Tracé Discontinu. EEG of a 27-week CA infant with severe HIE. Tracé discontinu (TD) is the earliest EEG activity that appears in viable neonate at 22–23 weeks of gestational age (GA). The waves are described as short bursts, consisting of slow and fast rhythm, interspersed against a flat or quiescent background of less than 25 μV.¹ At a very early age of 22–24 weeks GA may vary from 5 sec to 8 min.² As age increases, the periods of inactivity shorten. The longest acceptable single interburst interval (IBI) duration has been reported to be 26 weeks CA, 46 sec; 27 weeks CA, 36 sec; 28 weeks CA, 27 sec³; less than 30 weeks CA, 35 sec; 31–33 weeks CA, 20 sec; 34–36 weeks CA, 10 sec; and 37–40 weeks CA, 6 sec.^4–6

The TD is also the first EEG pattern occurring during quiet sleep that differentiates wakefulness from sleep in the premature infant at around 30–32 weeks CA.

Infants less than 30 weeks CA show interhemispheric hypersynchrony whereby the majority of bursts arising within the two hemispheres appear at the same time.⁵

Interhemispheric Hypersynchrony. EEG of an infant 28 weeks CA with apnea. The tracing is discontinuous. Infants at less than 30 weeks CA have “paradoxical hypersynchrony” whereby bursts of cerebral electrical activities are well synchronized and appear simultaneously between the two hemispheres. The pathophysiology of this phenomenon is unknown. After 30 weeks CA, bursts of cerebral activities appear to be more asynchronous. The bursts of cerebral activities during quiet sleep are synchronized in approximately 70% at 31–32 weeks CA, 80% at 33–34 weeks CA, and 100% after 37 weeks CA.

In infants less than 30 weeks CA, the average interburst interval (IBI) or quiescence is about 6–12 sec. The longest acceptable IBI is 30–35 sec.

Tracé Discontinu. EEG of a 28-week CA infant with severe HIE. Tracé discontinu (TD) is the earliest EEG activity that appears in the viable neaonate at 22–23 weeks of gestational age (GA). It is described as short bursts, consisting of slow and fast rhythm, interspersed against a flat or quiescent background of less than 25 μV.¹ At a very early age of 22–24 weeks GA, it may vary from 5 sec to 8 min.² As age increases, the periods of inactivity shorten. The longest acceptable single interburst interval (IBI) duration has been reported to be 26 weeks CA, 46 sec; 27 weeks CA, 36 sec; 28 weeks, 27 sec³; less than 30 weeks CA, 35 sec; 31–33 weeks CA, 20 sec; 34–36 weeks CA, 10 sec; and 37–40 weeks CA, 6 sec.^4–6

The TD is also the first EEG pattern occurring during quiet sleep that differentiates wakefulness from sleep in the premature infant at around 30–32 weeks CA.

Infants less than 30 weeks CA show interhemispheric hypersynchrony in which the majority of bursts arising within the two hemispheres appear at the same time.⁵ Note frequent low-voltage sharp waves at Cz.

Delta Brushes (Central Predominance). EEG of a 28-week CA infant with apnea.

Delta brushes (DBs) (“brushes,” “spindles-delta bursts,” “ripples of prematurity,” spindles-like fast waves,” and “beta-delta complexes”) first appear at 24–26 weeks CA and consist of a combination of a specific delta frequency transient (0.3–1.5 Hz, 50–250 mV) waves with a superimposed “buzz” of 8- to 22-Hz activity. DBs are symmetrical but asynchronous, except when they are associated with monorhythmic occipital delta activity. Before 33 weeks CA, they occur more frequently during active than quiet sleep. After that, they are more numerous in quiet sleep. In the youngest prematures, DBs are most commonly seen in the central and midline areas. At the peak of 32–34 weeks CA, they are seen in occipital and temporal regions. By term, DBs are only seen in the bursts of trace alternant. DBs disappear completely at 44 weeks CA.^4,5 If DBs are found at a frequency of greater than 2 every 10 sec at term, they should be considered abnormal.¹² DBs are prominent during REM and NREM sleep between 26–33 weeks CA and 33–37 weeks CA.⁵

Monorhythmic Occipital Delta Activity and Delta Brushes; Extremely Low-Voltage Background Activity. Monorhythmic occipital delta activity in an infant 31 weeks of conceptional age (CA) with severe hypoxic ischemic encephalopathy. This activity is described as a run of monomorphic, high amplitude (50–250 μV), surface-positive polarity, 0.3- to 1.5-Hz delta waves, occurring symmetrically and synchronously in the bi-occipital regions. The run can last from 2 to 60 sec, rarely longer than 5–6 sec in duration in an infant less than 28 weeks CA and commonly greater than 30 sec at 28–31 weeks CA. It is present at 23–24 weeks CA, peaks between 31 and 33 weeks, and then significantly fades by 35 weeks. It serves as the constituent of, and can intermix with, occipital delta brushes (arrow). Monorhythmic occipital delta activity is a principal landmark regional electrographic feature of the preterm infant. It is a strong rhythm and may persist in the presence of severe acute encephalopathy.^4,8

