at risk for seizures. Suppression of activity can be either focal or diffuse, and it indicates a severe derangement of brain function.
Indications for EEG. The main indications for obtaining an EEG are to assess for seizure disorders, intracranial disease processes, coma, and brain death. The most common reason for an EEG is to characterize a seizure disorder. The EEG is useful in defining epilepsy syndromes and for localization of a seizure focus. Because seizures occur infrequently, EEG activity is usually measured between seizures to determine whether characteristic waveforms, such as sharp waves or spikes, are present. These waveforms signify a predisposition to epilepsy. Capturing a seizure on EEG usually requires long-term monitoring with video. For some disease processes, the EEG shows specific diagnostic patterns, such as generalized periodic sharp waves in Creutzfeldt-Jakob disease. The EEG is also very useful in the evaluation of comatose patients. There may be distinctive patterns that can confirm that diagnosis of an underlying condition, such as triphasic waves in hepatic coma, spike discharges in nonconvulsive status epilepticus, and excessive beta activity associated with a benzodiazepine or barbiturate drug overdose. Finally, the EEG can be used to confirm brain death in patients in whom the EEG activity has ceased and the clinical criteria for brain death are present.
Seizures and Epilepsy
Epilepsy is medically defined as a condition characterized by an individual having two or more unprovoked seizures. A seizure is a paroxysmal disorder characterized by an abnormal excessive, hypersynchronous discharge of neurons that results in an alteration of normal brain function. This alteration of function can be quite dramatic, such as during a generalized tonic-clonic (GTC; grand mal) seizure, or much more subtle, such as during an absence (petit mal) seizure. If the seizures are consistently provoked, such as by fever or hypoglycemia, the term epilepsy should not be used. Epilepsy is not a single disorder, but rather a symptom of an underlying brain disorder. Epilepsy is a chronic disorder, although many children will go into remission. Although many people with epilepsy are normal in all other respects, approximately 50% will also have additional cognitive or behavioral impairments.
The history and neurologic examination are the cornerstones of neurologic diagnosis. When assessing when a patient may have had a seizure, it is important to obtain a description of a paroxysmal change in behavior, whether there was a loss of consciousness, the duration of the spell, and whether stimuli were encountered that might precipitate a seizure. A family history of epilepsy should always be ascertained. Of particular importance in the history is the description of the initial signs or symptoms. For example, the approach to a patient with an aura before a GTC is quite different from the patient who has a GTC without an aura. In the former case, it is likely that the patient has a focal onset to the seizure, increasing the chances that there is a structural lesion responsible for the seizure, while in the latter instance, it is likely the patient has a seizure-inducing stimulus, such as low blood sugar or perhaps an underlying genetic condition. Postictal features can also be helpful. Absence seizures of childhood are brief, typically lasting 30 seconds or less, and have a rapid offset, with the child quickly returning to normal mental status. Complex partial seizures are of longer duration, lasting 30 seconds to several minutes, and typically have some degree of confusion and tiredness after the event.
There are many episodic disorders that resemble seizures. Episodes such as night terrors, breath-holding spells, or syncope may resemble epileptic seizures. The timing of the event is important. When nocturnal, epileptic seizures typically occur in the early morning hours, while sleep disorders such as night terrors typically occur several hours after the child falls asleep. A young child for whom the event always occurs in association with provoked crying likely has breath-holding spells. Individuals who feel light-headed and clammy before losing conscious likely have syncope rather than epilepsy. If there is doubt about the diagnosis, it is usually better to wait before beginning therapy.
Seizures are classified into two major categories: focal and generalized. Focal seizures originate within a localized region of the brain, and may evolve into generalized convulsions. Generalized seizures rapidly engage both hemispheres of the brain. Generalized seizures are further classified into tonic, clonic, tonic-clonic, absence, myoclonic, and atonic.
