Seizure Classification and Diagnostic Approach

Introduction to Seizure Types

• Seizures are fundamentally classified as focal onset, generalized onset, or unknown onset, with this distinction being critical for treatment selection and prognosis, as stated by the American College of Radiology 1, 2

Distinguishing Focal from Generalized Seizures

• Focal seizures arise within networks of a single cerebral hemisphere and may remain localized or subsequently become more widely distributed, according to the American College of Radiology 1, 2
• Generalized seizures rapidly affect both hemispheres and both sides of the body from onset, even when caused by a focal lesion, as noted by the American College of Radiology 1, 2

Causes of Seizures

• Certain seizure types are likely associated with structural brain lesions including tumors, infection, infarction, traumatic brain injury, vascular malformations, developmental abnormalities, and seizure-associated brain pathology, as reported by the American College of Radiology 1, 3
• Trauma-related seizures subdivide into immediate and late seizures, with structural causes being more common with focal epilepsies, according to the American College of Radiology 1, 3

Investigation of Seizures

• Focal seizures show ictal discharges originating from one hemisphere, as stated by the American College of Radiology 1, 2
• Non-contrast CT has a central role in emergent acute seizures to rapidly identify structural pathology requiring immediate intervention, including intracranial hemorrhage, stroke, vascular malformation, hydrocephalus, and tumors, as recommended by the American College of Radiology 1, 3
• MRI is superior to CT and should be the primary imaging modality for newly diagnosed seizures, as it is more sensitive for detecting brain abnormalities, according to the American College of Radiology 4, 5
• Dedicated epilepsy protocols with 3T scanners, including T1-weighted volumetric acquisition with 1mm isotropic voxels and high-resolution coronal slices optimized for hippocampal pathology, are recommended for MRI, as stated by the American College of Radiology 6, 7

Neuroimaging for Generalized Seizures

• MRI is rarely indicated in neurologically normal patients with generalized seizures because the rate of positive findings is low given their genetic basis, as noted by the American College of Radiology 8, 9
• Only 2% of low-risk patients had abnormal MRI findings, according to the American College of Radiology 9, 4

Intractable/Refractory Epilepsy

• MRI with dedicated epilepsy protocols is the most sensitive and specific anatomic imaging technique for intractable seizures, with sensitivity of 84% and specificity of 70%, as reported by the American College of Radiology 6, 7

Focal Seizures: Clinical Features and Diagnostic Considerations

Core Distinguishing Characteristics

• The presence of any focal feature to a seizure is independently associated with clinically relevant abnormalities on neuroimaging, making recognition of these features critical, as recommended by the American College of Radiology 10
• Focal seizures have a recurrence rate up to 94%, considerably higher than generalized seizures at 72%, according to the American College of Radiology 10

Clinical Manifestations by Category

Motor Features

• Hemiparesis is the most frequent focal neurological sign when focal brain injury occurs from venous ischemia or hemorrhage, as noted by the American Heart Association/Stroke 11

Sensory and Cortical Signs

• Sensory symptoms referable to the affected brain region commonly occur alongside motor manifestations, as reported by the American Heart Association/Stroke 11
• Aphasia frequently accompanies focal seizures when language-dominant regions are involved, according to the American Heart Association/Stroke 11
• Other cortical signs, including visual field defects, psychosis, and specific sensory disturbances, may manifest depending on seizure localization, as described by the American Heart Association/Stroke 11

Altered Consciousness

• Focal seizures with impaired awareness represent a major subtype where consciousness is affected despite focal origin, as noted by the American College of Radiology 10

Temporal Presentation Patterns

• Focal seizures may present acutely, subacutely, or chronically, with subacute presentation being most common at 56%, as reported by the American Heart Association/Stroke 11
• Median delay from symptom onset to diagnosis is 7 days, highlighting the diagnostic challenge these seizures present, according to the American Heart Association/Stroke 11