Also note EEG signs, bitemporal attenuation, and interhemispheric hypersynchrony in an infant less than 28 weeks CA. Bitemporal attenuation probably reflects interhemispheric asynchrony and underdevelopment of the inferior frontal and superior temporal gyri.⁹ The physiology of interhemispheric hypersynchrony is unknown.⁴

Delta Brushes (Occipital Predominance); Monorhythmic Occipital Delta Activity. EEG of a 31-week CA infant with HIE showed bilateral synchronous monorhythmic occipital delta activity intermixed with delta brushes (DBs) in the occipital regions (arrow).

DBs (“brushes,” “spindles-delta bursts,” “ripples of prematurity,” “spindles-like fast waves,” and “beta-delta complexes”) first appear at 24–26 weeks CA and consist of a combination of a specific delta frequency transient (0.3–1.5 Hz, 50–250 mV) waves with a superimposed “buzz” of 8- to 22-Hz activity. DBs are symmetric but asynchronous, except when they are associated with monorhythmic occipital delta activity. Before 33 weeks CA, they occur more frequently during active than quiet sleep. After that, they are more numerous in quiet sleep.

In the youngest prematures, DBs are most commonly seen in the central and midline areas. At the peak of 32–34 weeks CA, they are seen in occipital and temporal regions. By term, DBs are only seen in the bursts of trace alternant. DBs disappear completely at 44 weeks CA.^4,5 If DBs are found more than 2 every 10 sec at term, they should be considered abnormal.¹² DBs are prominent during REM and NREM sleep between 26–33 weeks CA and 33–37 weeks CA.⁵

Markedly Excessive Discontinuity; Gross Interhemispheric Asynchrony; Extremely Low Voltage. (Same EEG recording as in Figure 3-5) A 31-week CA with severe HIE. EEG during comatose state shows extremely low-voltage background activity, excessive interburst interval of more than 2 min (not shown), and marked interhemispheric asynchrony. Also note monorhythmic occipital delta activity (open arrow), delta brushes in the occipital, temporal, and central regions (arrow), and STOP (double arrows). The patient died 1 day after this EEG.

Extremely low-voltage (<5 μV) background, markedly excessive discontinuity for age, burst suppression pattern, gross hemispheric asynchrony, depressed and undifferentiated, and isoelectric background activity are indicative of death or poor outcome.^4,8,10–13

Sharp Theta on the Occipitals of Prematures (STOP). STOP (open arrow) refers to 5–6 Hz sharp-contoured theta activity, either unilateral or bilateral and maximal in the occipital regions. The STOP pattern is similar to temporal theta bursts in configuration, but is faster in frequency and lower in amplitude. The incidence was highest at the youngest age, 22–25 weeks CA, decreasing to zero near term. In the youngest age these rhythms are mainly bilateral; in the older neonates they are unilateral. STOP is seen more often in active sleep, but is reduced in incidence in patients with abnormal slow waves, ictal or immature patterns. Right-sided STOP was more frequently associated with abnormal EEGs, especially in males with right-sided sharp waves, often noted in the patients with seizures and intraventricular hemorrhages.^2,14

Note excessive delta brushes, maximally expressed in central and temporal regions.

Sharp Theta Rhythm on the Occipital Areas of Prematures (STOP). STOP refers to 5–6 Hz sharp-contoured theta activity, either unilateral or bilateral and maximal in the occipital regions. The STOP pattern is similar to temporal theta bursts in configuration, but is faster in frequency and lower in amplitude. The incidence was highest at the youngest age, 22–25 weeks CA, decreasing to zero near term. In the youngest age these rhythms are mainly bilateral; in the older neonates they are unilateral. STOP is seen more often in active sleep, but is reduced in incidence in patients with abnormal slow waves, ictal or immature patterns. Right-sided STOP was more frequently associated with abnormal EEGs, especially in males with right-sided sharp waves, often noted in the patients with seizures and intraventricular hemorrhages.^2,14

Sharp Theta on the Occipitals of Prematures (STOP). A 39 weeks CA boy with severe hypoxic ischemic encephalopathy with shock, DIC, cardiorespiratory failure, and seizures. He recovered after the treatment. EEG shows frequent bursts of unilateral or bilateral dependent 5–6 Hz sharp-contoured theta activity in the occipital regions (arrow) superimposed on very low-voltage background activity.

