Symptomatic generalized epilepsy (SGE) encompasses a group of challenging epilepsy syndromes. As a group, SGE has 3 main features: (1) multiple seizure types, especially generalized tonic and atonic seizures; (2) brain dysfunction other than the seizures, in the intellectual domain (mental retardation or developmental delay) and in the motor domain (cerebral palsy); and (3) EEG evidence of diffuse brain abnormality.
The following are examples of epilepsy syndromes that are included in the category of SGE:
Early myoclonic encephalopathy
Early infantine epileptic encephalopathy with suppression bursts or Ohtahara syndrome
Epilepsy with myoclonic atonic seizures
Epilepsy with myoclonic absence
Progressive myoclonic epilepsies
See the following Medscape Reference epilepsy topics for more information on these conditions:
Overall, seizures are a paroxysm of high-frequency or synchronous low-frequency high-voltage electrical discharges that cause a sudden alteration in the CNS. Three conditions are involved: (1) population of pathologically excitable neurons, (2) an increase in excitatory glutaminergic activity, and (3) reduction of inhibitory GABAergic projections.  In symptomatic generalized epilepsy, an underlying structural or metabolic derangement is also present. EEG findings reflect age-related changes as the brain matures. 
Assessing the frequency of symptomatic generalized epilepsy (SGE) is difficult because the definition varies and can be more or less inclusive.
In a study by Ohtahara and Yamatogi, 12 of 16 patients with Ohtahara syndrome developed into West syndrome or infantile spasms between ages 3 and 6 months as defined by EEG findings.  Infantile spasms affect 1 in 2000 infants.  Lennox-Gastaut syndrome has an incidence of 1.9-2.1 cases per 100,000 children and accounts for 6-7% of intractable pediatric epilepsy; however, this depends of the definition used for Lennox-Gastaut syndrome.
A study of social outcomes in a Nova Scotia study found that 20 years after diagnosis of childhood-onset symptomatic generalized epilepsy (SGE), 25% die. All survivors have mental retardation and depend on parents and institutions in terms of living and finances.  Most patients with SGE grow up to have intractable epilepsy, with a small percentage who are seizure free with no antiepileptic drugs.  In a follow-up study of 14 patients with Ohtahara syndrome, 4 died early and the remaining 10 were severely handicapped. 
These refer to age at onset. In adulthood, most patients with symptomatic generalized epilepsy (SGE) have a less well-defined syndrome that still has the characteristics of SGE and is closest to a Lennox-Gastaut syndrome (intractable multiple seizure types, mental retardation, and cerebral palsy). Note the following progression:
Neonates - Early myoclonic encephalopathy, early infantine epileptic encephalopathy with suppression bursts, or Ohtahara syndrome
Infancy - West syndrome
Childhood - Epilepsy with myoclonic atonic seizures, epilepsy with myoclonic absence, Lennox-Gastaut syndrome
Adolescence-adults - Progressive myoclonic epilepsies
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