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

Omkar N. Markand, MD, FRCPC

Disclosures

Semin Neurol. 2003;23(1) 

In This Article

EEG in Neonates

In recent years there has been much interest in using EEG to evaluate full-term or premature neonates[111,112] due to the serious limitations in performing an adequate neurologic examination. The neonate may be confined to an isolette, may be intubated, or may be paralyzed for ventilatory control. Under such circumstances, EEG is a very important tool to assess an encephalopathic process or occurrence of epileptic seizures. In addition, the background abnormalities have been classified in neonates and used to predict neurologic outcome.

The EEG of a neonate shows distinctive patterns related to the conceptional age (CA) and the behavioral state (awake, active sleep, quiet sleep). Space does not permit a description of EEG patterns associated with different conceptional ages, but this is summarized in Table 2 . It is important to emphasize that the EEG maturation runs parallel in utero and in incubator; only minimal or no differences have been found between babies of the same CA born after different periods in utero.

In a full-term neonate four EEG patterns are observed related to the wakefulness/sleep cycle: (1) low-voltage irregular (LVI) is characterized by the presence of continuous low-amplitude (<50 µV), mainly widespread theta activity; (2) mixed pattern is characterized by continuous moderate amplitude (usually <100 µV) theta and delta activities; (3) high-voltage slow (HVS) consists of continuous high-amplitude semirhythmic, mostly delta activity (0.5 to 3.0 cps) in all regions with an amplitude of 50 to 150 µV; and (4) tracé alternant (TA) pattern is characterized by the occurrence of 3 to 5 second bursts of high amplitude (50 to 100 µV) slow activity (0.5 to 3.0 Hz), which occur at intervals of 3 to 10 seconds when the background is relatively low amplitude (10 to 25 µV) consisting of theta waves. In other words, there is an alteration of bursts of large-amplitude slow activity separated by quiescent or "flat" periods of low-voltage activity.

These four EEG patterns are recorded in different states. The LVI EEG is usually recorded in wakefulness and in active sleep. The mixed pattern can also be recorded in active sleep and relaxed wakefulness. The TA and HVS patterns are characteristic of quiet sleep. A unique characteristic of neonate sleep is that as the neonate falls asleep, he usually enters a period of active (REM) sleep. This differs from older infants, children, and adults who never begin their sleep with a period of active (REM) sleep. It is only by 10 to 12 weeks postterm that sleep onset changes from active to quiet (NREM) sleep.

A couple of perils and pitfalls need emphasized in interpreting the neonatal EEG. The TA pattern of quiet sleep in a normal full-term neonate and tracé discontinua pattern in a normal premature neonate during quiet sleep have a superficial resemblance to the suppression-burst pattern that carries a poor prognosis. Differentiation between normal and abnormal discontinuous patterns becomes an important challenge in interpreting the neonatal EEG. The suppression-burst pattern is invariant and nonreactive to stimulation. It usually signifies a severe encephalopathy (usually ischemic/hypoxic), although it may occur transiently due to recent intravenous sedative/hypnotic medication. On the other hand, the TA and tracé discontinua pattern associated with quiet sleep in full-term and premature neonates, respectively, are state dependent and react to stimulation. In addition, long recordings would demonstrate activities characteristic of wakefulness and active sleep in normal full-term and premature (over 32 weeks CA) neonates. The TA pattern gradually disappears over 6 weeks postterm when the HVS pattern becomes the sole EEG accompaniment of quiet sleep.[113]

Another striking feature of the neonatal EEG is the frequent occurrence of multifocal sharp transients during indeterminate and quiet sleep. These start appearing at 35 weeks CA and constitute a normal finding in full-term neonates. The clinical significance of these sharp transients remains controversial. It is difficult to clearly separate abnormal (pathologic) sharp waves from normal sharp transients of the neonate. In general, the normal sharp transients are infrequent in occurrence, usually blunt in morphology (rather than assuming spiky configuration), arise from any scalp location but are common over frontal and temporal regions, are truly random in occurrence, without persistent focality, and largely confined to the burst phase of the TA pattern. However, no universal criteria have been established to separate normal sharp transients of the newborn from the abnormal sharp wave activity. Unless the sharp wave discharges are repetitive, periodic, or localized over one region, an epileptogenic significance must not be assigned to them. When the multifocal sharp transients are very frequent and occur even during active sleep and wakefulness, the EEG is considered to be abnormal but suggestive merely of a nonspecific encephalopathic process.