Focal (Partial) Seizures
Focal seizures originate within networks of a limited region of the brain, often confined to one hemisphere. They can occur at any age. Focal seizures may be classified further into those without impairment of consciousness or awareness (simple partial seizures) and those with impairment of consciousness or awareness (complex partial seizures). Seizures without impairment of consciousness or awareness can be further subdivided into seizures with (1) observable motor or autonomic components or (2) subjective sensory or psychic phenomenon. The signs or symptoms of focal seizures depend on the location of the focus within the brain. Seizures involving the motor cortex most commonly consist of rhythmic or semirhythmic clonic movements of the face, arm, or leg. There is usually no difficulty in diagnosing this type of seizure. Seizures with somatosensory, autonomic, and psychic symptoms (hallucinations, illusions, déjà vu) may be more difficult to diagnose.
Most commonly, psychic symptoms occur as a component of a focal seizure with impaired consciousness or responsiveness. Focal seizures with impairment of consciousness or awareness (complex partial seizures), formerly termed temporal lobe or psychomotor seizures, are one of the most common seizure types encountered in both children and adults. The beginning of the focal seizure may serve as a warning to the patient (i.e., aura) that a more severe seizure is pending. It is important to recognize that the aura may enable the clinician to determine the cortical area from which the seizure is beginning.
The impairment of consciousness or awareness may be subtle. For example, the patient may either not respond to commands or respond in an abnormally slow manner. Although focal seizures with altered consciousness or awareness may be characterized by simple staring and impaired responsiveness, behavior is usually more complex during the seizure. Automatisms, semipurposeful behaviors of which the patient is unaware and subsequently cannot recall, are common during the period of impaired consciousness. Types of automatism behaviors are quite variable and may consist of activities such as facial grimacing, gestures, chewing, lip smacking, snapping fingers and repeating phrases. The patient does not recall fully this activity after the seizure. Most patients have some degree of postictal impairment, such as tiredness or confusion after the seizure.
The EEG in focal seizures is characterized by focal spikes or sharp waves. There is often a relationship between the location of the spikes and the seizure type, that is, occipital lobe spikes are associated with occipital lobe seizures, while frontal lobe spikes are associated with frontal lobe seizures.
Different types of seizures may evolve in temporal succession in the same patient. For example, a focal seizure starting with normal consciousness and awareness may become associated with alteration in consciousness and subsequently evolve to a generalized convulsive seizure as the seizure starts within a local neural circuit and then spreads to involve an increasing proportion of the brain and ultimately both hemispheres.
Generalized Seizures: Tonic-Clonic Seizure
A generalized tonic-clonic, or grand mal seizure, is the most severe type of seizure. It starts with a sudden loss of consciousness and generalized tonic stiffening and extension of the body secondary to a widespread contraction of the muscles. The patient may utter a piercing cry resulting from forced expiration of air from the lungs through closed vocal cords. Cessation of respirations with associated cyanosis is secondary to the tonic muscle contractions that prevent normal respiratory movements. The patient often bites his tongue during this phase of the seizure. Salivation occurs because the patient cannot swallow during the seizure. In addition, urinary incontinence is often present.
The initial tonic phase of the seizure is followed by the clonic phase, in which generalized bilaterally synchronous clonic jerks of the body alternate with brief periods of relaxation. As the periods of relaxation become more prolonged, the clonic movements gradually decrease and finally cease.
During the postictal period after the seizure, the patient is limp, obtunded, and unresponsive. The actual seizure may last about 1 to 2 minutes, while the postictal phase may last from 5 to 20 minutes. Afterward, the patient may arouse, but remains confused, and if left undisturbed, may sleep for an hour or so and awaken with a headache and generalized muscle soreness.
Generalized tonic-clonic seizures may occur at any age. They may be primary generalized seizures, which are generalized from onset, or secondary generalized seizures, which start as focal seizures and then become generalized as the seizure activity progresses to involve widespread areas of the brain.
The EEG of a GTC seizure shows various types of seizure activity that correspond to the different phases of the seizure. During the tonic phase, the EEG shows fast, repetitive, generalized spike discharges. During the clonic phase, the EEG shows spike-and-wave discharges, with the spike corresponding to the clonic jerks and the slow wave to the period of relaxation. Finally, during the postictal phase, the EEG shows generalized attenuation of background activity followed by slowing, which gradually decreases as the patient recovers from the seizure.
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