Pearls, Perils, and Pitfalls In the Use of the Electroencephalogram

Omkar N. Markand, MD, FRCPC


Semin Neurol. 2003;23(1) 

In This Article

EEG in Generalized Epilepsies

The epileptic process in generalized epilepsies involves large areas of the brain at the outset of the seizure, and the EEG is characterized by bilaterally synchronous generalized paroxysms of spikes and spike wave discharges. Generalized epilepsies are subcategorized as primary (idiopathic) and secondary (symptomatic).

A patient with primary generalized epilepsy (PGE) has no identifiable etiology, normal brain imaging, and normal neurocognitive functioning. The epilepsy has a strong genetic basis and is highly responsive to antiepileptic medication. The patient may suffer from absence (petit mal), myoclonic, and tonic-clonic seizures, among other generalized seizures. Many different syndromes of PGE have been recognized depending upon the predominant seizure type and the age of onset. Classically, the presence of rhythmic, anterior-dominant generalized bisynchronous 3 Hz spike wave discharges superimposed on a normal background are considered to be the EEG hallmark of PGE.

However, the most common EEG abnormality associated with PGE is the so-called "irregular" or "atypical" or "rapid spike" wave activity. This is characterized by generalized paroxysms of spikes or spike wave complexes occurring with an irregular frequency of about 3 to 5 Hz. Although some spike wave complexes will approximate 3 Hz, the overall impression is that the EEG abnormality is much less regular than the classic 3 Hz spike wave discharges (Fig. 21). Transient asymmetry of the bisynchronous spike wave activity and isolated "focal" spikes are common. Atypical generalized spike waves are not only seen in PGE but also in secondary generalized epilepsies such as progressive myoclonus epilepsies of different etiologies.

Figure 21.

EEG of a 46-year-old patient with primary generalized epilepsy, showing atypical generalized bisynchronous spike wave activity.

Besides the presence of brief (1 to 3 seconds) generalized spike wave discharges, there are no interictal epileptiform abnormalities that are specific for individual syndromes included under PGE (childhood absence epilepsy, juvenile absence epilepsy, juvenile myoclonic epilepsy, epilepsy with myoclonic absences, and generalized tonic-clonic seizures on awakening). There are a few EEG features that are more common with certain syndromes: (1) polyspike wave discharges are more common with myoclonic epilepsies; (2) paroxysms of occipital-dominant rhythmic delta activity in the EEG is a feature most commonly encountered with childhood absence epilepsy; (3) short paroxysms of spike wave discharges of higher frequency (4.0 to 4.5 Hz) are more common with PGE manifesting primarily with generalized tonic-clonic seizures; and (4) PPR is most common with juvenile myoclonic epilepsy.

In patients with PGE, a routine EEG may capture one or more absence seizures or epileptic myoclonic jerks. In children an absence seizure may be induced during the EEG study by hyperventilation with characteristic generalized 3 Hz spike wave discharge, which is sustained for more than 3 seconds in duration. Epileptic myoclonic jerks are associated in the EEG with high-amplitude generalized polyspike wave discharges in association with myoclonic jerks. Not well recognized is the fact that the EEG in patients with PGE may record focal or lateralized spikes in addition to the overwhelming generalized bisynchronous spike wave activity.[84,85] Similarly, spike or spike wave activity occurring bilaterally but restricted to the frontal areas (where the generalized paroxysms are usually maximum) is also common. Such discharges are often called "abortive" spike wave complexes. Roughly one quarter of patients with 3 Hz spike wave activity in their EEG may show such focal or lateralized discharges,[84] which should generally be viewed as isolated fragments or limited expression of what is fundamentally a generalized epileptic abnormality. Such focal epileptiform discharges often shift from one electrode to the other and from one side to the other.

