What investigations beyond electroencephalogram (EEG) are necessary to identify potential underlying causes of recurrent focal sensory seizures?

Investigations Beyond EEG for Recurrent Focal Sensory Seizures

MRI head with a dedicated epilepsy protocol is the essential investigation beyond EEG for recurrent focal sensory seizures, as it detects structural abnormalities in 55% of patients with focal seizures compared to only 18-30% with CT. 1, 2

Primary Neuroimaging Investigation

MRI Head with Dedicated Epilepsy Protocol

MRI should be the primary imaging modality for all patients with recurrent focal sensory seizures unless contraindicated. 1, 2 The dedicated protocol must include:

• Coronal T1-weighted imaging (3mm slices) perpendicular to the hippocampal long axis 2
• High-resolution 3D T1-weighted gradient echo with 1mm isotropic voxels 2
• Coronal T2-weighted sequences 2
• Coronal and axial FLAIR sequences 2
• 3T MRI is preferred over 1.5T when available for superior lesion detection 2

The superiority of MRI is substantial: it demonstrates focal brain abnormalities in 55% of children with seizures versus only 18% detected by CT, and 29% of abnormal findings on MRI are completely missed by initial CT. 1

Structural Pathologies to Identify

MRI can detect the underlying causes of focal sensory seizures including:

• Hippocampal sclerosis 1, 3
• Focal cortical dysplasia 1, 3
• Developmental abnormalities 1, 4
• Neoplasms 1
• Vascular malformations 4
• Gliosis from prior injury 1
• Infarction, hemorrhage, or thrombosis 1

When to Add IV Contrast to MRI

IV contrast is not routinely necessary but should be added when: 2

• Neoplasm is suspected based on clinical features or initial non-contrast images 2
• Inflammatory or autoimmune encephalitis is considered 1, 2
• Infection is suspected 2
• Initial non-contrast images are insufficient for diagnosis 2

Laboratory Investigations

Essential Blood Tests

Check serum glucose and sodium in all patients with focal seizures, as these are the most frequent metabolic abnormalities causing focal neurologic deficits. 4

Autoimmune and Inflammatory Workup

When MRI shows inflammatory changes or clinical features suggest autoimmune encephalitis, obtain: 1

• Serum neuronal autoantibodies 1
• Lumbar puncture with CSF analysis including neuronal autoantibodies, inflammatory markers (IgG index and oligoclonal bands), and infection studies 1

This is particularly important because autoimmune encephalitis is a major cause of new-onset refractory seizures and requires immunotherapy rather than antiepileptic drugs alone. 1

Advanced Functional Imaging for Surgical Planning

FDG-PET/CT Brain

FDG-PET should be obtained when MRI is normal but seizures persist and surgical treatment is being considered. 1, 2, 5

• Sensitivity is 87-90% for temporal lobe epilepsy 2, 5
• Sensitivity is 38-55% for extra-temporal lobe epilepsy 2, 5
• The hallmark finding is reduced glucose metabolism at the epileptic focus, often more extensive than the structural abnormality 3

Ictal SPECT

SPECT with statistical ictal SPECT co-registered to MRI (SISCOM) identifies the hyperperfusion focus in 84% of patients when surgical planning is needed. 2, 5

• Requires injection during a seizure captured on video-EEG monitoring 3
• Must be compared with interictal SPECT and correlated with MRI 3
• Interictal SPECT alone is not reliable 3

When CT Head is Appropriate

CT head without contrast should only be used in emergent situations requiring rapid assessment: 2, 4, 5

• Acute trauma with suspected intracranial hemorrhage or mass effect 1
• Patient is unstable or requires close monitoring during scanning 2
• Immediate intervention may be needed 4, 5

CT has critical limitations: it misses 29% of abnormalities visible on MRI and has poor detection of orbitofrontal and medial temporal lesions. 2, 4

Neoplasm Screening in Specific Contexts

If autoimmune encephalitis is diagnosed, screen for associated neoplasm with: 1

• CT chest, abdomen, and pelvis 1
• Mammogram/breast MRI in women 1
• Pelvic or testicular ultrasound 1
• Body PET if initial screening is negative 1

Common Pitfalls to Avoid

Do not assume a normal CT excludes structural abnormality—MRI may still reveal significant pathology in up to 29% of cases. 2, 4

Do not delay MRI in favor of repeated EEGs when focal seizures are recurrent—the structural lesion detection rate is 55% and directly impacts treatment decisions. 1, 2

Do not order routine MRI protocols—specifically request a dedicated epilepsy protocol with the sequences listed above, as standard protocols may miss subtle cortical dysplasia. 1, 2