How is a definitive diagnosis of focal seizures reached in a 2‑year‑6‑month‑old girl with no prior medical history?

Definitive Diagnosis of Focal Seizures in a 2.5-Year-Old Child

The definitive diagnosis of focal seizures in a 2.5-year-old child is established through the combination of characteristic clinical features (focal motor, sensory, or behavioral manifestations) and electroencephalogram (EEG) demonstrating focal spikes, sharp waves, or ictal discharges originating from one cerebral hemisphere. 1, 2

Core Diagnostic Components

Clinical Assessment

The clinical presentation must demonstrate focal features, which may include:

• Focal motor manifestations: Unilateral clonic movements, limb posturing, or hemiparesis 1, 2, 3
• Behavioral arrest with altered awareness: Hypomotor seizures characterized by cessation of activity with indeterminate consciousness level, which in this age group may be associated with focal EEG patterns in 70% of cases 4
• Orofacial involvement: Mouth deviation or facial twitching 5
• Versive movements: Head and eye deviation, which reliably indicates contralateral hemisphere involvement 4, 6
• Failure to return to baseline alertness within several hours: This suggests underlying structural pathology requiring urgent evaluation 7, 5

Critical pitfall: In children under 3 years, generalized-appearing motor seizures may actually arise from focal EEG abnormalities in 37% of cases, making clinical observation alone insufficient for definitive diagnosis. 7, 4

Electroencephalogram (EEG) - The Diagnostic Standard

EEG is recommended as part of the neurodiagnostic evaluation and is essential for definitive diagnosis. 7

The hallmark EEG finding is:

• Focal spikes or sharp waves originating from a portion of one cerebral hemisphere 2
• Ictal discharges originating from one hemisphere during recorded seizures 1
• In infants and toddlers, focal motor seizures are reliably associated with focal EEG seizures in the contralateral hemisphere 4

Important caveat: A normal routine EEG cannot rule out focal seizures, as interictal recordings may miss epileptiform activity. Video-EEG monitoring may be critical for clarifying the focal nature of epilepsy in this age group. 2, 4

Neuroimaging - MRI as the Preferred Modality

MRI with an epilepsy protocol is the preferred neuroimaging study and should be seriously considered in any child with a seizure of partial (focal) onset. 7, 1

Specific indications for MRI in this 2.5-year-old include:

• Any focal seizure presentation: Focal features show considerably higher positive neuroimaging yields (50% on CT, even higher on MRI) compared to generalized seizures 1
• Failure to return to baseline within several hours: Emergent neuroimaging should be performed 7, 5
• Age under 3 years: Children aged <1 year warrant serious consideration for nonurgent MRI, and this principle extends to very young children 7

MRI superiority over CT: MRI detected abnormalities not identified by CT in 47% of children with focal seizures, and 29% of abnormal intracranial findings were missed on initial CT. 1

The recommended MRI protocol includes:

• 3T scanner with dedicated epilepsy protocol 1
• T1-weighted volumetric acquisition with 1mm isotropic voxels 1
• High-resolution coronal slices optimized for hippocampal pathology 1

Laboratory Evaluation

Laboratory studies should be targeted based on clinical circumstances:

• Serum glucose: Hypoglycemia can present with focal neurologic deficits and should be checked in all patients with focal features 1
• Electrolytes (sodium, calcium, magnesium): Glucose abnormalities and hyponatremia are the most frequent metabolic causes of focal deficits, though usually predicted by history and physical examination 1
• Toxicologic screening: Should be considered if there is any question of drug exposure 7

Key principle: Laboratory abnormalities are rarely the sole cause of focal seizures in otherwise healthy children, but metabolic derangements must be excluded. 1

Lumbar Puncture - Limited Role

Lumbar puncture is of limited value in the child with a first non-febrile seizure and should be used primarily when there is concern about possible meningitis or encephalitis. 7

Consider LP when:

• Fever is present with focal seizures 7
• Altered mental status persists 1
• Clinical suspicion for CNS infection exists (e.g., anti-NMDA receptor encephalitis can present with new-onset focal seizures) 3

Diagnostic Algorithm for This 2.5-Year-Old

1. Document the clinical seizure semiology: Identify any focal features (unilateral movements, versive head/eye deviation, focal weakness, behavioral arrest)
2. Obtain EEG: Standard EEG is recommended; video-EEG monitoring if routine EEG is non-diagnostic but clinical suspicion remains high 7, 4
3. Perform MRI brain with epilepsy protocol: This is the definitive imaging study for focal seizures 7, 1
4. Check targeted laboratory studies: Glucose, basic metabolic panel if clinical features suggest metabolic derangement 1
5. Consider LP only if: Fever, altered mental status, or clinical concern for CNS infection 7

Structural Causes to Consider

Approximately 4% of children with first-time afebrile focal seizures have urgent intracranial pathology, most commonly:

• Infarction, hemorrhage, and thrombosis 1
• Focal cortical dysplasia: Can present with focal seizures and may be subtle on imaging 8
• Tumors, vascular malformations, developmental abnormalities 1
• Seizure-associated brain pathology 1

The combination of characteristic focal clinical features plus focal EEG abnormalities establishes the definitive diagnosis, with MRI providing essential information about underlying structural etiology. 1, 2, 4