A unique EEG pattern of pathologic significance is the presence of positive Rolandic sharp waves (PRS).[114] PRS may be confined to one hemisphere; if they are bilateral they may be consistently more abundant over one hemisphere. They are usually maximum at CZ but may be lateralized to C3 or C4 electrodes (Fig. 32). Their positive polarity is a distinctive feature in addition to their localization. Although initially described as the EEG correlates of the intraventricular hemorrhage, PRS waves may be seen in other conditions, including periventricular leukomalacia, parenchymal hemorrhage, hydrocephalus, hypoxic/ischemic insult, and other conditions.[112,115,116,117] Presently, the view is that PRS waves represent a marker of different white matter lesions rather than being specific for periventricular/intraventricular hemorrhage. Positive polarity sharp waves at other locations (especially in the temporal region) have no distinctive significance and may just be a part of multifocal sharp transients in the neonates.

Figure 32.

EEG of a neonate of 35 weeks conceptional age, showing low-amplitude positive Rolandic sharp wave discharges at the vertex (CZ) electrode. Patient had grade IV intraventricular hemorrhage and was unresponsive.

Abnormalities of the background activity in full-term neonates are usually classified as either severe or mild[118,119,120] and are summarized in Table 3 . The severe EEG abnormalities indicate severe impairment of brain function and carry a poor prognosis for survival and/or neurologic development. Severely abnormal EEG patterns consist of: (1) isoelectric EEG showing activity consistently below 5 µV; (2) persistent low-voltage tracing, EEG showing activity between 5 and 15 µV with very little variability or sleep/wake differentiation; (3) paroxysmal tracing or suppression-burst pattern (discontinuous, invariant, and nonreactive pattern characterized by 1 to 10 second paroxysms of polymorphic activities such as sharp waves, spikes, and theta-delta activities interspersed with long quiescent periods as long as > 20 seconds); (4) invariant high-amplitude delta activity (persistent and nonchanging high-amplitude 0.5 to 3.0 Hz generalized activity); and (5) the presence of gross asynchrony and asymmetry of the EEG activity over the two sides of the head. Studies have established that the presence of these EEG abnormalities in a full-term neonate, particularly if the abnormalities have been persistent in serial EEGs, carry a very poor prognosis for survival or future neurologic development; over 90% of neonates with such severe abnormalities have an unfavorable outcome.[118,119]

Mild abnormalities of the background activities include more than the usual asynchrony and/or asymmetry; EEG being immature for the conceptional age; lack of recognizable sleep states; excessive discontinuity ("flat" periods longer than 30 seconds); abnormal monorhythmic activities; and excessive multifocal sharp transients. The presence of more than one mild abnormality may suggest an underlying encephalopathic process of varying severity, particularly if the EEG shows persistent abnormality on serial studies. Several of the above mild abnormalities do occur in neonates who are heavily sedated; hence, iatrogenic causes need to be excluded before ascribing them to permanent neurologic insult.

Neonatal Seizures

One of the major reasons an EEG is performed is if a neonate is suspected of having epileptic seizures. In neonates, epileptic seizures are often characterized clinically by subtle motor behavior such as elevation of a limb, eye deviation, eyelid flutter, tonic posturing, bicycling movements of the legs, apnea, and so on. The EEG is indispensable in establishing the epileptic nature of the motor activity by demonstrating an associated ictal pattern. There are many unique features of neonatal seizures that are different from the seizures encountered in older children and adults. The International Classification of Epileptic Seizures is obviously inappropriate for neonates. The immature brain at this age is unable to initiate and sustain generalized epileptic discharges as in older children; hence, typical tonic-clonic seizures do not occur. Many of the neonatal seizures are subtle seizures as described above. At least some of these do not show a close relationship to an EEG change. The significance of such stereotypic motor events with no concomitant EEG changes becomes a controversial issue regarding diagnosis and management. Whether these events represent "epileptic" dysfunction (not "picked up" by scalp electrodes) or whether these stereotypic behaviors signify episodes of brain stem release phenomena has yet to be resolved.