Secondary generalized epilepsy (SGE) is a more serious disorder, secondary to known diffuse cerebral hemispheric insult. Patients are children who have frequent seizures of generalized type, usually medically refractory. Many have significant developmental delay and neurocognitive deficits. In contrast to PGE, the background activity of the EEG in SGE syndrome is disorganized and there are variable degrees of slowing. In addition, there are several paroxysmal EEG patterns associated with SGE syndrome: (1) irregular bisynchronous spike wave activity described above, which can occur both with PGE or SGE; (2) slow spike wave (2.5 Hz or slower in frequency) discharges; (3) hypsarrhythmia; (4) independent multifocal spike discharges (IMSD); and (4) generalized paroxysmal fast activity (GPFA). These EEG patterns are largely nonspecific for etiology but are expressions of severe neocortical insult. Many of these EEG patterns are also age-dependent.

West's syndrome (infantile epileptic encephalopathy) is characterized clinically by infantile spasms (jackknife seizures). The EEG usually shows a distinctive interictal pattern called hypsarrhythmia. It consists of very-high-amplitude, asynchronous slow activity superimposed on frequent multifocal spikes, polyspikes, or sharp waves or generalized spike wave complexes. The abundance of epileptiform activity, the entire absence of any organization ("chaotic" appearance), and absence of normal activities (e.g., alpha rhythm or sleep spindles) are constant features of a typical hypsarrhythmic pattern (Fig. 22). When some of the characteristic features are lacking or are less prominent, some would interpret the EEG as showing "modified hypsarrhythmia." Often the classical hypsarrhythmic pattern occurs only during NREM sleep, and the awake tracing showing diffuse slowing with minimal epileptiform activity. During an actual infantile spasm, there is an abrupt generalized attenuation of the background (i.e., an electrodecremental response) (Fig. 23). This may be preceded by a high-voltage, usually generalized biphasic slow wave complex. During the electrodecrement there may be low-amplitude beta activity with varying spatial distribution. These electrodecremental events occur often during sleep but without behavioral accompaniment.

Figure 22.

EEG of a 6-month-old infant with developmental delay and infantile spasms, showing typical hypsarrhythmic pattern.

Figure 23.

EEG of a 5-month-old infant, showing electrodecremental response during an infantile spasm monitored on the last channel.

Hypsarrhythmia, which is an EEG pattern, and infantile spasms do not have an absolutely constant relationship and are not interchangeable terms. Typical and modified hypsarrhythmia occurs in two thirds of the EEGs of infants with infantile spasms, whereas the remaining one third show generalized slow spike wave discharges (described below).[86] Besides various pathologic conditions associated with a severe cortical insult, hypsarrhythmic pattern is often encountered in infants suffering from tuberous sclerosis or genetically determined metabolic conditions such as non-ketotic hyperglycenemia.[87] Children with Aicardi's syndrome (agenesis of corpus callosum, mental retardation, infantile spasms, choreoretinal lesions) show a markedly asymmetric hypsarrhythmic pattern with virtually complete interhemispheric asynchrony of a suppression-burst-like background.[88]

The hypsarrhythmic pattern is a maturational pattern most commonly expressed between the ages of 4 and 12 months. As the infant grows older, beyond the age of 2 years, it is rare to encounter typical hypsarrhythmia, although infantile spasms may still continue. Hypsarrhythmia is replaced by different EEG patterns such as a diffusely slow tracing, slow spike wave discharges as seen with Lennox-Gastaut syndrome, IMSD, and, rarely, a normal tracing.

Adrenocorticotrophic hormone therapy often has a dramatic effect on infantile spasms as well as the hypsarrhythmic EEG pattern, which may virtually disappear in a matter of a few days to a few weeks after initiation of therapy. However, despite these clinical and EEG improvements, the long-term neurocognitive development remains subnormal.

Lennox-Gastaut syndrome (childhood epileptic encephalopathy) is another common form of SGE manifesting in early childhood with developmental delay, neurocognitive deficits, and frequent generalized seizures including tonic seizures. The EEG shows generalized, slow spike wave discharges (1.5 to 2.5 Hz) superimposed on abnormally slow background activity (Fig. 24).[89,90] It is important to distinguish these EEG findings from those seen with primary generalized epilepsy where the background activity is normal for age and the generalized spike wave discharges are usually of faster frequency (3 to 5 Hz). Although appearing widespread and bilaterally synchronous, the slow spike wave activity is usually higher in amplitude over the anterior head regions (in 90% of patients); less commonly the amplitude is highest over the occipital areas. The duration of the paroxysms varies widely from isolated complexes to almost continuous slow spike wave activity, commonly without an identified behavioral or awareness change. Hence, the slow spike wave activity in Lennox-Gastaut syndrome is considered an interictal pattern, although it must be understood that subtle changes of behavior in retarded and uncooperative children are hard to recognize.

