EEG Atlas Part I Normal&Bengin V اطلس رسام المخ الكهزبائى الجزء الاول

FIGURE 1.1. Normal Alpha Rhythm and Squeak Effect. An alpha rhythm appears immediately after eye closure and disappears with eye opening. Immediately after eye closure, alpha frequency may be accelerated for 0.5–1 sec. Therefore, alpha frequency assessment should not be done during this period. This is called the “squeak effect.”
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I Normal & Benign variants
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Alpha rhythm or posterior dominant rhythm (Figures 1-1 to 1-5, 1-11, and 1-16)
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• Monomorphic either sinusoidal or having sharp points at the top or bottom, 8–13 Hz in older children and adults during relaxed wakefulness with eyes closed.
• Eye opening attenuates alpha rhythm (AR) and eye closure accentuates AR.
• AR also attenuates with:
1- Drowsiness 
2- Concentration 
3- Stimulation 
4- Visual fixation
5- Anxiety
6- Eye closure with mental calculation
• AR responsive to eye opening occurs in 75% of infants between 3rd and 4th months.

Frequency
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• Mean AR frequency: 
• 4 months – 4 Hz
• 12 months – 6 Hz
• 36 months – 8 Hz
• 9 years – 9 Hz
• 10 years – 10 Hz  Elderly – above 9 Hz
• Abnormal AR:
• 1 year: 50% is seen in 1.5% in all ages.
• Voltage asymmetry >20% is seen in 5% of normal children.
• Persistent asymmetry of 50% or more is considered abnormal.
• Persistent asymmetry of 35–50% is considered suspect if the lower voltage AR is on the right side.
• Symmetry is best measured in referential montage to avoid phase cancellation.
• The same rule is applied to mu and temporal theta activity.

Squeak effect
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• Immediately after eye closure, alpha frequency may be accelerated for 0.5–1 sec. Therefore, alpha frequency assessment should not be done during this period.

Paradoxical AR
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• AR presents with eye opening if the environment is devoid of light as the result of partial alerting. Paradoxical AR is seen in drowsiness and sedation.

Beating or waxing and waning of the AR
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• Effect of two separate alpha frequencies.

Bancaud phenomenon
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• When unilateral cerebral lesions or transient cerebral dysfunction (such as migraine or TIA) are present in the occipital or, less commonly, parietal or temporal lobes, the side of defective reactivity (eye opening and alerting) occurs ipsilateral to the side of the lesion. When both phenomena exist, the same side of the brain is affected.

Beta activity
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• ≥ 13 Hz; most common 18–25 Hz; less common 14–16 Hz; rare 35–40 Hz.
• First develops between 6 months and 2 years
• Distribution: frontocentral >widespread>posterior.
• Voltage 35% is indicative of:
• Cortical injury
• Transient conditions such as postictal state
• Subdural or epidural fluid collection
• More sensitive than focal polymorphic delta activity (PDA)
• Amplitude asymmetry of >35% is considered abnormal.
• Focal increased amplitude is seen in
• Skull defect (breach rhythm)
• Focal structural abnormality, especially focal cortical dysplasia
• Presence of beta activity is almost always a good prognostic sign.

FIGURE 1.2. Alpha Rhythm in Subdural EEG. Subdural recording shows the alpha rhythm in the right occipital lobe with reaction to eye opening and eye closure. Harmonic of the alpha rhythm is frequently seen in an intracranial EEG. In addition, alpha rhythms usually are sharper in morphology because the scalp and skull act as a high-frequency filter and pass lower frequencies more efficiently than higher frequencies. 2 All the normal EEG rhythms seen in the scalp EEG can be seen in the intracranial EEG.

FIGURE 1.3. Beating (Waxing and Waning of Amplitude). The beating or waxing and waning of the alpha rhythm is the effect of two separate alpha frequencies.

FIGURE 1.4. Alpha and Mu Rhythm. Eye opening (open arrow) attenuates the alpha rhythm but reveals a prominent mu rhythm (C3 and C4) at the same frequency (11 Hz). Note lateral eye movement (X) after the eye opening.

