eMedicine Specialties > Neurology > Electroencephalography Atlas
Generalized Epilepsies on EEG
Updated: Sep 10, 2008
Introduction
The International Classification of Epileptic Syndromes and Epilepsies1 classifies the epilepsies along 2 dichotomies: partial (ie, localization-related) versus generalized, and idiopathic versus cryptogenic or symptomatic. This double dichotomy conveniently allows the epilepsy classification system to be presented in a simple table, as follows:
Classification of the Epilepsies (Adapted from Tich and Pereon, 19992)
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Table
| Generalized | Localization-related | |
|---|---|---|
| Idiopathic (genetic) | Childhood absence epilepsy Juvenile absence epilepsy Juvenile myoclonic epilepsy Epilepsy with grand-mal seizures on awakening Other idiopathic generalized epilepsies | Benign focal epilepsy of childhood (2 types) ADNFLE* Primary reading epilepsy |
| Symptomatic or cryptogenic | West syndrome Lennox-Gastaut syndrome Other symptomatic generalized epilepsies | Mesiotemporal lobe epilepsy Neocortical focal epilepsy |
| Generalized | Localization-related | |
|---|---|---|
| Idiopathic (genetic) | Childhood absence epilepsy Juvenile absence epilepsy Juvenile myoclonic epilepsy Epilepsy with grand-mal seizures on awakening Other idiopathic generalized epilepsies | Benign focal epilepsy of childhood (2 types) ADNFLE* Primary reading epilepsy |
| Symptomatic or cryptogenic | West syndrome Lennox-Gastaut syndrome Other symptomatic generalized epilepsies | Mesiotemporal lobe epilepsy Neocortical focal epilepsy |
*ADNFLE - Autosomal dominant nocturnal frontal lobe epilepsy
The term idiopathic often is misunderstood in this setting and requires clarification. Whereas the term idiopathic in medicine usually means "of unknown cause," idiopathic epilepsies are not truly of unknown cause (this confusing terminology will most likely be corrected in the upcoming ILAE classification system3). In epilepsy, idiopathic seizures are genetically determined and have no apparent structural cause, with seizures as the only manifestation of the condition. Findings of the neurologic examination and neuroimaging studies are normal, and EEG findings also are normal other than the epileptiform abnormalities. In some syndromes, the genetic substrate has even been identified.
Most idiopathic epilepsies are generalized, but a few genetic epilepsies are focal. Nonidiopathic epilepsies are by definition not genetic (although some may be associated with a minor genetic predisposition), but are the result of a brain insult or lesion. If the damage is focal, it results in a localization-related epilepsy; if it is diffuse, it results in a generalized epilepsy. The difference between symptomatic and cryptogenic is subtle: symptomatic means that the etiology is known, while cryptogenic means that an underlying etiology is apparent but cannot be documented objectively. Thus, the boundary between the 2 is largely dependent on our diagnostic and imaging techniques.
This review discusses EEG findings in the generalized epilepsies.
For further information see Medscape's Epilepsy Resource Center.
Waveform Descriptions
Spikes and sharp waves are sharp transients that have a strong association with epilepsy. The two are distinguished by their duration (spikes <70 ms, sharp waves 70-200 ms), but no difference is noted in terms of clinical significance. Several characteristics distinguish these from benign epileptiform variants (see article EEG Atlas: Focal (Nonepileptic) Abnormalities), including high amplitude that makes them "stand out" from ongoing background activity and aftergoing slow waves, which give the appearance of their "disrupting" background activity (see Media files 1-2).
Spike-wave complexes (SWC) are the repetitive occurrence of a spike followed by a slow wave. Since any significant spike or sharp wave usually is followed by a slow wave (see above), a run of 3 seconds is required to classify a record as SWC, as opposed to the categories already mentioned (spike or sharp wave). SWC can be divided further into 2 more specific types, as follows:
- The 3-Hz SWC: This pattern is characterized by a frequency of 2.5-4 Hz and a very monomorphic ("perfectly regular") morphology (see Media file 3). It occurs in very discrete bursts, and between bursts the EEG is normal.