Figure 24.

EEG of a 16-year-old child with mental retardation and tonic seizures, showing slow spike wave activity superimposed on a slow background.

When one encounters prolonged episodes of slow spike wave activity lasting several seconds to minutes, the interpretative challenge is to decide if these electrographic events represent an ictal pattern (atypical absences or nonconvulsive status) or they simply represent more pronounced interictal pattern. A history of similar long episodes of slow spike wave activity in one or more previous EEGs would support an interictal finding. Also, giving a small dose of lorazepam intravenously will have no affect on an interictal pattern but will usually abort an ictal pattern, at least temporarily.

If a tonic seizure is recorded during the EEG of a patient with Lennox-Gastaut syndrome, the characteristic finding is an electrodecrement or "flattening" lasting several seconds. In addition, high-frequency rhythmic activity in the alpha-beta frequency range commonly occurs during the electrodecrement.

Another distinctive EEG pattern of a symptomatic generalized epilepsy syndrome is IMSD characterized by the presence of three or more independent and noncontiguous foci of spike or spike wave activity with at least one focus in one hemisphere (Fig. 25).[91,92] As expected, the background activity is invariably disorganized and slow in frequency.

Figure 25.

EEG of a 7-month-old child, showing independent multifocal spike discharges.

There is a close correlation between the three EEG patterns of hypsarrhythmia, slow spike wave, and IMSD associated with SGE. All of them are associated with diffuse or multifocal cerebral abnormalities and have similar clinical correlates of mental retardation, multiple and medically intractable seizure types, and a high incidence of neurologic deficits.[91] Furthermore, serial studies over time may show a change of one pattern to the other in the same patient. Also, in the same EEG study, more than one of these patterns may coexist (e.g., IMSD during wakefulness and slow spike wave activity during sleep). It is very well known that at least 20% of infants with hypsarrhythmia may show slow spike wave usually by the second to fourth year of life. Both of these patterns may further change to IMSD in early childhood. Thus, these three EEG patterns have a common physiopathologic basis and are probably dependent more on cerebral maturation than on a particular kind of cerebral pathologic process.[91] Hypsarrhythmia is usually seen in the later half of the first year of life in response to a cerebral insult prenatally, perinatally, or in the immediate postnatal period. It rarely results from cerebral insults after the second year of life. The slow spike wave pattern associated with Lennox-Gastaut syndrome is commonly observed between the ages of 2 and 5 years. The IMSD pattern is seen commonly throughout the first decade of life.

A unique EEG pattern of GPFA is seen predominantly during sleep consisting of high-frequency, 12 to 25 Hz repetitive spike discharges occurring synchronously over both hemispheres (Fig. 26).[93] It is associated most commonly with SGE (usually Lennox-Gastaut syndrome) but it may rarely occur also with PGE or in patients with focal seizures, particularly with a frontal lobe focus. This EEG pattern is usually not associated with an obvious clinical change, although subtle tonic seizures (opening of eyes and jaw, eye deviation upward) may be missed. In rare patients with PGE and 3 Hz generalized spike wave, the awake EEG may appear rather benign but the presence of GPFA during sleep is a warning that more severe encephalopathy may be present. In such patients, motor seizures are common and the disorder is likely to persist in adulthood.[94]

EEG in Focal or Localization-Related Epilepsies

Focal epilepsies are usually categorized into two subgroups: (1) asymptomatic (secondary) localization-related epilepsies due to acquired focal cortical processes; and (2) idiopathic (primary) localization-related epilepsies, which are largely age-dependent and genetically based disorders, the best-known being benign Rolandic epilepsy (BRE).