Mu is an arc-like central rhythm with negative sharp component and positive slow component. The frequency is similar to alpha rhythm and it is intermixed with 20-Hz beta activity. It is located at the C3, C4, and Cz electrodes. It is not blocked by eye opening but is attenuated by movement of extremities or thinking about moving with greater effect on opposite hand. The apiculate phase may resemble spikes.

FIGURE 1.5. Alpha and Mu Rhythm. Eye opening attenuates the alpha rhythm (open arrow), and eye closure accentuates the alpha rhythm. Eye opening or closure does not affect the mu rhythm (C3 and C4). Mu is an arc-like central rhythm with negative sharp component and positive slow component. The frequency is similar to alpha rhythm and it is intermixed with 20-Hz beta activity. It is located at C3, C4, and Cz electrodes and is not blocked by eye opening but attenuated by movement of extremities or thinking about moving with greater effect on opposite hand. The apiculate phase may resemble spikes.

FIGURE 1.6. Slow Alpha Variant. Slow alpha variant (open arrow) is described as rhythmic sinusoidal, notched theta or delta activities, which have a harmonic relationship with the alpha rhythm (one-third or, more commonly, one-half the frequency). Slow alpha variant is a rare physiologic variant of alpha rhythm (less than 1% of normal adults) seen during relaxed wakefulness, has a harmonic relationship and is interspersed with the normal alpha rhythm, and shows similar distribution and reactivity as a normal alpha rhythm. 4, 5 It should not be misinterpreted as occipital intermittent rhythmic delta (OIRDA) or theta activity activities, pathologic findings seen in children and adults. Slow alpha variant may be differentiated from pathologic slow waves by morphology (notched appearance), frequency (subharmonic of normal alpha rhythm), reactivity to eye opening, and disappearance with sleep. It sometimes mimics RTTD, except that it occurs only over the posterior head regions.
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Slow alpha variant (Figures 1-6 to 1-7)
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• A rare physiologic variant of AR (less than 1% of normal adults), seen during relaxed wakefulness, has a harmonic relationship and interspersed with the normal AR, and shows similar distribution and reactivity as a normal AR.
• Usually alternates with AR.
• Rhythmic sinusoidal, notched theta, or delta activities that have a harmonic relationship with the AR (one-third or, more commonly, one-half the frequency).
• Should not be misinterpreted as occipital intermittent rhythmic delta (OIRDA) or theta activity activities, and pathologic findings seen in children and adults.
• Slow alpha variant may be differentiated from pathologic slow waves by:
• Morphology (notched appearance)
• Frequency (subharmonic of normal AR)
• Reactivity to eye opening
• Disappearance with sleep
• Sometimes mimics rhythmic temporal theta bursts of drowsiness (RTTD), except that it occurs only over the posterior head regions.

FIGURE 1.7. Slow Alpha Variant. EEG of a 7-year-old boy with recurrent syncope showing semi-rhythmic notched theta activity, a subharmonic of the baseline alpha rhythm at channels 4, 8, 14, and 18, which appears immediately following the eye blink. Slow alpha variant is a rare, benign EEG variant (less than 1% of normal adults), has a harmonic relationship with the alpha rhythm, and shows similar distribution and reactivity as a normal alpha rhythm, reactivity to eye opening and eye closure (arrow), and disappearance with sleep. 4 It should not be misinterpreted as occipital intermittent rhythmic delta activity (OIRDA) or theta activity, pathologic findings seen in children and adults.

FIGURE 1.8. Fast Alpha Variant. The fast alpha variant pattern (arrow) is a harmonic of the alpha rhythm that has a frequency approximately twice that of alpha rhythm, usually within the range of 16 to 20 Hz, with a voltage of 20-40 μV. It is usually intermingled with alpha rhythm and shows reactivity and a distribution similar to that of alpha rhythm.
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Fast alpha variant pattern (Figures 1-8 to 1-10)
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•Harmonic of the AR that has a frequency approximately twice that of AR, usually within the range of 16–20 Hz, with a voltage of 20–40 μV.

• Usually intermingled with AR and shows reactivity and a distribution similar to AR.

FIGURE 1.9. Fast Alpha Variant. The fast alpha variant pattern (within the rectangle) is a harmonic of the alpha rhythm that has a frequency approximately twice that of alpha rhythm, usually in the range of 16 to 20 Hz, with a voltage of 20-40 μV. It is usually intermingled with alpha rhythm and shows reactivity and distribution similar to that of alpha rhythm.