- Slow SWC: This pattern is not only slower (<2.5 Hz) but also more irregular (less monomorphic) than the 3-Hz SWC. Bursts are less discrete than the 3-Hz SWC, and between bursts other abnormalities are seen in symptomatic/cryptogenic epilepsies of the Lennox-Gastaut type (see Media files 4-5).
Polyspikes are multiple repetitive spikes occurring at about 20 Hz (see Media file 6).
Generalized epileptiform discharges (ie, spikes, sharp waves, SWCs) are usually maximal in the frontal regions, with typical phase reversals at the F3 and F4 electrodes (see Media file 7).
Hypsarrhythmia is defined as continuous (during wakefulness), high-amplitude (>200 Hz), generalized polymorphic slowing with no organized background and multifocal spikes (see Media files 8-9).
Electrographic seizures
- Electrodecrement consists of abrupt attenuation (flattening) of background activity, often preceded by a high-amplitude transient (see Media file 10). This typically is associated with infantile spasms or atonic seizures.
- Generalized paroxysmal fast activity (GPFA) consists of bursts of fast (10 Hz) activity and typically is associated with tonic seizures.
Clinical Correlation
Idiopathic generalized epilepsies
These syndromes, formerly called primary generalized epilepsies, are the best-known group of idiopathic epilepsies. They epitomize the meaning of the term idiopathic: genetic basis, normal neurologic examination findings, and normal intelligence. EEG shows generalized epileptiform discharges and may show photosensitivity. Seizure types include generalized tonic-clonic (GTC), absence, and myoclonic. Accordingly, EEG typically shows generalized spikes or sharp waves, 3-Hz or faster SWCs (clinically associated with absence seizures), and polyspikes (clinically associated with myoclonic seizures). The EEG is normal (ie, no abnormal slowing) except for the epileptiform abnormalities.
Within the group of idiopathic generalized epilepsies, distinct entities are distinguished, primarily on the basis of predominant seizure type(s) and age of onset. Some syndromes are very well individualized, while others have less clear boundaries. The major and well-defined types of idiopathic generalized epilepsies include childhood absence epilepsy, juvenile myoclonic epilepsy, and epilepsy with grand mal seizures (sometimes referred to as grand mal on awakening).
Symptomatic generalized epilepsies
These are associated with diffuse brain dysfunction. The cause may be known (symptomatic), such as anoxic birth injury or a metabolic or chromosomal defect, or it may be unknown (cryptogenic). Accordingly, clinical evidence of diffuse brain dysfunction is usually present, either intellectual (ie, developmental delay or mental retardation) or motor (ie, developmental delay or cerebral palsy). Similarly, the EEG shows evidence of diffuse brain dysfunction in addition to the epileptiform abnormalities, in the form of slowing. The clinical and EEG manifestations are not specific as to etiology, but vary tremendously with age, and thus are said to be age dependent.
West syndrome is the phenotype of symptomatic or cryptogenic generalized epilepsy in the first year of life and is characterized by infantile spasms, hypsarrhythmia, and developmental delay. It is an age-specific response of the immature brain to a nonspecific focal or generalized insult. Age of onset peaks between 3 and 7 months.
Lennox-Gastaut syndrome (LGS) has an early childhood onset (age 1-8 y) and consists of multiple seizure types, mental retardation, and typical EEG findings dominated by generalized slow SWC. Seizure types include atypical absences, tonic, atonic, myoclonic, and GTC seizures. The atonic, myoclonic, tonic, and GTC seizures of LGS frequently result in unprotected falls (referred to as "drop attacks") with injury. Besides the classic EEG pattern of generalized slow SWC, other frequent but less specific EEG findings include background slowing, generalized slowing, and multifocal spikes. During sleep, the EEG may show polyspikes and slow waves. Another typical feature of LGS is generalized paroxysmal fast activity (>10 Hz) during sleep. Many patients with symptomatic generalized epilepsy do not meet all the criteria for LGS.