The hallmark of focal epilepsy in the interictal EEG is the presence of a focus of epileptiform activity (i.e., spikes, spike waves, or spike wave complexes). The interictal paroxysmal activity is characteristically random, occurring at inconstant intervals. Intuitively, the positive correlation of an abundance of epileptiform discharges with a more frequent occurrence of clinical seizures would be expected. This is, however, not true. Abundance and rate of repetition of epileptiform discharges correlate very poorly with the frequency of clinical seizures. It is not uncommon to see rare interictal spikes in a patient who has frequent complex partial seizures. The reverse is also true, as in BRE where the child may have only rare clinical seizures but very abundant epileptiform discharges, particularly during sleep. An important fact regarding interictal epileptiform discharges is that they become much more frequent in the EEG immediately after a clinical focal seizure.[77]

Another characteristic of the interictal EEG is the presence of a focal abnormality of the background activity. This may take the form of intermittent low-voltage slow waves and attenuation of faster rhythm with the same localization as that of the epileptiform discharges. In general, when the focal epileptiform activity occurs along with abnormal focal organization, the possibility of a structural lesion is more likely and the focal epilepsy is most likely to be symptomatic. On the other hand, occurrence of random focal spikes without any accompanying features of focal disorganization of the background activity is usual with idiopathic or primary localization-related epilepsies such as BRE or benign occipital epilepsy.

The majority (85%) of patients with complex partial seizures have temporal lobe onset, whereas a small proportion (15%) have frontal lobe onset partial epilepsy. Temporal lobe epilepsy, which is the most common type of focal epilepsy in adults, is associated with interictal epileptiform discharges over one or both anterior temporal regions (Fig. 27). The peak of the potential field involves inferior frontal (F7 or F8), midtemporal (T3 or T4), and the ear lobe electrodes (A1 or A2). The F7 and F8 electrodes of the 10 to 20 electrode placement system more commonly record activity originating in the anterior temporal lobe rather than in the frontal areas. In suspected patients with temporal lobe epilepsy, recording from additional TI and T2 electrodes must be performed to optimally elicit epileptiform abnormalities of temporal lobe origin. In patients undergoing presurgical evaluation, anterior sphenoidal electrodes are commonly inserted, which markedly increase the yield of demonstrating temporal epileptiform abnormalities. In frontal lobe epilepsy, interictal epileptiform discharges are recorded over the frontal region but often the EEGs, even on several occasions, may remain normal.[95,96] Special attention must be paid to the midline electrodes (FZ and CZ), which can demonstrate low-amplitude epileptiform discharges. Also, additional supraorbital and midfrontopolar (FPZ) electrodes may be helpful in demonstrating epileptiform abnormalities in frontal lobe epilepsy.

Figure 27.

EEG of a 68-year-old patient with a long history of complex partial seizures, showing a focus of sharp waves and low-amplitude slow activity over the right anterior temporal region.

BRE constitutes the most common primary (idiopathic) localization-related partial epilepsy with onset between the ages of 4 and 10 years. Virtually all recover by the age of 15 or 16 years when the patients become seizure-free and their EEGs revert to normal. The background EEG activity in wakefulness and sleep is normal. There are epileptiform discharges over the central or centrotemporal region, which are markedly activated during sleep (Fig. 28).[97] In some children, epileptiform discharges are restricted to sleep recording only.

Figure 28.

EEG of a 9-year-old child with benign Rolandic epilepsy, showing a focus of right centrotemporal spike discharges. The right half of the figure shows a spike discharge with horizontal dipole distribution.

A horizontal dipole field is a highly characteristic feature of the centrotemporal epileptiform discharges associated with BRE.[98] The negative end of the dipole is located at the centrotemporal area, and the positive end toward the frontal regions bilaterally (see right half of Fig. 28). In 10 to 15% of patients with BRE, additional EEG abnormalities are seen. Independent spike foci may occur over the occipital and less commonly over the frontal regions. Interestingly, bisynchronous spike wave discharges also occur in approximately 10% of patients with BRE during routine EEG recordings.[99]

Centrotemporal or Rolandic spikes are not specific for BRE. Children with acquired brain insults involving fronto-centro-temporal regions (who may also have neurologic deficits, e.g., cerebral palsy) may show epileptiform activity over one or both central areas. In contrast to BRE such "lesional" patients usually have abnormal background activity, either focally or diffusely, and have secondary (symptomatic) partial epilepsy.