The opposite situation is also common. An electrographic ictal pattern may occur without an obvious clinical change. Such "subclinical" seizures are common with a pharmacologic neuromuscular blockade, stupor and coma following severe hypoxic/ischemic encephalopathy, multifocal status epilepticus, and following apparently successful treatment of status epilepticus using antiepileptic drugs.

The EEG ictal pattern is highly variable but consists of rhythmic activity of some sort, which is well localized to a relatively small area of the brain. With rare exception, the ictal pattern in neonates is focal, unifocal, or multifocal. When multifocal, the ictal pattern may occur over two different regions at the same time but the discharges have different waveforms and different repetition rates. Multifocal seizures simultaneously occurring on the two sides is a unique feature of neonatal epileptogenesis.

Interictal epileptiform abnormalities are rarely present to aid in the diagnosis of neonatal seizures. Multifocal sharp transients over the frontal and temporal regions are common even in healthy neonates and, as mentioned above, do not correlate with present or future occurrence of epileptic seizures. It appears that in neonates the epileptic process exhibits "all or none" features: either a seizure manifests overtly with appropriate electrographic features or has no interictal epileptiform markers.

Ictal EEG patterns associated with neonatal seizures are of four basic types[111]:

  1. Focal spikes or spike wave discharges superimposed on a more or less normal background is an ictal pattern most commonly located over the Rolandic region (C3 or C4), and the frequency of the ictal discharge is usually over two per second (Fig. 33). In neonates, focal EEG discharges and clinical seizures do not necessarily imply focal brain lesions. Common etiologies include metabolic disturbances, such as hypocalcemia or hypoglycemia, and subarachnoid hemorrhage. Such an ictal pattern, when associated with normal a EEG background, is prognostically favorable.

  2. Another focal ictal pattern consists of slow-frequency sharp waves or complex waveforms repeating approximately one per second, never recruiting at a faster rate (Fig. 34). This pattern is similar to PLEDs. The background activity is almost always low in amplitude. This pattern, called "depressed" brain discharges,[121] is associated with a severe cerebral insult (e.g., hypoxic/ischemic encephalopathy, encephalitis, cerebrovascular accident, etc.). The accompanying clinical seizures are usually subtle or the EEG discharges are entirely subclinical.

  3. Focal monorhythmic pattern in the beta, theta, and delta frequency is a unique ictal pattern in neonates. It may start with low-amplitude focal 8 to 14 Hz activity that slows down to 4 to 7 Hz and then to 0.5 to 3.0 Hz rhythmic pattern. All types of combinations of the frequency band are possible but some ictal discharges may remain essentially monorhythmic ("alpha band" seizures) during a given seizure (Fig. 35). The background is always abnormal and usually low in amplitude. This type of pattern has been referred to as pseudo-beta-alpha-theta-delta ictal pattern[111] and is usually associated with subtle seizures, tonic or myoclonic seizures, or no behavioral clinical change. This ictal pattern is associated with severe CNS dysfunction and correlates with a poor outcome.

  4. Multifocal ictal pattern is characterized by an abnormal background activity and the development of an ictal pattern independently or, rarely, simultaneously over two or more areas of one or both hemispheres. Two or more focal seizures may appear concomitantly in the same or, more commonly, the opposite hemisphere and appear to progress independently. This ictal EEG pattern is usually associated with subtle seizures; the underlying pathology consists of severe encephalopathy due to infection, congenital anomalies, birth trauma, or anoxia. This pattern carries a poor prognosis for normal neurologic development.

Technical Aspects

Several technical points are of crucial importance to optimize neonatal EEG recording. The study should be long enough to include active and quiet sleep; the total duration of the recording may exceed the usual 30 minutes recommended in adults. In most neonates it may be necessary to record the EEG for 45 to 60 minutes. The presence of sleep differentiation is an important maturational feature for EEG interpretation. Some abnormal patterns such as the degree of discontinuity, asynchrony and asymmetry, presence of multifocal sharp transients, or delta brushes can be evaluated only in quiet sleep. Additionally, polygraphic variables must be routinely recorded in addition to several channels of scalp EEG. These include respiration, extraocular movements, EKG, and chin activity. These non-EEG variables are critical in identifying different states (awake, active sleep, or quiet sleep) and recognition of various artifacts. A neonatal EEG lacking such polygraphic variables is difficult to interpret unless it is grossly abnormal.

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