Periodic Epileptiform Discharges 41 Introduction

Periodic discharges, as their name implies, are those that recur at regular intervals. In general, the term is used to refer to discharges that appear on the order of every second to every

Useful Terms Epilepsy

Fig. 13. Rhythmic theta. This tracing is from a 54-yr-old woman with refractory right temporal lobe epilepsy. This depicts an evolving electrographic seizure involving rhythmic and sharply contoured theta activity over right temporal channels, acquired during a complex partial seizure recorded during long-term monitoring.

Fig. 13. Rhythmic theta. This tracing is from a 54-yr-old woman with refractory right temporal lobe epilepsy. This depicts an evolving electrographic seizure involving rhythmic and sharply contoured theta activity over right temporal channels, acquired during a complex partial seizure recorded during long-term monitoring.

3hz Periodic Lateralized Discharge

Fig. 14. Rhythmic beta. This tracing is from a 12-mo-old girl with infantile spasms. This particular page illustrates a sudden burst of left hemispheric rhythmic beta activity at 13 to 14 Hz for 6 s, associated with sudden dystonic arm movements. The burst of rhythmic beta activity begins with a sudden high-amplitude sharp wave discharge, maximal at T3.

Fig. 14. Rhythmic beta. This tracing is from a 12-mo-old girl with infantile spasms. This particular page illustrates a sudden burst of left hemispheric rhythmic beta activity at 13 to 14 Hz for 6 s, associated with sudden dystonic arm movements. The burst of rhythmic beta activity begins with a sudden high-amplitude sharp wave discharge, maximal at T3.

several seconds, but not fast enough to be considered ongoing seizure activity. In some instances, periodic discharges on EEG are so characteristic of particular clinical diagnoses as to be nearly pathognomonic, whereas, in other cases, periodic discharges may be the nonspecific result of a variety of cerebral insults.

4.2. Patterns of Periodic Discharges

4.2.1. Periodic Generalized Sharp Waves With One-Per-Second Frequency

This pattern is the characteristic EEG finding in Creutzfeldt-Jakob Disease (CJD), a progressive spongiform encephalopathy caused by prions that leads to dementia and myoclonus, among other features (Fig. 15). More than 90% of all CJD patients will demonstrate this finding on EEG by the time the disease is advanced, but in the first few weeks or months the periodic discharges may be unilateral, not as prominent, or not present at all. They typically occur on a background that is slow and disorganized, and such an encephalo-pathic background may be the only EEG sign early in the disease course. New variant CJD does not seem to be associated with these periodic discharges.

4.2.2. Periodic Generalized Sharp Complexes Occurring Every 5 to 20 s

Subacute sclerosing panencephalitis, a late sequela of childhood measles infection, is characterized by these periodic EEG discharges that are separated by relatively long intervals. A noteworthy feature is that clinically observed myoclonic jerks, which are a prominent sign of the disease, can occur in temporal association with these generalized complexes, which are usually of very high voltage and occur on a slow, disorganized background.

4.2.3. Periodic Lateralized Epileptiform Discharges

These are aptly named unilateral sharply contoured waveforms that recur at a regular frequency, usually every one to few seconds. The waveform appearance is commonly that of a broad, high-voltage sharp wave, sharp-and-slow wave complex, or sharply contoured triphasic wave (Fig. 16). The intervening background on the side of the discharges is abnormal, often slow, and of low voltage. Most commonly, periodic lateralized epileptiform discharges (PLEDs) are observed in the setting of an acute or subacute structural lesion of the cerebral hemisphere, often large and destructive in nature. Thus, stroke (both ischemic and hemor-rhagic), tumor, abscess, and encephalitis are among the common etiologies. In general, PLEDs are not thought to represent ongoing ictal activity, although a high percentage of patients with PLEDs have seizures at other times. Sometimes PLEDs are observed in late status epilepticus or during a recovery phase after status epilepticus, and less commonly a pattern of PLEDs may increase in frequency and evolve into clear ictal activity. PLEDs can persist for days to weeks after a cerebral insult, although their amplitude and frequency usually decrease during that period of time. Periodic epileptiform discharges occurring independently over the two cerebral hemispheres have been termed bilateral independent PLEDs; their presence sometimes suggests CNS infection.

The occurrence of PLEDs that are predominantly over the temporal regions suggests the diagnosis of herpes simplex encephalitis, a hemorrhagic infection caused by herpes simplex virus that has a predilection for the orbitofrontal and temporal regions and can rapidly lead to encephalopathy, seizures, coma, and death. Because of the fulminant nature of this disorder, the appearance of any periodic discharges over the temporal regions should immediately raise suspicion for this diagnosis in the appropriate clinical setting.

