Interictal Epileptiform Abnormalities 21 Spikes and Sharp Waves

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2.1.1. Description

Spikes and sharp waves are sharply contoured waveforms that are distinct from the EEG background and usually have a negative polarity (Fig. 1). They can be of any voltage and can occur either singly or in repetitive runs with varying frequencies. They can be focal or generalized in distribution. Spikes, by definition, have a duration shorter than 70 ms, whereas sharp waves have a duration between 70 ms and 200 ms. These discharges classically have an asymmetric appearance, with the initial deflection characterized by a sharper slope than the return to baseline. They may be observed as isolated waveforms or they can be followed by a slow wave. Although similar deflections with a positive polarity are sometimes referred to as "positive spikes," nearly all epileptiform discharges of clinical interest are of negative polarity.

From: The Clinical Neurophysiology Primer Edited by: A. S. Blum and S. B. Rutkove © Humana Press Inc., Totowa, NJ

Spike And Wave Discharges

Fig. 1. Sharp waves. Note the run of sharp wave discharges evident phase reversal at F7.

over the anterior left temporal channels, with

Fig. 1. Sharp waves. Note the run of sharp wave discharges evident phase reversal at F7.

over the anterior left temporal channels, with

2.1.2. Physiological Basis and Significance

A spike is thought to be generated by the synchronous depolarization of thousands of cortical neurons located within an area of at least 6 cm2. Sharp waves are thought by some to result from the synchronous activation of either a smaller pool of neurons or a pool of neurons further from the recording electrode, such as below the cortical surface. However, most electroencephalographers consider spikes and sharp waves to have the same physiological significance: they are interictal epileptiform discharges that serve as markers for epileptoge-nesis, either focal or generalized, and are suggestive of an underlying propensity toward seizures. They are observed only rarely in healthy individuals.

2.1.3. The Spike-and-Slow-Wave Complex

This term refers to the occurrence of a spike followed immediately by a slow wave, which can be of varying frequency and amplitude, and is usually distinct from the underlying background (Fig. 2). Sharp-and-slow wave complexes include a sharp wave as the initial waveform, rather than a spike. The following slow wave in these discharges may represent inhibition and subsequent hyperpolarization of cortical neurons that follows the initial synchronous depolarization. Spike-and-slow wave complexes are strongly suggestive of an underlying epileptic disorder.

2.1.4. Polyspikes

These are discharges characterized by multiple spikes observed in rapid succession, typically at frequencies of 10 Hz or faster (Fig. 3). They may be followed by a slow wave. Polyspikes may be observed in many generalized seizure disorders, particularly those in which myoclonus is a feature, and at times may occur in temporal association with a clinically noted myoclonic jerk.

2.1.5. Differential Diagnosis

Because of the clinical and physiological significance of spikes and sharp waves, it is important to recognize the features that distinguish them from other similar waveforms. Many of these are also discussed in Chapter 7, on normal EEG variants.

2.1.6. Vertex Waves

Vertex waves are broad, sharply contoured waveforms of negative polarity that are typically central in location and observed mostly during Stage II sleep (Fig. 4). Individual vertex waves can appear more prominently over one hemisphere, as long as they are not persistently unilateral. Because sleep can be associated with an exacerbation of epileptiform activity, distinguishing central spikes from vertex waves may be difficult in certain cases. However, a vertex wave usually has a symmetric morphology, whereas the initial deflection of a spike is usually steeper than the return to baseline.

2.1.7. Wicket Spikes

Wicket spikes are sharp negative-polarity waveforms that can be of high amplitude and are typically observed in the temporal regions of older adults (Fig. 5). Most commonly, they arise out of a high-amplitude background that is already somewhat sharp in morphology. They are of uncertain clinical significance but do not seem to suggest an underlying epileptic disorder.

2.1.8. Small Sharp Spikes, or Benign Epileptiform Transients of Sleep

Small sharp spikes, or benign epileptiform transients of sleep (BETS), are low-amplitude, short-duration waveforms that are symmetrically biphasic. As their name suggests, they are

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Benign Epileptiform Transients

Page -1- Papcr-spcccf = 3D Time t TD Date = 200SI 11:11 Falicnl - spikeiLslowwnvo

Fig. 2. Spike and slow wave. Note the characteristic spike followed by a slow wave, in this instance distributed bi-anteriorly at close to 4 Hz. Spikes may sometimes seem to be "buried" by the slow wave component, giving rise to a notched appearance, as seen in portions of this burst.

Page -1- Papcr-spcccf = 3D Time t TD Date = 200SI 11:11 Falicnl - spikeiLslowwnvo

Fig. 2. Spike and slow wave. Note the characteristic spike followed by a slow wave, in this instance distributed bi-anteriorly at close to 4 Hz. Spikes may sometimes seem to be "buried" by the slow wave component, giving rise to a notched appearance, as seen in portions of this burst.