STOP refers to 5–6 Hz sharp-contoured theta activity, either unilateral or bilateral and maximal in the occipital regions. The STOP pattern is similar to temporal theta bursts in configuration, but is faster in frequency and lower in amplitude. The incidence was highest at the youngest age, 22–25 weeks CA, decreasing to zero near term. In the youngest age these rhythms are mainly bilateral; in the older neonates they are unilateral. STOP is seen more often in active sleep, but is reduced in incidence in patients with abnormal slow waves, ictal or immature patterns. Right-sided STOP was more frequently associated with abnormal EEGs, especially in males with right-sided sharp waves, often noted in patients with seizures and intraventricular hemorrhages.^2,14

STOP and Temporal Theta Bursts Patterns. (Same recording as in Figure 3-10) The EEG shows both temporal theta bursts (double arrows) and sharp theta on the occipitals of prematures (STOP) patterns (open arrow).

Temporal theta bursts are rhythmic 4- to 6-Hz, sharply contoured theta waves in short bursts of 1–2 sec with voltage varying from 20 to 200 μV, arising independently in the left and right midtemporal regions. This activity appears at the age of 26 weeks CA, is expressed maximally around 29 weeks,¹⁵ 29–31 weeks,² or between 30 and 32 weeks CA,⁵ then rapidly disappears, and is replaced by temporal alpha bursts at 33 weeks CA.^5,16

STOP refers to 5 to 6 Hertz sharp-contoured theta activity, either unilateral or bilateral and maximal in the occipital regions. The STOP pattern is similar to temporal theta bursts in configuration, but is faster in frequency and lower in amplitude. The incidence was highest at the youngest age, 22–25 weeks CA, decreasing to zero near term.^2,14

Extremely Low Voltage Background; Monorhythmic Occipital Delta Activity and Delta Brushes. EEG of a 31-week CA newborn with severe HIE. DWI MI shows bilateral basal ganglia hyperintensity (open arrow). EEG shows monorhythmic occipital delta activity and delta brushes superimposed on extremely low-voltage background activity with amplitude less than 5 μV. The patient died 1 day after this EEG.

Monorhythmic occipital delta activity is a principal landmark regional electrographic feature of the preterm infant. It is a strong rhythm and may persist in the presence of severely acute encephalopathy. Extremely low-voltage (<5 μV) background, in the absence of drug intoxication, acute hypoxemia, hypothermia, severe metabolic disturbances, and postictal state, along with markedly excessive discontinuity for age, burst suppression pattern, gross hemispheric asynchrony, depressed and undifferentiated, and isoelectric background activity are indicative of death or poor outcome.^4,8,10–13

Monorhythmic Occipital Delta Activity and Delta Brushes. Monorhythmic occipital delta activity in an infant 31 weeks of conceptional age (CA) with moderate hypoxic ischemic encephalopathy. This activity is described as a run of monomorphic, high-amplitude (50–250 μV), surface-positive polarity, 0.3- to 1.5-Hz delta waves, occurring symmetrically and synchronously in the bi-occipital regions. The run can last from 2 to 60 sec, rarely longer than 5 or 6 sec in an infant less than 28 weeks CA and commonly greater than 30 sec at 28–31 weeks CA. It is present at 23–24 weeks CA, peaks between 31 and 33 weeks, and then significantly fades by 35 weeks. It serves as the constituent of, and can intermix with, occipital delta brushes (arrow). Monorhythmic occipital delta activity is a principal landmark regional electrographic feature of the preterm infant. It is a strong rhythm and may persist in the presence of severely acute encephalopathy.^4,8

Also note EEG signs, bitemporal attenuation, and interhemispheric hypersynchrony in infants less than 28 weeks CA. Bitemporal attenuation probably reflects interhemispheric asynchrony and underdevelopment of the inferior frontal and superior temporal gyri.⁹ The physiology of interhemispheric hypersynchrony is unknown.⁴

Temporal Theta Bursts (Temporal Sawtooth Waves, Temporal Sharp Transients). Temporal theta bursts are rhythmic, 4- to 6-Hz, sharply contoured theta waves in short bursts of 1–2 sec with voltage varying from 20 to 200 μV, arising independently in the left and right midtemporal regions. This activity appears at the age of 26 weeks CA, is expressed maximally around 29 weeks,¹⁵ 29–31 weeks² or between 30 and 32 weeks CA,⁵ then rapidly disappears, and is replaced by temporal alpha bursts at 33 weeks CA.^5,16

