What are the guidelines for imaging to rule out a brain mass in an 18-month-old with a first-time focal seizure?

Neuroimaging Guidelines for an 18-Month-Old with First-Time Focal Seizure

MRI is strongly recommended as the primary neuroimaging modality for an 18-month-old with a first-time focal seizure to rule out brain mass, as it has significantly higher sensitivity (55%) compared to CT (18%) for detecting clinically relevant abnormalities. 1

Rationale for Neuroimaging in Focal Seizures

Focal seizures in children carry a much higher risk of underlying structural abnormalities compared to generalized seizures:

• Approximately 4% of children with first-time afebrile seizures with focal manifestations have urgent intracranial pathology, most commonly infarction, hemorrhage, and thrombosis 1, 2
• Young age (≤18 months) is a specific risk factor for clinically urgent intracranial pathology 2
• Positive yields from neuroimaging in focal seizures are considerably higher than in generalized seizures with normal neurologic examination 1
• The frequency of recurrence for focal seizures is up to 94%, which is significantly greater than for generalized seizures (72%) 1

Imaging Modality Selection

MRI (Preferred)

• First-line recommendation: MRI is the preferred neuroimaging modality 1, 3
• MRI demonstrates focal brain abnormalities in 55% of children with seizures, compared to only 18% with CT 1
• 29% of abnormal intracranial findings in children with new-onset afebrile seizures with focal features are not detected on initial CT but are visible on MRI 1
• MRI is superior for detecting developmental abnormalities, subtle lesions, and peri-ictal cortical abnormalities 1

CT Considerations

• Limited role in first-time seizure evaluation - 78.8% of CT scans in children with new-onset seizures show no imaging findings 1
• May be appropriate in acute settings when MRI is not immediately available or if there are concerns about acute hemorrhage or mass effect requiring urgent intervention 1
• Should be followed by MRI even if initially normal, as nearly 30% of abnormalities are missed on CT 1

Specialized MRI Protocol Recommendations

For optimal detection of epileptogenic lesions:

• Use optimized epilepsy protocol with adequate spatial resolution
• Include multiplanar reformatting
• Incorporate coronal T1, 3D volumetric T1 acquisition, and FLAIR sequences (both coronal and axial) 3
• Select specific protocols based on clinical and EEG findings that identify the region of seizure onset 1

Risk Factors Requiring Special Attention

Pay particular attention to:

• Age ≤18 months (your patient falls in this high-risk category) 2
• Persistent Todd's paresis (post-ictal focal weakness) 2
• Focal neurologic deficits on examination 1, 4
• Focal EEG abnormalities 5

Exceptions to Imaging

Certain well-characterized seizure syndromes may not require imaging:

• Benign rolandic seizures
• Benign occipital epilepsy with classic EEG findings
• These syndromes can be diagnosed clinically or through specific EEG patterns 1

Clinical Approach Algorithm

1. Confirm focal seizure characteristics through detailed seizure semiology
2. Perform neurological examination to identify any focal deficits
3. Obtain EEG to identify focal abnormalities
4. Proceed with MRI as the primary imaging modality
5. Consider CT only if MRI is not immediately available and there is concern for acute intervention
6. If CT is performed initially, follow up with MRI even if CT is normal

This approach maximizes the likelihood of detecting clinically significant abnormalities that could impact treatment decisions and long-term prognosis in this high-risk age group.