Poly Spike And Wave
Fig. 3. Polyspike and slow wave. A bi-anterior burst of polyspikes appear, followed by a prominent slow wave. These are more commonly observed in generalized epilepsies, but are not unique to such.
Polyspikes
Fig. 4. Waveforms with epileptiform appearance: vertex waves. Vertex waves are typically central in location and are observed mostly during Stage II sleep. Here, vertex waves appear in a clustered fashion, with phase reversal about the vertex channels, as illustrated in this transverse montage.

Fig. 5. Waveforms with epileptiform appearance: wicket spikes. Wicket spikes are of uncertain clinical significance but are typically observed in temporal channels in older adults. From Goldensohn et al., 1999 with permission.

observed during sleep, and are most common over the temporal regions, although they can be more widespread, unilateral or bilateral, and can have a horizontal dipole distribution. They are usually a normal EEG variant of no known clinical significance.

2.1.9. Rhythmic Midtemporal Theta Bursts, or Psychomotor Variant

Rhythmic midtemporal theta bursts, or psychomotor variant, are sharp in morphology and occur classically at a 6-Hz frequency. A characteristic feature is the notched appearance of the waveforms (Fig. 6). They most commonly occur in the temporal regions of young adults during sleep. They are a normal EEG variant of no clinical significance.

Normal Eeg Tracings

Fig. 6. Waveforms with epileptiform appearance: rhythmic midtemporal theta bursts. This pattern is also known as the "psychomotor variant." Note the run of unchanging 5-Hz activity over the left temporal channels lasting 3 s in this tracing. The patient is drowsy, as is commonly the case for this pattern. This pattern is no longer considered pathological.

Fig. 6. Waveforms with epileptiform appearance: rhythmic midtemporal theta bursts. This pattern is also known as the "psychomotor variant." Note the run of unchanging 5-Hz activity over the left temporal channels lasting 3 s in this tracing. The patient is drowsy, as is commonly the case for this pattern. This pattern is no longer considered pathological.

Psycho Motor Varient

Fig. 7. Waveforms with epileptiform appearance: 14-and-6 Hz positive bursts. Fourteen- and 6-Hz positive bursts are seen with shifting laterality in this tracing. They are downward deflections, very sharply contoured, broadly distributed, and best seen with long-distance referential recording technique. They are a normal variant pattern. From Goldensohn et al., 1999 with permission.

Fig. 7. Waveforms with epileptiform appearance: 14-and-6 Hz positive bursts. Fourteen- and 6-Hz positive bursts are seen with shifting laterality in this tracing. They are downward deflections, very sharply contoured, broadly distributed, and best seen with long-distance referential recording technique. They are a normal variant pattern. From Goldensohn et al., 1999 with permission.

2.1.10. Fourteen- and 6-Hz Positive Bursts

Fourteen- and 6-Hz positive bursts are distinguished from epileptiform spikes by their positive polarity and their characteristic mixture of 14-Hz and 6-Hz frequencies (Fig. 7). They usually occur in the recordings of adolescents and young adults during sleep, and are best observed using montages with long interelectrode distances or ear references. Although, in the past, electroencephalographers have associated 14- and 6-Hz positive bursts with a variety of neurological abnormalities, they are now thought to be a normal variant.

2.1.11. Occipital Spikes of Blindness

Occipital spikes of blindness can be observed in patients with acquired visual loss in childhood. They are epileptiform in appearance and often quite narrow but are not thought to suggest an underlying seizure disorder.

2.1.12. Six-Hertz Spike-and-Wave, or "Phantom" Spike-and-Wave

Six-Hertz spike-and-wave, or "phantom" spike-and-wave, is a discharge characterized by repetitive spike-and-slow wave complexes occurring at a 6-Hz frequency, with the spike being of very low amplitude compared with the following slow wave. It is typically observed in a generalized distribution but may be posteriorly predominant. Six-Hertz spike-and-wave is not thought to be associated with clinical seizures.

2.1.13. Sharply Contoured Artifact

Finally, sharply contoured artifact can also be confused for true spikes and sharp waves. ECG artifact is frequently sharp in morphology and is identified by a clear temporal association with the QRS complex on the cardiac rhythm strip. Muscle artifact can be quite sharp but is usually of an extremely fast frequency that distinguishes it from cerebrally generated activity. Electrode dysfunction or "pop" artifact can also be initially confused for epileptiform activity. However, this artifact characteristically lacks the appropriate field expected in a brain-derived discharge. It is also often much steeper at the outset than bona fide epileptiform discharges, with an exponential decay.

2.2. Epileptiform Discharges by Location

Spikes and sharp waves from several brain regions deserve particular mention because of their special significance or the frequency with which they are observed.