Many studies[100,101,102] have stressed that a small proportion of epileptics have midline spike foci; the epileptiform abnormalities are localized at the midline electrodes with some spread to parasagittal electrodes on both sides. They are best demonstrated in the coronal montage, which includes midline (FZ, CZ, and PZ) electrodes; the amplitude may be somewhat asymmetric in parasagittal electrodes. The midline spike foci have highest amplitude at the CZ electrode in most of the patients. Such foci are common in children, and at least half of the patients show these discharges only during sleep. Certainly, caution must be exercised in distinguishing midline CZ spikes from normal sleep potentials such as vertex sharp transients. Midline spike foci have a high correlation with clinical seizures of focal or generalized type, and a significant proportion show evidence of cognitive or neurologic deficits.

Rarely, a focal seizure is recorded in a routine EEG in a patient with focal epilepsy. As mentioned before, the hallmark of an ictal EEG pattern associated with a focal seizure is the sudden appearance of focal rhythmic activity that evolves during a short period with a progressive change in the amplitude and frequency with a topographic spread. Postictally, there may be a period of "flattening" followed by variable degrees of slow wave activity, which may be generalized initially but usually becomes lateralized or focal later in the postictal period before the preictal EEG pattern becomes reestablished. A transient delta focus after the seizure is a very reliable sign of a localized lesion or at least a focal origin of the previous epileptic seizure. The postictal focal slowing may persist for a few seconds to a few hours or even a few days, depending upon such factors as the age of the patient, size of the lesion, duration, and number of seizures.

The ictal EEG pattern of complex partial seizures of temporal lobe onset usually starts with a focal rhythmic 4 to 7 Hz theta activity over one temporal region, which then spreads bilaterally, becoming higher in amplitude, slower in frequency, and often interspersed with sharp wave components.

Complex partial seizures of frontal lobe onset have a unique semiology compared with the complex partial seizures of temporal lobe onset.[95,96] The former are brief (less than 1 minute), commonly nocturnal, often occurring in clusters with many per day, have complex motor automatisms with kicking and thrashing, sexual automatism, vocalization, and with very short or no postictal period of confusion after the seizure. The frantic and often bizarre motor thrashing and kicking behavior and rapid return of awareness at the end of the seizure frequently leads to an erroneous diagnosis of pseudoseizures. Diagnostic difficulty is further compounded by the fact that routine EEG, even on many occasions, may be nonrevealing in patients with frontal lobe seizures.[95,96] Interictal abnormalities, when present, are often seen near the vertex bifrontally so that localization is not easy. Furthermore, the ictal EEG during a frontal lobe complex partial seizure often fails to provide diagnostic information because of marked contamination with ongoing movement artifacts during the seizure. Careful evaluation of the ictal EEG using coronal montage and high-frequency filtering may reveal rhythmic ictal patterns in frontal/central electrodes near the midline, spreading bilaterally to the parasagittal region. The frontal lobe complex partial seizures not only resemble pseudoseizures clinically but also lack diagnostic interictal and ictal EEG findings. Their brief duration, emergence out of sleep, and stereotypic semiology are highly characteristic features that help distinguish them from pseudoseizures.

Certain EEG Patterns Morphologically Similar to Interictal/ictal Epileptiform Discharges But Unrelated to Epilepsy

There are some EEG patterns that, by virtue of having spike or sharp wave components or being rhythmic, closely resemble diagnostically important epileptiform discharges. Although they share some of the same electrographic features, their presence in the EEG has no correlation with epileptic seizures. These patterns have been reviewed in detail.[103]

  1. The 14 and 6 Hz positive spikes are brief paroxysms, usually less than 1 second, of positive spikes with a frequency of 6 Hz at some times and 14 Hz at others. They are distributed over the posterior hemispheric regions, occurring usually unilaterally. If enough of them are recorded in a tracing, they are expressed on both sides of the head. They are most commonly seen in children and adolescents during drowsiness and light sleep. This pattern has been the subject of many reviews and long-drawn controversy but it is now held to be a normal sleep activity or a normal variant.