Sharp Waves Eeg

Fig. 15. Periodic generalized sharp waves in Creutzfeldt-Jakob disease. This tracing derives from a 68-yr-old man with a rapidly progressive dementia and myoclonus. The EEG demonstrates continuous, high-amplitude, periodic sharp waves at 1 to 1.5 Hz that are bilateral, although maximal over the left hemisphere. The technologist noted occasional clinical myoclonic jerking of the limbs during this acquisition.

Fig. 15. Periodic generalized sharp waves in Creutzfeldt-Jakob disease. This tracing derives from a 68-yr-old man with a rapidly progressive dementia and myoclonus. The EEG demonstrates continuous, high-amplitude, periodic sharp waves at 1 to 1.5 Hz that are bilateral, although maximal over the left hemisphere. The technologist noted occasional clinical myoclonic jerking of the limbs during this acquisition.

Epileptiform Discharge Eeg

Fig. 16. Periodic lateralized epileptiform discharges (PLEDs). PLEDs are regularly recurring sharply contoured waveforms seen unilaterally. This sample is from a 66-yr-old man after resection of a parafalcine meningioma complicated by intracranial hemorrhage. There are fairly monotonous, recurrent sharp wave discharges from the left frontal region at 1.4 Hz.

Fig. 16. Periodic lateralized epileptiform discharges (PLEDs). PLEDs are regularly recurring sharply contoured waveforms seen unilaterally. This sample is from a 66-yr-old man after resection of a parafalcine meningioma complicated by intracranial hemorrhage. There are fairly monotonous, recurrent sharp wave discharges from the left frontal region at 1.4 Hz.

SUGGESTED READING

Ebersole JS, Pedley TA. Current Practice of Clinical Electroencephalography. 3rd ed. Lippincott Williams & Wilkins, Philadelphia, PA, 2002.

Goldensohn ES, Legatt AD, Koszer S, Wolf SM. Goldensohn's EEG Interpretation: Problems of Overreading and Underreading. 2nd ed. Futura Publishing Company, New York, NY, 1999.

Niedermeyer E, Lopes da Silva F. Electroencephalography: Basic Principles, Clinical Applications, and Related Fields. 5th ed. Lippincott Williams and Wilkins, Philadelphia, PA, 2004.

Tyner FS, Knott JR, Mayer WB Jr. Fundamentals of EEG Technology: Basic Concepts and Methods. Lippincott Williams and Wilkins, Baltimore, MD, 1983.

REVIEW QUESTIONS

1. What are the definitions of a spike and a sharp wave?

2. What is the presumed basis for the slow wave after a spike or sharp wave?

3. When are polyspike discharges most commonly encountered?

4. Are occipital spikes of blindness indicative of an epileptic tendency?

5. What are the characteristic features of Rolandic spikes?

6. What are common frequencies of generalized spike and wave discharges and in what setting are they observed?

7. When are electrodecremental patterns encountered?

8. What are the features of sharp waves in CJD?

9. What are the main features of an EEG pattern that suggest an ictal event?

10. When are PLEDs encountered?

REVIEW ANSWERS

1. A spike is defined to be less than 70 ms in duration. Sharp waves have a duration between 70 and 200 ms.

2. The following slow wave is thought to derive from the synchronous repolarization of a collection of cortical neurons associated with a previous wave of cortical excitation.

3. Polyspikes are most frequently encountered in generalized epilepsies and in myoclonic epilepsies.

4. No, occipital spikes of blindness are observed with patients with acquired vision loss early in life. They are not thought to be predictive of epilepsy.

5. Rolandic spikes are high amplitude, biphasic spikes or sharp waves, often with a following slow wave, with surface negativity maximal involving the centrotemporal channels, and often exhibiting a horizontal dipole. They are very sleep activated.

6. Generalized spike and wave discharges are sometimes encountered at approx 3 Hz in childhood absence epilepsy, at 4 to 6 Hz in juvenile myoclonic epilepsy, and at less than 3 Hz in Lennox-Gastaut syndrome.

7. Electrodecremental patterns are observed in infantile spasms, tonic, and atonic seizure types. A generalized slow wave may occasionally precede the sudden, low-voltage, fast activity in this pattern.

8. CJD is frequently associated with periodic sharp waves at approx 1 Hz. This pattern may be unilateral early in the course of the disease, but later becomes generalized.

9. Rhythmicity and evolution of pattern are pivotal features that allow recognition of an electro-graphic seizure.

10. PLEDs are usually encountered in acute or subacute cerebral injuries. Etiologies may include stroke, tumor, abscess, and encephalitis, among others. Temporal lobe PLEDs raise a concern of herpes encephalitis. PLEDs may also be observed in the setting of recent status epilepticus.

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