2.2.1. Temporal Spikes

The majority of partial seizures in adults stem from the temporal lobes, and, thus, temporal spikes and sharp waves are the most frequently observed focal interictal discharges (Fig. 1). There is a large literature on the significance of interictal temporal discharges in helping to determine the laterality of temporal lobe seizure onset, and synchronous bitemporal discharges or even unilateral discharges from the side contralateral to ictal onset can be observed, presumably because the two mesial temporal lobes are so intimately connected. Extra electrodes are frequently used to provide more coverage of temporal areas: "true anterior temporal" scalp electrodes are placed by some laboratories (referred to as T7 and T8 in American Clinical Neurophysiology Society nomenclature), whereas many use sphenoidal needle electrodes, which are inserted several centimeters deep to the surface below the zygomatic arch bilaterally and record from near the mesial temporal lobes.

2.2.2. Centrotemporal (or "Rolandic") Spikes

These are characteristic of the childhood syndrome of benign epilepsy with centrotemporal spikes (Rolandic epilepsy) and are observed over the centrotemporal region either unilaterally or bilaterally (Fig. 8). These are typically high-amplitude biphasic or, less often, polyphasic spikes or sharp waves, with a following slow wave. In patients with this syndrome, these discharges can occur quite frequently, particularly during sleep. Often the

Centrotemporal Spikes

Fig. 8. Centrotemporal ("Rolandic") spikes. This bipolar tracing is from a 5-yr-old boy with a history of spells of convulsive activity involving the left arm. Note the frequent high-amplitude sharp and slow waves appearing independently from bicentral channels with some spread to involve the temporal channels as well, better seen on the right. These discharges are often sleep activated.

Fig. 8. Centrotemporal ("Rolandic") spikes. This bipolar tracing is from a 5-yr-old boy with a history of spells of convulsive activity involving the left arm. Note the frequent high-amplitude sharp and slow waves appearing independently from bicentral channels with some spread to involve the temporal channels as well, better seen on the right. These discharges are often sleep activated.

distribution suggests a horizontal dipole (i.e., a phase reversal in a referential recording or two simultaneous phase reversals of opposite polarity in a bipolar recording), although this is not invariable (Fig. 9). The clinical syndrome consists of simple partial seizures of the face, sometimes affecting speech or swallowing functions, and occasional generalized tonic-clonic (GTC) seizures. The syndrome is considered benign, in that underlying structural lesions are rarely observed (despite the clear focality of the discharges) and seizures do not usually persist beyond childhood. Rolandic epilepsy is thought to be inherited in an autosomal dominant fashion with incomplete penetrance, such that the characteristic EEG discharges can be observed in some children without clinical seizures.

2.2.3. Occipital Spikes

These are observed in a syndrome analogous to, but less well-defined than, Rolandic epilepsy, called benign childhood epilepsy with occipital paroxysms. The occipital discharges in this syndrome attenuate with eye opening. Importantly, these patients' seizures, which are characterized by visual hallucinations and headache, are not typically photosensitive, and the EEG does not show an abnormal response to intermittent photic stimulation. As noted above, occipital spikes in patients with acquired blindness do not necessarily imply an underlying tendency toward seizures.

2.2.4. Frontal Sharp Waves

These can be a normal EEG finding in neonates between 28 and 42 wk conceptional age and, in this age group, do not necessarily imply an epileptic disorder, as long as there is no persistent unilaterality. They are also termed encoches frontales, because of their classic "check-mark" appearance.

2.2.5. Generalized Spikes

In adults, generalized epileptiform discharges often have a bifrontal predominance, sometimes to such an extent that the sharp or spike morphology is only truly observed frontally and not in more posterior regions. Most electroencephalographers consider these bifrontally predominant discharges to be manifestations of generalized epileptogenesis, however.

Generalized spikes are typically suggestive of an underlying generalized epilepsy. As noted above, a bifrontal predominance is common. In most idiopathic generalized epilepsies, spike-and-slow wave complexes are observed, sometimes repeating at frequencies that are characteristic of particular epilepsy syndromes. For example, interictal runs of generalized spike-and-slow wave discharges recurring three times per second are most strongly associated with childhood or juvenile absence epilepsy (Fig. 10), whereas faster generalized discharges (four to six per second) are typically observed with the syndrome of juvenile myoclonic epilepsy (Fig. 11). "Slow" (less than three per second) generalized spike-and-slow wave discharges (Fig. 12) are an interictal pattern observed in Lennox-Gastaut syndrome, a childhood disorder characterized by multiple seizure types and developmental delay.

Occasionally in patients with partial epilepsy, a physiologically focal epileptiform discharge can spread so rapidly to involve the entire brain that, on visual analysis of the EEG, it seems generalized in onset, a phenomenon called secondary bilateral synchrony.