Patient Education
For excellent patient education resources, visit eMedicine's Procedures Center and Brain and Nervous System Center. Also, see eMedicine's patient education articles Electroencephalography (EEG) and Epilepsy.
Multimedia
 | Media file 1: Spike, generalized. Note the high amplitude and the aftergoing background suppression and slow wave. |
 | Media file 2: Spike, generalized. Significant spikes usually are followed by a slow wave, as shown here. This example also illustrates that generalized spikes are typically maximal frontally. This is typical of the primary (ie, idiopathic, genetic) epilepsies. If the burst lasted 3 seconds or more, it could be classified as spike-wave complexes. |
 | Media file 3: Sharp waves, multifocal. Sharp waves are seen at T4, T6, T5, and F3 on this 9-second segment. With other findings, this often is seen in the symptomatic/cryptogenic epilepsies of the Lennox-Gastaut type. |
 | Media file 4: Slow spike-wave complexes. In addition to being slower, this is also less monomorphic than the 3-Hz spike-wave complexes. With other findings, this often is seen in the symptomatic/cryptogenic epilepsies of the Lennox-Gastaut type. |
 | Media file 5: Slow spike-wave complexes (SWC). In addition to being slower, this is also less monomorphic than the 3-Hz SWC. With other findings, this often is seen in the symptomatic/cryptogenic epilepsies of the Lennox-Gastaut type. |
 | Media file 6: Polyspike, generalized. Note the aftergoing slow wave. This is associated with the "primary" or idiopathic (genetic) generalized epilepsies, most typically juvenile myoclonic epilepsy. |
 | Media file 7: Generalized 3-Hz spike-wave complexes (SWC). This pattern is very monomorphic, with a maximum (shown here by a phase reversal) frontally, typically at F3/F4. This is typical of idiopathic (ie, genetic) generalized epilepsies, such as absence epilepsy. The 3-Hz SWC is often faster at onset (4-5 Hz), as shown here. |
 | Media file 8: Hypsarrhythmia. High-amplitude slowing with no organized background, and multifocal spikes (left and right frontal in this sample). This is a phenotype of the first year of life and is associated with West syndrome (ie, infantile spasms). |
 | Media file 9: Hypsarrhythmia. High-amplitude slowing (note the scale) with no organized background, and multifocal spikes (right frontal and left occipital in this sample). This is a phenotype of the first year of life and is associated with West syndrome (ie, infantile spasms). |
 | Media file 10: Generalized paroxysmal fast activity and electrodecrement. This pattern is characteristic of the symptomatic/cryptogenic epilepsies of the Lennox-Gastaut type and may be subclinical or associated with tonic or atonic seizures. |
Keywords
EEG atlas, spikes and sharp waves, spike-wave complexes, SWC, 3-Hz SWC, slow SWC, polyspikes, generalized epileptiform discharges, hypsarrhythmia, idiopathic generalized epilepsies, childhood absence epilepsy, juvenile myoclonic epilepsy, epilepsy with grand mal seizures, symptomatic generalized epilepsies, West syndrome, cryptogenic generalized epilepsy, Lennox-Gastaut syndrome, LGS
References
International League Against Epilepsy, Commission on Classification and Terminol. Proposal for revised classification of epilepsies and epileptic syndromes. Epilepsia. Jul-Aug 1989;30(4):389-99. [Medline].
Tich SN, Pereon Y. Semiological seizure classification. Epilepsia. Apr 1999;40(4):531. [Medline].
Engel J Jr. Classifications of the International League Against Epilepsy: time for reappraisal. Epilepsia. Sep 1998;39(9):1014-7. [Medline].
Appleton RE, Beirne M. Absence epilepsy in children: the role of EEG in monitoring response to treatment. Seizure. Jun 1996;5(2):147-8. [Medline].