Temporal Theta Bursts. Temporal theta bursts are rhythmic 4- to 6-Hz, sharply contoured theta waves in short bursts of 1–2 sec with voltage varying from 20 to 200 μV, arising independently in the left and right midtemporal regions. This activity appears at the age of 26 weeks CA, is expressed maximally around 29 weeks,¹⁵ 29–31 weeks,² or between 30 and 32 weeks CA,⁵ then rapidly disappears, and is replaced by temporal alpha bursts at 33 weeks CA.^5,16

Temporal Theta Burst. Temporal theta bursts (arrow) are rhythmic 4- to 6-Hz, sharply contoured theta waves in short bursts of 1–2 sec with voltage varying from 20 to 200 μV, arising independently in the left and right midtemporal regions. This activity appears at the age of 26 weeks CA, is expressed maximally around 29 weeks,¹⁵ 29–31 weeks,² or between 30 and 32 weeks CA,⁵ then rapidly disappears, and is replaced by temporal alpha bursts at 33 weeks CA.^5,16 Also note delta brushes (arrow head) in the left frontal and temporal regions.

Temporal Theta Bursts (Temporal Sawtooth Waves). Temporal theta bursts are rhythmic 4- to 6-Hz, sharply contoured theta waves in short bursts of 1–2 sec with voltage varying from 20 to 200 μV, arising independently in the left and right midtemporal regions. This activity appears at the age of 26 weeks CA, is expressed maximally around 29 weeks,¹⁵ 29–31 weeks,² or between 30 and 32 weeks CA,⁵ then rapidly disappears, and is replaced by temporal alpha bursts at 33 weeks CA.^5,16

Temporal Theta Bursts. Temporal theta bursts are rhythmic 4- to 6-Hz, sharply contoured theta waves in short bursts of 1–2 sec with voltage varying from 20 to 200 μV, arising independently in the left and right midtemporal regions. This activity appears at the age of 26 weeks CA, is expressed maximally around 29 weeks,¹⁵ 29–31 weeks,² or between 30 and 32 weeks CA,⁵ then rapidly disappears, and is replaced by temporal alpha bursts at 33 weeks CA.^5,16

Temporal Alpha Bursts; Normal Newborn: 33 Weeks Conceptional Age (CA). Temporal alpha bursts have similar character of amplitude, burst duration, and spatial distribution as temporal theta bursts (as described in Fig. 3-14 to 3-18). The presence of temporal alpha bursts is a very specific developmental marker for 33 weeks CA, because they appear at 33 weeks CA and disappear by 34 weeks CA.⁵

Trace Alternant. Background activity in NREM sleep is discontinuous before 36 weeks CA. Between 36 and 38 weeks CA, two NREM EEG patterns present (1) continuously diffuse high-voltage, slow-wave activity and (2) trace alternant—this consists of bursts of activity (delta activity admixed with faster frequencies) with an amplitude of 50–300 μV, interspersed between periods of decreased amplitude (resembles the EEG during wakefulness and active sleep and consists of mixed frequencies with amplitudes of 25–50- μV). Trace alternant begins to wane by 38–40 weeks CA and disappears by 44–46 weeks CA when it is replaced by continuous slow-wave activity. After that, sleep spindles present at around 46 weeks CA.

The distinction between trace alternant and tracé discontinu is the amplitude of IBI (interburst interval). In tracé discontinu, it is less than 25 μV, whereas in trace alternant, it is more than 25 μV.^2,4,5

Trace Alternant. Background activity in NREM sleep is discontinuous before 36 weeks CA. Between 36 and 38 weeks CA, two NREM EEG patterns present (1) continuously diffuse high-voltage, slow-wave activity and (2) trace alternant—this consists of bursts of activity (delta activity admixed with faster frequencies) with an amplitude of 50–300 μV, interspersed between periods of decreased amplitude (resembles the EEG during wakefulness and active sleep and consists of mixed frequencies with amplitudes of 25–50- μV). Trace alternant begins to wane by 38–40 weeks CA and disappears by 44–46 weeks CA when it is replaced by continuous slow-wave activity. After that, sleep spindles present at around 46 weeks CA.

The distinction between trace alternant and tracé discontinu is the amplitude of IBI (interburst interval). In tracé discontinu, it is less than 25 μV, whereas in trace alternant, it is more than 25 μV.^2,4,5

Frontal Sharp Transients (encouche frontales) (FSTs); Temporal Sharp Waves. A 39-week CA infant with apnea. EEG shows frontal sharp transients (open arrow) and left anterior temporal sharp waves. The rest of EEG recording is within normal limits.

