Periodic discharges usually indicate cortical damage, and can be due to stroke, anoxia, infection, degenerative disorders, and other conditions. The periodic patterns can be focal, regional, or generalized, with regional
distribution being the most common. Periodic lateralized epileptiform
discharges PLEDs are high-amplitude sharp waves that recur at a rate of 0.5-3.0/sec. They are prominent over one hemisphere or one region. When bilateral, they are independent, thereby keeping the term lateralized.
PLEDs are a sign of parenchymal destruction and most commonly seen in strokes. Other important causes include head injury, abscess, encephalitis, hypoxic encephalopathy, brain tumors, and other focal cerebral lesions. It is impossible to distinguish definitively
between causes on the basis of waveform. Of the encephalitides, herpes simplex most commonly produces PLEDs. Other viral infections produce slowing without PLEDs. The PLEDs have an amplitude of 100-300 µV. An early negative component is followed by a positive wave. The discharge may be
complex, with additional sharp and slow
components
superimposed on the waveform.
Figure 5-8: Hypsarrhythmia
Hypsarrhythmia, seen typically in children with infantile spasms. The high-amplitude bursts with interburst interval is characteristic. Left and right lateral portion of the LB montage.
Figure 5-9: PLEDs
Periodic lateralized epileptiform discharges (PLEDs) in a patient with herpes simplex encephalitis. Left medial portion of the LB montage.
Patients with PLEDs may have myoclonic jerks that are either synchronous with the jerks or independent. When the jerks are independent, the generator for the myoclonus is probably deep. Even when they are synchronous, the generator is probably subcortical. The cortical discharge reflects projections from the deep generator.
Herpes simplex encephalitis
HSV encephalitis usually shows PLEDs on EEG during some phase of the illness, although at other times, there is slowing in the theta and subsequently delta range. The PLEDs are sharply contoured slow waves with a frequency of 2-4 Hz. The duration of each wave is often more than 50 msec. This relatively slow frequency of repetition helps to differentiate PLEDs in herpes encephalitis from the higher frequency discharges of SSPE.
Neonates with herpes encephalitis may have necrosis that is not confined or even most prominent in the temporal region. These patients often do not have PLEDs. The EEG may show a poorly organized background with slow activity in the delta range predominating.
Anoxic encephalopathy
Anoxic encephalopathy is also sometimes called hypoxic-ischemic encephalopathy, since the most common cause is cardiac arrest, where there is not only loss of oxygenation but also loss of blood flow to the brain.
The background is disorganized with diffuse slowing and suppression. Periodic sharp waves are often seen and may predominate in the record. They look similar to PLEDs, except that they are synchronous between the hemispheres. Patients may have myoclonus associated with the discharges. These probably represent the extreme of the burst
suppression pattern, seen often in patients with anoxic encephalopathy.
Burst-suppression pattern
The burst-suppression pattern occurs in patients with severe encephalopathies. The finding is not specific as to etiology but is most often seen in patients with hypoxic- ischemic damage and in barbiturate coma. The burst-suppression
Figure 5-10: Burst-suppression pattern
Burst-suppression pattern, which some people call the suppression-burst pattern since the bursts occpy a lesser temporal proportion of the recording than the suppressions. Right medial portion of the LB montage.
pattern in different clinical conditions can look very similar. In fact, the burst suppression pattern can look similar to the markedly discontinuous pattern of 29-week conceptional age newborns. Bursts of slow waves with superimposed sharp activity are superimposed on a very suppressed background. The background is not flat, but rather is very low voltage, composed of a mixture of frequencies.
Subacute sclerosing panencephalitis
SSPE has almost disappeared as a result of measles immunization. Periodic complexes are seen in most patients at an intermediate stage. Early on, there may be only mild slowing, with disorganization of the background. Late in the course, the periodic complexes may completely disappear, leaving the recording virtually isoelectric. The discharges are slow waves with sharp components. The duration of the complex is up to 3-sec, and the interval between complexes is 5-15 sec. The background during the interval is disorganized and generally suppressed. Myoclonus is typically synchronous with the discharge.
EEG in SSPE resembles the burst-suppression pattern. The background is usually more suppressed with burst suppression than SSPE. The two patterns are more easily
differentiated by clinical presentation. Patients with burst suppression usually have a known history of hypoxia or severe metabolic derangement. Patients with SSPE have a typical history of a progressive neurologic disorder with intellectual deterioration and seizures. SSPE is very rare.
Creutzfeldt-Jakob disease CJD produces an evolving pattern of EEG findings which depend on stage of the disease. At some point in the disease process, a periodic pattern is seen, composed of a sharp wave or sharply-contoured slow wave. The interval between discharges is 500-2,000 ms. The discharges are maximal in the anterior regions and may occasionally be unilateral. Only laterally are the discharges
prominent posteriorally and when so are commonly associated with blindness. The discharges may or may not be temporally locked to myoclonus. These discharges are superimposed on an abnormal background
Figure 5-11: Creutzfeld-Jakob disease
Periodicc discharges in a patient with CJD. The discharges are seen from both sides. These are the frontopolar and frontal channels of the TB montage.
characterized by low-voltage slowing in the theta and delta range. The periodic complexes abate in sleep.
Early in the course, the periodic complexes cannot be seen and the only finding may be focal or generalized slowing. About 10-15% of patients may not show periodic patterns during their course.