Associated Seizure Activity

• Focal or generalized seizures occur in 40% of patients with cerebral venous thrombosis, making seizures a frequent feature when focal pathology is present, as noted by the American Heart Association/Stroke 11
• Focal seizures may evolve to bilateral tonic-clonic seizures, which starts focally then propagates to both hemispheres, as described by the American College of Radiology 10

Underlying Structural Associations

• Focal seizures show considerably higher positive neuroimaging yields compared to generalized seizures in neurologically normal patients, according to the American College of Radiology 10
• Approximately 4% of children with first-time afebrile focal seizures have urgent intracranial pathology, most commonly infarction, hemorrhage, and thrombosis, as reported by the American College of Radiology 10

Focal Features in Seizure Diagnosis

Diagnostic Principles

• Focal seizure features show considerably higher positive neuroimaging yields compared to generalized seizures, with detection rates of 50% on CT when neurologic findings are focal versus only 6% without focal features, as recommended by the American College of Radiology 12
• MRI detected abnormalities not identified by CT in 47% of children with focal seizures, and 29% of abnormal intracranial findings were not seen on initial CT in children with new-onset afebrile seizures with focal features, according to the American College of Radiology 12
• Patients with altered mental status, fever, or a new focal neurologic deficit require more extensive evaluation beyond routine laboratory testing, as suggested by the American College of Emergency Physicians 13
• The American College of Emergency Physicians recommends checking glucose in patients with focal features, as hypoglycemia can present with focal neurologic deficits 13

Laboratory Evaluation

• Glucose abnormalities and hyponatremia are the most frequent abnormalities identified in patients with focal neurologic deficits, and are usually predicted by history and physical examination, as noted by the American College of Emergency Physicians 13

Guidelines for Diagnosis of Focal Seizures in Young Children

Clinical Assessment

• In children younger than 3 years, seizures that appear generalized may actually originate from focal cortical activity in ≈ 37 % of cases, indicating that clinical observation alone is insufficient for a definitive diagnosis. 14

• Failure of a child to regain baseline alertness within several hours after a seizure suggests an underlying structural brain abnormality and warrants urgent neuroimaging. 14

Electroencephalography (EEG)

• EEG is recommended as a core component of the neuro‑diagnostic work‑up and is essential for confirming a focal seizure diagnosis. 14

• When a routine EEG is nondiagnostic but clinical suspicion for focal epilepsy remains high, video‑EEG monitoring should be pursued to clarify seizure origin. 14

Neuroimaging

• MRI with an epilepsy protocol is the preferred imaging modality for any child with a seizure of partial (focal) onset. 14

• In children under 3 years, especially those younger than 1 year, MRI (even if non‑urgent) should be strongly considered to detect subtle structural lesions. 14

• If a child does not return to baseline alertness within several hours, emergent MRI is indicated to evaluate for acute intracranial pathology. 14

Laboratory Evaluation

• Toxicology screening should be performed when there is any suspicion of drug or toxin exposure contributing to the seizure. 15

Lumbar Puncture

• Lumbar puncture has limited value for a first non‑febrile seizure and should be reserved for cases with concern for meningitis or encephalitis, such as when fever accompanies focal seizures. 14

• Consider lumbar puncture if the child presents with fever plus focal seizure activity, persistent altered mental status, or other clinical signs suggestive of central nervous system infection. 14

Diagnostic Algorithm (Key Steps)

• Document seizure semiology to identify focal features (e.g., unilateral movements, versive head/eye deviation).

• Obtain EEG as the first neurophysiologic test; proceed to video‑EEG if the routine study is nondiagnostic. 14

• Perform MRI brain with an epilepsy protocol as the definitive imaging study for focal seizures. 14

• Order targeted laboratory studies (glucose, electrolytes) when metabolic derangement is clinically suspected.

• Reserve lumbar puncture for cases with fever, ongoing altered mental status, or strong suspicion of CNS infection. 14