Abnormal neonatal EEG patterns fall into at least one of the following types: • Abnormality of maturation
• Epileptiform activity • Background abnormality
Lombroso developed a numeric classification of abnormalities, but this is not in widespread use. The classification scheme is presented on the CD.
Abnormalitites of maturation
Dysmature means that the EEG pattern is not appropriate for the conceptional age. For example, a discontinuous pattern with an interburst interval of 1 minute is normal in a preemie o 29 weeks conceptional age. This same pattern would be very abnormal in a term patient and would be indicative of encephalopathy. Persistent dysmaturity is associated with poor neurologic outcome. Transient dysmaturity may be due to non- neurologic causes and is not necessarily associated with brain damage.
Visual analysis of neonatal EEG allows for detection of only great discrepancies, but this is usually sufficient for routine interpretation. Quantitative analysis is possible but seldom needed and not in routine use.
Abnormalities of state are difficult to diagnose in routine EEG. An invariant pattern is abnormal, but state change may not necessarily be captured during a routine 20-minute EEG.
Background abnormalities Background abnormalities include: • Excessive slow activity
• Low voltage background • Burst-suppression pattern • Asymmetric patterns
Excessive slow activity is difficult to discern, since neonates have prominent delta activity already. Some infants with brain damage may have widespread delta, however. The slow background is present in wake and sleep states and reacts poorly to exogenous stimuli. This pattern is differentiated from normal delta activity by its widespread distribution and lack of reactivity. Normal delta is prominent anteriorally and attenuated by exogenous stimuli.
Amplitude asymmetries are significant only if they approach 50% of more. The
asymmetry usually indicates focal cerebral damage in the region of suppressed voltage. A common pitfall is misinterpretation of asymmetries due to extracranial hematomas or fluid collections. Subdural hematomas may suppress activity from one or both sides. The isoelectric EEG is a confirmatory test for brain death. Guidelines for determination of brain death are presented in Chapter 7.
The low-voltage record is unusual in neonates and suggests abnormalities in generation of electrical activity in the cortex. The technician needs to ensure that non-REM sleep is recorded, since normal REM sleep has a low-voltage background. Bilateral subdural hematomas may also produce bilateral attenuation of the background.
Epileptiform activity
Epileptiform activity in the neonate can look very different from epileptiform activity in older children and adults. The epileptiform activity may be focal, multifocal, or
generalized. Immaturity of cerebral maturation usually does not allow for generalization of epileptiform activity.
Focal discharges occur usually in central region, more often on the right than the left. The discharges may occur singly or in trains at 5-10/sec. Focal epileptiform activity is
differentiated from normal frontal sharp transients and multifocal sharp transients by consistent lateralization. Also, normal sharp transients never occur in trains. The focal discharges occasionally have a smooth contour and could be confused with an alpha or theta rhythm. Sustained rhythmic activity is never normal in neonates of any conceptional age, however. The rhythm must be differentiated from the fast component of delta
brushes by the absence of an underlying slow wave and the longer duration of the epileptiform discharge than the fast component of a delta brush.
Focal discharges are usually associated with focal clonic seizures. The location of the focus may not necessarily correlate well with the clinical seizure activity. The prognosis for favorable neurologic outcome is good, since focal discharges in neonates do not necessarily indicate a focal structural lesion.
Most focal sharp waves are surface negative. Surface-positive waves are seen in some neonates with intracerebral hemorrhage. If the sharp wave is followed by a slow wave, the hemorrhage is most likely subarachnoid. If the sharp wave is not followed by a slow component, the hemorrhage may still be subarachnoid, but is more likely to be
intraventricular, subependymal, or intraparenchymal. The specificity of positive sharp waves for hemorrhage is controversial, however.
Multifocal discharges are usually associated with an abnormal background, characterized by disorganization or
suppression. The spikes can e single or multiple,
occurring in trains similar to those of unifocal discharges. The prognosis for good neurologic outcome is poorer for multifocal discharges than unifocal discharges. Seizures are usually clonic and may be subtle. The chief differential diagnosis for multifocal discharges is normal multifocal sharp transients. The abnormal background is key to differentiation
between these patterns. Pseudo-beta-alpha-theta-delta is a descriptive term for a discharge that begins at 8-12/sec and gradually slows to 0.5-3.0/sec. The discharge may have a sharp appearance but alternatively may have a smooth contour. This is an ictal pattern, with typical seizures being tonic, myoclonic, or subtle. The pseudo-beta-alpha-theta-delta rhythm usually indicates a poor prognosis and is commonly seen in patients with perinatal asphyxia. The evolution of changing frequency is common, especially to frequencies that are a
subharmonic of the original frequencies.
Rarely, neonates may manifest seizures without any perceptible alteration in background. The generator of epileptiform activity is probably subcortical, and the discharges are not projected to the surface. These infants usually have severe brain damage, explaining the lack of rostral projection of the activity.
Figure 6-3: Rhythmic discharges in a neonate
Rhythmic activity in the alpha range is not normal during stage of the sleep-wake cycle in a neonate. This is a seizure discharge. The generalized spike-wave patterns of seizures in older children and adults are not seen in neonates.