FIGURE 1.10. Fast Alpha Variant. The fast alpha variant pattern (box) is a harmonic of the alpha rhythm that has a frequency approximately twice that of alpha rhythm, usually in the range of 16 to 20 Hz, with a voltage of 20-40 μV. It shows reactivity and a distribution similar to that of alpha rhythm.

FIGURE 1.11. Low-Voltage Background Activity. EEG of a 16-year-old-boy with recurrent syncope. Low-voltage EEG during wakefulness characterized by activity of voltage ≤ 20 μV over all head regions. With higher gain, a wide variety of different frequency waveforms are noted including beta, theta, and, to a lesser degree, delta waves with or without a posterior alpha rhythm, over the posterior areas. 6 Waves of higher amplitude can sometimes be activated by hyperventilation, photic stimulation, and sleep. Low voltage EEG is a normal EEG variant and does not represent an abnormality unless the frequency band shows abnormal local or diffuse slowing, asymmetries, or paroxysmal events. The prevalence of low-voltage EEG was 1% between ages 1 and 20 years, 7% between 20 and 39 years, and 11% between 40 and 69 years. 7 The prevalence increases sharply after the age 13. 8 Low-voltage EEG in children below age 10 years is considered abnormal if neither hyperventilation nor non-REM sleep changes the low voltage character. Low voltage EEG can be seen in adults with chronic vertebrobasilar artery insufficiency and chronic alcoholism. 9

FIGURE 1.12. Posterior Slow Waves of Youth. EEG of a 9-year old boy with recurrent headaches and numbness shows bilateral occipital slow waves (Box) intermixed with and briefly interrupting the alpha rhythm.

“Posterior slow waves of youth” (youth waves or polyphasic waves) are physiologically high-voltage theta or delta waves accompanied by the alpha rhythm and creating spike wave-like phenomenon. They are most commonly seen in children aged 8 to 14 years but are uncommon in children under 2 years. They have a 15% incidence in healthy individuals aged 16 to 20 years but are rare in adults after age 21 years. They are typically seen both unilaterally and bilaterally in a single recording. They are always accompanied by the alpha rhythm, attenuated with eye opening, disappear with the alpha rhythm during drowsiness and light sleep, and may be accentuated by hyperventilation.
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Posterior slow waves of youth (youth waves or polyphasic waves) (Figures 1-12 to 1-15)
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• Physiologically high-voltage theta or delta waves accompanied by the AR and creating spike wave-like phenomenon

• Most commonly seen in children aged 8–14 years and are uncommon in children under 2 years.

• A 15% incidence in healthy individuals aged 16–20 years but rare in adults above 21 years of age.

• Typically seen both unilaterally and bilaterally in a single recording. They are always accompanied by the AR, attenuated with eye opening, disappear with the AR during drowsiness and light sleep, and may be accentuated by hyperventilation and stress.

• Characteristic findings:

• Monorhythmic occipital rhythm attenuates with eye opening

• Normal slower waveforms rarely >1.5 times the amplitude of AR.

• Normal slower waveforms attenuate with AR during alerting.

• Slower waveforms has the same asymmetry in the ongoing AR

• Index of abnormality of theta/delta slowing.

• Complexity and variability of waveforms

• Incidence (how often slow waves occur)

• Voltage ratio (normal slow waves rarely >1.5 times the amplitude of AR)

• Persistence with eye opening

• Symmetry (consistently predominant on one side).

FIGURE 1.13. Posterior Slow Waves of Youth; Attenuated with Eye Opening. EEG of a 10-year-old boy with syncope showing occipital slow theta and delta waves (arrows) mixed with and briefly interrupting the alpha rhythm in both occipital regions but maximally expressed in the left hemisphere. These are so-called “posterior slow waves of youth,” which are physiologic findings seen commonly in children aged 8 to 14 years. They are always accompanied by the alpha rhythm, are attenuated with eye opening (open arrow). and disappear with the alpha rhythm during drowsiness and light sleep. 10–12

FIGURE 1.14. Intermittent Right Occipital Delta Slowing; Simulating Posterior Slow Wave of Youth. An 8-year-old boy with autism and few generalized tonic-clonic seizures. The 24-hour ambulatory EEG performed to rule out ESES persistently shows decreased alpha reactivity to eye closure (open arrow) and intermittent polymorphic delta slowing (arrow head) in the right occipital region without shifting lateralization. This EEG can simulate “posterior slow waves of youth,” which is a physiologic finding. However, persistent lateralization raises a concern of abnormality in the right posterior quadrant.