2.3. Activation Procedures and Précipitants

The two commonly used activation procedures during a routine EEG, hyperventilation and intermittent photic stimulation, can increase the chance of detecting interictal epileptiform

Centrotemporal Spikes
Fig. 9. Centrotemporal spikes. This referential tracing from the same patient as in Fig. 8 illustrates the horizontal dipole associated with centrotemporal discharges. Maximal negativity is seen from the peri-Rolandic channels, with the positive pole of the dipole appearing over anterior channels.
Interictal Spikes

Fig. 10. Three-Hertz generalized spike-and-slow wave activity. This tracing derives from a 12-yr-old girl with staring spells and a positive family history. Note that this 8-s burst of bilaterally synchronous 3-Hz spike and wave activity arises during hyperventilation. The subject does not respond to the cue until the cessation of the spike and wave activity. The burst is maximal over bi-anterior channels.

Fig. 10. Three-Hertz generalized spike-and-slow wave activity. This tracing derives from a 12-yr-old girl with staring spells and a positive family history. Note that this 8-s burst of bilaterally synchronous 3-Hz spike and wave activity arises during hyperventilation. The subject does not respond to the cue until the cessation of the spike and wave activity. The burst is maximal over bi-anterior channels.

Sharp Waves And Spike Activity 24houreeg
Fig. 11. Fast spike-and-wave activity. This tracing is from a 17-yr-old girl with juvenile myoclonic epilepsy. Note the synchronous, bi-anterior spike and wave activity at 4 to 5 Hz.
Sharp Waves And Spike Activity 24houreeg

Fig. 12. "Slow" spike-and-slow wave activity. This tracing is from a 36-yr-old man with Lennox-Gastaut Syndrome (LGS). In this case, there is a run of 2- to 21/2-Hz synchronous bi-anteriorly predominant sharp and slow wave activity lasting 7 s. Such activity, when prolonged, may appear during atypical absence seizures common to patients with LGS.

Fig. 12. "Slow" spike-and-slow wave activity. This tracing is from a 36-yr-old man with Lennox-Gastaut Syndrome (LGS). In this case, there is a run of 2- to 21/2-Hz synchronous bi-anteriorly predominant sharp and slow wave activity lasting 7 s. Such activity, when prolonged, may appear during atypical absence seizures common to patients with LGS.

discharges in susceptible patients. Hyperventilation can bring out both focal and generalized discharges, although it is most useful in eliciting the three-per-second spike-and-slow wave pattern observed with absence seizures. Intermittent photic stimulation usually triggers generalized discharges in patients with photosensitive generalized epilepsies.

Drowsiness and sleep are also precipitants of epileptiform discharges. For this reason, routine EEGs in patients with suspected seizure disorders are often performed after sleep deprivation, to maximize the chances of recording an adequate period of sleep. The various stages of non-REM sleep, in particular, seem to be associated with increased epileptiform activity. Some patients may only have epileptiform discharges during sleep.

2.4. Clinical Interpretation of Epileptiform Abnormalities

The identification and characterization of interictal epileptiform discharges are useful clinically in several ways. First, their mere presence can help to confirm the diagnosis in a patient in whom a seizure disorder is suspected. However, their limitation in this regard must be understood: only approx 50% of patients with known epilepsy have interictal discharges on their first routine EEG. This number increases to 70% or greater with repeat studies. However, patients with mesial temporal lobe seizures may not have visible discharges if only standard scalp electrodes are used, and frontal lobe seizure patients also have a low incidence of interictal discharges. Elderly epilepsy patients have a lower incidence of interictal discharges than do younger patients. Finally, approx 1% of patients who do not have clinical seizures may have epileptiform discharges on their EEGs. In summary, then, although the finding of interictal epileptiform discharges is fairly specific for the diagnosis of epilepsy, a single EEG is not an exquisitely sensitive test for epilepsy, particularly in certain situations or populations.

Interictal discharges can be critical, however, in the characterization of a patient's epilepsy syndrome. For some patients, it is not possible on clinical grounds alone to determine whether seizures are partial or generalized in onset, and some syndromes have characteristic interictal EEG discharges that form part of their definition.

Thirdly, a clinician may wish to assess the frequency of interictal discharges in a known epilepsy patient, if there is a question of worsening seizures or frequent subclinical seizures, for example (surprisingly, however, frequent discharges do not necessarily correlate with frequent seizures in many cases). In addition, patients with epilepsy who have unexplained cognitive dysfunction or sleep disturbances may sometimes have very frequent interictal discharges during sleep, although the causal relationship between these discharges and their symptoms is not completely understood.

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  • Sabrina
    What do intermittent sharply contoured waveforms indicate?
    5 years ago

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