  2. Small sharp spikes or benign epileptiform transients of sleep are spike potentials that are recorded in adults during drowsiness and light sleep. They are sporadic and shift from one side to the other, and have widespread distribution over the scalp despite low amplitude. The spikes are usually low amplitude (less than 50 µV), very short duration (less than 50 milliseconds), and mono- or diphasic potentials with no or minimal slow wave component following the spikes (Fig. 29). They are never associated with focal slowing or other abnormalities of the background activities. Their special distribution on the scalp may suggest a horizontally oriented dipole generator extending across the sagittal midline with opposite polarity on the two sides of the head. They are especially frequent in the EEG performed after a period of sleep deprivation and are considered to have no correlation with epileptic seizure disorder.[104]

  3. The 6 Hz spike wave or phantom spike wave consists of bilaterally synchronous, short (less than 1 second) paroxysms of 5 to 7 Hz spike wave activity.[105] Although widespread in distribution, they are usually best developed over the posterior hemispheric region (Fig. 30). The spike component is usually low in amplitude, whereas the slow wave following it is more prominent; hence, the term "phantom spike wave." It occurs in young adults and is best expressed during drowsiness. There is still some controversy regarding their clinical significance, especially the ones that are anteriorly dominant. It has been suggested[106] that the 6 Hz spike wave activity that occurs with high amplitude and predominance over the anterior hemisphere and is recorded during wakefulness may have a high correlation with seizures, whereas predominantly occipital, low-amplitude, spike wave discharges occurring in drowsiness have no correlation with epileptic seizures.

  4. "Psychomotor variant" or rhythmic midtemporal discharges is a rare EEG finding observed in less than 1% of the patient population coming to an EEG lab. It is characterized by long (several seconds to a minute) paroxysms of 5 to 7 Hz rhythmic activity with an admixture of sharp components occurring over the midtemporal (T3/T4) regions (Fig. 31).[107] The paroxysms are unilateral or bilateral. Bilateral paroxysms may appear independently on the two sides or simultaneously with variable asymmetry. The pattern is invariant and without evolution from beginning to end. This EEG finding occurs during drowsiness or light sleep and has no correlation with epileptic seizure disorder.

  5. The wicket temporal pattern is characterized by negative sharp transients that occur in runs and have a wicket shape. The repetition rate varies between 6 and 11 Hz in different and even in the same bursts. They are best seen in T3/T4 or F7/F8 electrodes with variable spread to A1/A2. They are usually bitemporal, occurring independently on the two sides. The pattern occurs exclusively in adults and is seen in both wakefulness and sleep.[108]

  6. Subclinical rhythmic EEG discharges of adults is a rare EEG pattern[109,110] that closely resembles an ictal pattern, but there is no evidence that it represents an epileptic seizure. It occurs mainly in elderly subjects and may extend beyond 1 minute but without any clinical change. It is predominant over the posterior head regions and usually bilateral but may be asymmetric. In the most characteristic form the pattern begins with a series of rhythmic sharp-contoured waves that gradually merge into a sustained theta frequency. It may subside abruptly or gradually.[103]

  7. There are many normal activities, particularly during sleep in children that, being sharp in configuration, are often mistaken for epileptiform discharges. These include vertex sharp transients and positive occipital sharp transients. In children, the vertex sharp transients may be quite high in amplitude and spiky in configuration, very closely resembling epileptiform discharges. One has to be very careful in interpreting sharp waves or spikes in children of Rolandic location expressed only during sleep. Normal mu rhythm in central leads and breach rhythm recorded in patients with iatrogenic or acquired skull defect may also consist of sharp components that may be mistaken for epileptiform discharges. Lambda waves, which are sharply contoured activity in the occipital region seen when a person has eyes open and scans the surrounding environment, need to be differentiated from posterior hemispheric epileptiform discharges.


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