FIGURE 1.15. Asymmetric Alpha Rhythm. (same EEG recording as in Fig. 1-14) EEG shows a train of spikes in the right parietal region (open arrow) as well as theta and polymorphic delta slowing in the right occipital region. Persistent lateralization of theta and delta slowing is a red flag for posterior slow wave of youth and should raise the concern of focal abnormality in that area.

FIGURE 1.16. Squeak Effect. EEG of a healthy 10-year-old boy with migraine. Immediately after eye closure, the alpha frequency may be accelerated for 0.5–1 sec; therefore, alpha frequency assessment should not be done during this period. This is called the “squeak effect.”

FIGURE 1.17. Lambda Waves. Lambda waves are “sharp transients occurring over the occipital region of the head of waking subjects during visual exploration, mainly positive relative to other areas and time locked to saccadic eye movement. Amplitude varies, but is generally below 50 μV.” 6 Lambda waves do not occur before 1 year of age and are most common during the middle years of childhood. The prevalence of lambda waves between 3 and 12 years is about 80%. Lambda waves have been described as biphasic or triphasic; their predominant positive component is preceded and followed by a negative component. They may be asymmetrical on the two sides or may be present only on one side. Strictly speaking, they are bilaterally synchronous. The most important precipitating factor of lambda waves is voluntary scanning eye movements. Lambda waves usually occur as random and isolated waveforms but may recur at intervals of 200–500 msec as in this EEG page. 9 Sometimes, especially when present unilaterally, they may be mistaken for focal abnormalities, but the distinction can be made by replacing the geometric image with a blank surface.
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Lambda waves (Figures 1-17 to 1-19)
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• Sharp transients of sawtooth shape (biphasic or triphasic) occurring over the occipital region of waking subjects during visual exploration (scanning complex picture), mainly positive relative to other areas and time locked to saccadic eye movements.

• Amplitude varies, but is generally below 50 μV. Duration is 100–250 msec except in 1–3 years that can be up to 400 msec.

• Resemble positive occipital sharp transients of sleep (POSTS) and visual evoked potential. Subjects with prominent lambda waves also have prominent POSTS.

• In children, highest amplitude and sharpest component is surface negative in the occipital region.

• Random and isolated waveforms but may be recur at intervals of 200–500 msec.

• Visual evoked potentials occur in association with saccadic eye movement.

• Do not occur before 1 year of age.

• Most common during the middle years of childhood. The prevalence of lambda waves between 3 and 12 years of age is about 80%.

• Lambda waves have been described as biphasic or triphasic; their predominant positive component is preceded and followed by a negative component.

• Strictly bilateral synchronous although may be asymmetrical on the two sides. Rarely present only on one side.

• Marked asymmetry indicates an abnormality on the side of lower amplitude.

• The most important precipitating factor is voluntary scanning eye movements.

• Lambda wave is attenuated by:

• Darkening room

. • Staring at a blank card

. • Eye closure

• Lambda waves usually occur as random and isolated waveforms but may recur at intervals of 200–500 msec.

• Accompanied by eye movement and eyeblink artifacts.

• Sometimes, especially when present unilaterally, they may be mistaken for focal abnormalities, but the distinction can be made by replacing the geometric image with a blank surface.

• Marked and persistent asymmetry indicates an abnormality on the side of lower amplitude.

FIGURE 1.18. Lambda Waves and Alpha Rhythm. Lambda waves (open arrow) are “sharp transients occurring over the occipital region of the head of waking subjects during visual exploration,” mainly positive relative to other areas and time locked to saccadic eye movement. Amplitude varies, but is generally below 50 μV. 6 Lambda waves have been described as biphasic or triphasic; their predominant positive component is preceded and followed by a negative component. They are most commonly seen in children aged 2–15 years. They may be asymmetrical, appearing bilaterally, or may be present only on one side. Strictly speaking, they are bilaterally synchronous. The most important precipitating factor of lambda waves is voluntary scanning eye movements. 9 Note alpha rhythm with eye closure (double arrows).

