What test is used to verify a seizure?

Diagnostic Tests for Seizure Verification

Electroencephalography (EEG) is the gold standard test for verifying seizures, with continuous EEG monitoring being particularly valuable for detecting nonconvulsive seizures in critically ill patients. 1, 2

Primary Diagnostic Approach

EEG Testing

• Standard EEG: First-line test to detect epileptiform activity
• Continuous EEG monitoring: Essential for:
  • Patients with suspected nonconvulsive status epilepticus
  • Patients with subtle convulsive status epilepticus
  • Patients who have received long-acting paralytics
  • Patients in drug-induced coma 1

Timing of EEG Detection

• 88% of seizures are detected within the first 24 hours of continuous EEG monitoring
• 5% are detected on monitoring day 2
• 7% are detected after 48 hours of monitoring 2
• Comatose patients are more likely to require >24 hours of monitoring to detect the first seizure (20% vs 5% of non-comatose patients) 2

Neuroimaging Studies

Acute Setting

• CT Head (without contrast): Initial imaging of choice in emergency situations
  • Rapidly identifies structural pathology requiring immediate intervention
  • Detects intracranial hemorrhage, stroke, vascular malformations, hydrocephalus, tumors
  • Limited sensitivity (30%) for detecting lesions in focal epilepsies 1

Non-Emergent Setting

• MRI Head: Imaging study of choice when not in an emergency
  • Superior to CT for identifying and characterizing focal causative lesions
  • Essential for determining prognosis and treatment strategy
  • Should include specific seizure protocol sequences 1

Laboratory Testing

Essential Tests

• Serum glucose (to identify hypoglycemia)
• Electrolytes (especially sodium)
• Complete blood count
• Calcium levels
• Renal function tests (BUN, creatinine)
• Magnesium levels (particularly in alcoholics)
• Toxicology screen when substance use is suspected 3

Diagnostic Yield

• Laboratory testing has relatively low yield in otherwise healthy patients with new-onset seizures who have returned to baseline mental status
• Most abnormalities can be predicted by history and physical examination
• Only a small percentage of cases reveal unexpected metabolic abnormalities 3, 1

Advanced Imaging Techniques

Nuclear Medicine Studies

• FDG-PET: Useful for localizing epileptogenic foci in refractory cases
• Perfusion SPECT: Valuable for ictal imaging
  • Ictal SPECT: 73% sensitivity, 75% specificity
  • Inter-ictal SPECT: 50% sensitivity, 75% specificity
  • More effective in temporal lobe epilepsy than extra-temporal lobe epilepsy 1
• SISCOM (Subtraction of ictal and inter-ictal SPECT co-registered to MRI): Improves sensitivity and specificity of seizure-related perfusion networks 1

Risk Factors for Seizures on EEG

The following factors increase the likelihood of detecting seizures on EEG:

• Coma (OR 7.7)
• Age <18 years (OR 6.7)
• History of epilepsy (OR 2.7)
• Convulsive seizures during current illness prior to monitoring (OR 2.4) 2

Common Pitfalls and Caveats

1. Nonconvulsive seizures: 92% of seizures detected on continuous EEG are exclusively nonconvulsive and would be missed without EEG monitoring 2

2. Delayed detection: Some seizures are only detected after prolonged monitoring, especially in comatose patients 2

3. Misdiagnosis: Many conditions can mimic seizures, including pseudoseizures, syncope, migraine, cerebrovascular disease, movement disorders, and sleep disorders 4

4. Limited EEG availability: Local access to neurologic and EEG expertise, technical personnel, and equipment may limit performance of EEGs in emergency departments 1

5. Overreliance on imaging: While neuroimaging is important, it cannot replace EEG for seizure verification 1

By following this diagnostic approach, clinicians can effectively verify seizures and determine their underlying causes, leading to appropriate treatment decisions that improve patient outcomes.