FSTs are physiological findings described as isolated high-amplitude, broad, biphasic transients with blunt configuration, seen maximally in frontal–prefrontal regions. They usually either are isolated or occur in brief runs, alone or in combination with anterior slow dysrhythmia, symmetrically, bilaterally, and synchronously. They occur in transition from active to early quiet sleep. Although the premature form, which is polyphasic and of very high amplitude, may appear very early at 26–31 wks CA, the typical biphasic FSTs are maximally expressed at 35–36 wks, are diminished in number and voltage after 44 wks CA, are rarely seen during sleep after 46 wks CA, and are absent at 48 wks CA.¹⁷

Minor criteria for pathologic FSTs include (1) excessive amount, (2) excessive duration duration, (3) high amplitude, (4) occurrence in active sleep/wakefulness, (5) atypical morphology, (6) marked asymmetry, and (7) FSTs beyond 48 weeks.^{4,5,7,8,16,18}

Although there are no rigid criteria to distinguish abnormal from normal focal sharp waves in neonates, some findings, especially in combination, may be helpful to identify abnormal sharp waves: (1) complex morphology, especially polyphasic, followed by extremely high-voltage slow wave, (2) spikes rather than sharp waves, (3) positive polarity, (4) repetitive runs especially burnt-out sharp waves, (5) persistent localization or lateralization (even as early as 30 weeks CA), (6) occurrence during wakefulness (especially in term infants) term infants, (7) moderately or severely abnormal background activity, (8) frequency of more than 1 per 1 min, (8) any sharp waves or spikes after 2 months CA, (9) high amplitude (>150 μV) and long duration (>150 msec), and (10) multifocal sharp waves.^2,4,5,19

Frontal Sharp Transients (encouche frontales). Frontal sharp transients (encoches frontales) are physiologic findings described as isolated high amplitude (50 to >150 μV), broad, biphasic transients (either negative–positive or positive–negative) with blunt configuration, seen maximally in frontal–prefrontal regions (Fp3–Fp4). They usually are isolated or occur in brief runs, alone or in combination with anterior slow dysrhythmia, symmetrically, bilaterally, and synchronously. They occur in transition from active to early quiet sleep. Although the premature form, which are polyphasic and very high amplitude may appear very early at 26–31 weeks CA, the typical biphasic FST are maximally expressed at 35–36 weeks, are diminished in number and voltage after 44 weeks CA, are rarely seen during sleep after 46 weeks CA, and are absent at 48 weeks CA.¹⁷

Minor criteria for pathologic FSTs include (1) excessive amount, (2) excessive duration, (3) high amplitude, (4) occurrence in active sleep/wakefulness, (5) atypical morphology, (6) marked asymmetry, and (7) FSTs beyond 48 weeks.^{4,5,7,8,16,18}

Frontal Sharp Transients (encouche frontales); Anterior Slow Dysrhythmia. Frontal sharp transients (FST) or encoche frontales (arrow) are physiologic sharp waves occurring between 34 and 44 weeks CA. They are high-voltage, broad-based, biphasic, or, less commonly, triphasic sharp waves maximally seen in the prefrontal regions. FST occur symmetrically and synchronously on both sides during sleep, particularly during transition from active to quiet sleep. An immature form of FST described as very high-voltage polymorphic sharp waves can be seen at earlier ages between 27 and 31 weeks CA. The frequency of FST sharply declines between 43 and 45 weeks CA and disappears at 48 weeks CA. They may occur alone or in combination with “anterior slow dysrhythmia” (arrow head).

Premature Frontal Sharp Transients (FST) in 31 weeks CA Infant. Frontal sharp transients (encoches frontales) are physiologic findings described as isolated high amplitude (50 to >150 μV), broad, biphasic transients (either negative–positive or positive–negative) with blunt configuration, seen maximally in frontal–prefrontal regions (Fp3–Fp4). They usually are isolated or occur in brief runs, alone or in combination with anterior slow dysrhythmia, symmetrically, bilaterally, and synchronously. They occur in transition from active to early quiet sleep. Although the premature form which is polyphasic and very high amplitude may appear very early at 26–31 weeks CA, the typical biphasic FST are maximally expressed at 35–36 weeks, are diminished in number and voltage after 44 weeks CA, are rarely seen during sleep after 46 weeks CA, and are absent at 48 weeks CA.¹⁷

Eye Movement Artifact. A 38-week CA infant with apnea. EEG shows biphasic sharp theta activity in bilateral prefrontal regions that simulate frontal sharp transients. Eye lead channel can differentiate between these two conditions. When activity points in the different directions as in this EEG, it indicates that this is eye movement artifact rather than brain wave.