FIGURE 1.19. Lambda Waves. Lambda waves (A) are “sharp transients occurring over the occipital region of the head of waking subjects during visual exploration,” mainly positive relative to other areas and time locked to saccadic eye movement. Amplitude varies, but is generally below 50 μV. 6 Lambda waves have been described as biphasic or triphasic; their predominant positive component is preceded and followed by a negative component. They are most commonly seen in children aged 2–15 years. They may be asymmetrical on the two sides or may be present only on one side although are strictly bilateral synchronous. The most important precipitating factor of lambda waves is voluntary scanning eye movements. 9 POSTS (B, sample from the other EEG), also known as “lambdoid waves” are usually monophasic, sharply contoured electropositive waves seen mainly during light to moderate levels of sleep.

FIGURE 1.20. Positive Occipital Sharp Transients of Sleep (POSTs). EEG of a 4-year-old asymptomatic male during drowsiness. Characteristics of POSTS include sharply-contoured, surface positive, occurring in trains with a repetitive rate of 4–5 Hz, and monophasic checkmark-like waveform seen singularly or in clusters over the occipital regions. POSTS are always bilaterally synchronous but are commonly asymmetric on the two sides and should not be misinterpreted as epileptiform activity or focal nonepileptiform activity. 10,14 POSTS occur during drowsiness and stage 2 sleep.
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Positive occipital sharp transients of Sleep (Figures 1-20 to 1-24)
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• Best seen at the age of 15–35 years and rarely ❤ years.

• Seen in 50–80% of healthy adults.

• Amplitude 20–75 μV; duration 80–200 msec.

• Absent in individuals with poor central vision.

• Sharply-contoured, surface positivity, occurring in trains with a repetitive rate of 4–5 Hz, and monophasic checkmark-like waveform seen singularly or in clusters over the occipital regions.

• Always bilaterally synchronous but are commonly asymmetric on the two sides. Asymmetry of 50% is normal.

• POSTS occur during deep drowsiness and stage 2 sleep. Rare in REM sleep.

FIGURE 1.21. Positive Occipital Sharp Transients of Sleep (POSTs). EEG of a 3-year-old asymptomatic male during stage 2 sleep. Characteristics of POSTS are sharp-contoured, surface positivity, occurring in trains with a repetitive rate of 4–5 Hz, and monophasic checkmark-like waveform seen in singly or in clusters over the occipital regions. POSTS are always bilaterally synchronous but are commonly asymmetric on the two sides and should not be misinterpreted as epileptiform activity or focal nonepileptiform activity.

POSTS occur during, drowsiness and stage 2 sleep.

FIGURE 1.22. Posterior Occipital Sharp Transients of Sleep (POSTS). POSTS (Box) can simulate epileptiform activity. Their triangular morphology, persistent lack of slow wave following sharp transients, positive polarity, constant symmetry, and occurrence during sleep differentiate them from epileptiform activity.

FIGURE 1.23. Aymmetric Posterior Occipital Sharp Transient of Sleep (POSTS). A 7-year-old boy with recurring staring episodes and behavioral issues. The routine EEG during sleep shows bilaterally synchronous but asymmetric POSTS. Characteristics of POSTS are surface positivity, occurring in trains with a repetitive rate of 4–5 Hz, and monophasic checkmark-like waveforms. POSTS are always bilaterally synchronous but are commonly asymmetrical on the two sides and should not be misinterpreted as epileptiform activity or focal nonepileptiform activity.

FIGURE 1.24. Pathologically Asymmetry of Positive Occipital Sharp Transients of Sleep (POSTS). A 7-year-old girl born 24 weeks gestational age with grade 4 intraventricular hemorrhage (IVH). Subsequently, she developed spastic quadriparesis and global developmental delay. Cranial MRI showed periventricular leukomalacia with bilateral white matter involvement, greater on the left. Prolonged 72-h-video-EEG demonstrates persistent suppression of POSTS and anterior beta activity in the left hemisphere throughout the drowsy and sleep EEG recording. Although persistent asymmetric POSTS in this case are pathologic, physiologic POSTS can be quite asymmetric and may be present on only one side in the routine EEG. Therefore, asymmetric POSTS without other associated abnormalities should not be misinterpreted as abnormal.