Beaumanoir A, Bureau M, Deonna T, et al. Continuous spikes and waves during slow sleep. Electrical status epilepticus during slow sleep. In: Acquired Epileptic Aphasia and Related Conditions. London: John Libbey; 1995.
Benbadis SR, Luders HO. Epileptic syndromes: an underutilized concept. Epilepsia. Nov 1996;37(11):1029-34. [Medline].
Benbadis SR, Luders HO. Generalized epilepsies. Neurology. Apr 1996;46(4):1194-5. [Medline].
Benbadis SR, Tatum WO. Overintepretation of EEGs and misdiagnosis of epilepsy. J Clin Neurophysiol. Feb 2003;20(1):42-4. [Medline].
Benbadis SR, Wyllie E. Pediatric epilepsy syndromes. In: Levin KH, Luders HO, eds. Comprehensive Clinical Neurophysiology. Philadelphia, Pa: WB Saunders; 2000:468-80.
Berkovic SF, Andermann F, Andermann E, et al. Concepts of absence epilepsies: discrete syndromes or biological continuum?. Neurology. Jun 1987;37(6):993-1000. [Medline].
Brenner RP, Atkinson R. Generalized paroxysmal fast activity: electroencephalographic and clinical features. Ann Neurol. Apr 1982;11(4):386-90. [Medline].
Grunewald RA, Panayiotopoulos CP. Juvenile myoclonic epilepsy. A review. Arch Neurol. Jun 1993;50(6):594-8. [Medline].
Holmes GL, McKeever M, Adamson M. Absence seizures in children: clinical and electroencephalographic features. Ann Neurol. Mar 1987;21(3):268-73. [Medline].
Hrachovy RA, Frost JD Jr, Kellaway P. Hypsarrhythmia: variations on the theme. Epilepsia. Jun 1984;25(3):317-25. [Medline].
Lancman ME, Asconape J, Brotherton T, et al. Juvenile myoclonic epilepsy: an underdiagnosed syndrome. J Epilepsy. 1995;8:215-218.
Lancman ME, Asconape JJ, Penry JK. Clinical and EEG asymmetries in juvenile myoclonic epilepsy. Epilepsia. Mar-Apr 1994;35(2):302-6. [Medline].
Loiseau P, Duche B, Pedespan JM. Absence epilepsies. Epilepsia. Dec 1995;36(12):1182-6. [Medline].
Lombroso CT. A prospective study of infantile spasms: clinical and therapeutic correlations. Epilepsia. Apr 1983;24(2):135-58. [Medline].
Luders H, Noachtar S, eds. Atlas and Classification of Electroencephalography. Philadelphia, Pa: WB Saunders; 2000.
Markand ON. Slow spike-wave activity in EEG and associated clinical features: often called 'Lennox' or "Lennox-Gastaut' syndrome. Neurology. Aug 1977;27(8):746-57. [Medline].
Reutens DC, Berkovic SF. Idiopathic generalized epilepsy of adolescence: are the syndromes clinically distinct?. Neurology. Aug 1995;45(8):1469-76. [Medline].
Thomas P, Beaumanoir A, Genton P, Dolisi C, Chatel M. 'De novo' absence status of late onset: report of 11 cases. Neurology. Jan 1992;42(1):104-10. [Medline].
Wyllie E. Classification of seizures. In: Wyllie E, ed. The Treatment of Epilepsy: Principles and Practice. 2nd ed. Baltimore, Md: Lippincott Williams & Wilkins; 1997:355-7.
Further Reading
Keywords
EEG atlas, spikes and sharp waves, spike-wave complexes, SWC, 3-Hz SWC, slow SWC, polyspikes, generalized epileptiform discharges, hypsarrhythmia, idiopathic generalized epilepsies, childhood absence epilepsy, juvenile myoclonic epilepsy, epilepsy with grand mal seizures, symptomatic generalized epilepsies, West syndrome, cryptogenic generalized epilepsy, Lennox-Gastaut syndrome, LGS
