What are generalized onset seizures?

Generalized Onset Seizures

Generalized onset seizures are seizures that rapidly affect both hemispheres of the brain and both sides of the body from onset, characterized by diffuse or generalized involvement of the brain on EEG or clinically. 1

Definition and Classification

• Generalized onset seizures originate at a point within the brain and rapidly engage bilaterally distributed networks, as opposed to focal seizures which originate within networks limited to one hemisphere 2
• According to the International League Against Epilepsy (ILAE) classification, generalized seizures are categorized into motor and nonmotor (absence) seizures 1, 3
• The term "generalized" implies diffuse involvement of the brain, differing from focal seizures with secondary generalization (now known as focal to bilateral tonic-clonic), which start focally and then propagate to both hemispheres 4

Types of Generalized Onset Seizures

Motor Seizures

• Generalized motor seizures can manifest as:
  • Tonic-clonic (grand mal)
  • Tonic
  • Clonic
  • Myoclonic
  • Myoclonic-tonic-clonic
  • Myoclonic-atonic
  • Atonic
  • Epileptic spasms 3

Nonmotor (Absence) Seizures

• Nonmotor generalized seizures include:
  • Typical absence
  • Atypical absence
  • Absence with myoclonic features
  • Absence with eyelid myoclonia 3

Clinical Presentation

• Most generalized seizures involve loss of awareness 2
• Primary generalized epilepsy syndromes include juvenile myoclonic epilepsy and childhood absence epilepsy 4
• Tonic-clonic seizures may be precipitated by alcohol or sleep deprivation in adult-onset cases 5
• Generalized seizures can occasionally have focal evolution with semiology suggestive of focal seizures, which may lead to misdiagnosis 6, 7

Diagnostic Approach

• Electroencephalography (EEG) is essential for diagnosis, showing generalized spike-and-wave and/or polyspike-and-wave discharges 6
• MRI is rarely indicated in evaluation of a neurologically normal patient presenting with generalized seizures because the rate of positive intracranial findings is low, given their genetic underpinnings 4
• CT has limited role in evaluation of generalized seizures, with only about 6% of CT examinations showing positive findings for generalized seizures 4

Etiology and Genetics

• Many generalized seizures have genetic underpinnings 4
• Adult-onset idiopathic generalized epilepsy appears to have a genetic etiology similar to classical IGE with onset before age 20 5
• Generalized seizures can occur in both genetic (idiopathic) and structural/metabolic (symptomatic) forms of epilepsy 6

Treatment Considerations

• Correct classification is crucial for proper treatment selection 1
• Patients with generalized seizures typically respond better to medications effective against generalized absence and myoclonic seizures 7
• Misdiagnosis of generalized seizures as focal seizures may lead to inappropriate medication selection 6, 7

Important Clinical Pitfalls

• Generalized onset seizures with focal evolution can be misdiagnosed as complex partial (focal) seizures, leading to inappropriate treatment 6, 7
• In children, variable expression at different stages of brain maturation can challenge proper classification 2
• Adult-onset generalized epilepsy may be misdiagnosed as non-lesional partial epilepsy; early postictal EEG and sleep deprivation studies may improve detection 5