FIGURE 1.25. Posterior Slow-Wave Transients (Occipital Sharp Transients); Associated with Eye Movements. Posterior slow-wave transients associated with eye movements is an EEG pattern consisting of a monophasic or biphasic slow transient with a duration of 200–400 msec and a voltage of up to 200 μV in the occipital regions (*). The latency of 100–500 msec is noted after the eyeblinks or eye movements. The initial component of the transient is surface positive. The ascending phase is steeper than the descending phase. This EEG pattern is seen in children age 6 months to 10 years, but seen most commonly in children aged 2–3 years. This EEG pattern is a normal phenomenon but may be misinterpreted as epileptiform activity.
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Posterior slow-wave transients associated with eye movements (Figures 1-25 to 1-28)
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• Seen in children age 6 months to 10 years, but most commonly in children aged 2–3 years.

• Consisting of a monophasic or biphasic slow transient with a duration of 200–400 msec and a voltage of up to 200 μV in the occipital regions.

• A latency of 100–500 msec is noted after the eyeblinks or eye movements.

• The initial component of the transient is surface positive. The ascending phase is steeper than the descending phase.

FIGURE 1.26. Posterior Slow-Wave Transients (Occipital Sharp Transients); Associated with Eye Movements. Posterior slow-wave transients associated with eye movements is an EEG pattern consisting of a monophasic or biphasic slow transient with a duration of 200–400 msec and a voltage of up to 200 μV in the occipital regions (open arrow). The latency of 100–500 msec is noted after the eyeblinks or eye movements. The initial component of the transient is surface positive. The ascending phase is steeper than the descending phase. This EEG pattern is seen in children age 6 months to 10 years, but seen most commonly in children aged 2–3 years. This EEG pattern is a normal phenomenon but may be misinterpreted as epileptiform activity. 9,10

FIGURE 1.28. Posterior Slow-Wave Transients (Occipital Sharp Transients); Associated with Eye Movements. Posterior slow-wave transients associated with eye movements is an EEG pattern consisting of a monophasic or biphasic slow transient with a duration of 200–400 msec and a voltage of up to 200 μV in the occipital regions (*). The latency of 100–500 msec is noted after the eyeblinks or eye movements. The initial component of the transient is surface positive. The ascending phase is steeper than the descending phase. This EEG pattern is seen in children aged 6 months to 10 years, but seen most commonly in children aged 2–3 years. This EEG pattern is a normal phenomenon but may be misinterpreted as epileptiform activity. 9,10

FIGURE 1.29. Occipital Slow Transients; Cone Wave and Diphasic Slow Transient. In children, the transition from light to deep sleep may be associated with bilateral high-voltage slow transients in the occipital regions. These waves vary from a cone-shaped configuration (double arrows) to a biphasic slow transient (open arrow). These transients occur every 3–6 sec during light sleep and more frequently during deeper stage of sleep.
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Occipital slow transients (Figures 1-29 to 1-31)
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• Physiologic waves presenting during non-REM sleep, especially a transition from light to deep sleep in infancy until 5 years of age

• Bilateral, isolated, medium- to high-amplitude, monomorphic, triangular-shaped, delta waves with a typical duration greater than 250 msec in the occipital regions.

• These waves vary from a cone-shaped configuration (cone waves or “O” waves) to a biphasic slow transient. These transients occur every 3–6sec during light sleep and more frequently during deeper stages of sleep.

FIGURE 1.30. Occipital Slow Transients; Cone Waves. In children, the transition from light to deep sleep may be associated with bilateral high-voltage slow transients in the occipital regions. These waves vary from a cone-shaped configuration to a biphasic slow transient. These transients occur every 3–6 sec during light sleep and more frequently during deeper stages of sleep. 10 Cone waves or “O” waves (arrow) are physiologic waves presenting during non-REM sleep from infancy until 5 years of age. They are isolated, medium- to high-amplitude, monomorphic, triangular shaped, delta waves with a typical duration greater than 250 msec that occur over the occipital region.