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KEY FACTS

Imaging
- Nuclear imaging is not part of routine evaluation of isolated seizures - Plays important role in multidisciplinary/multimodality work-up of intractable epilepsy
- Nuclear imaging may identify structurally inconspicuous epileptogenic foci
- SPECT: Radiotracer deposition reflects regional cerebral blood flow - Interictal: Epileptogenic focus appears as area of decreased radiotracer deposition (hypoperfusion) - Ictal: Epileptogenic focus appears as area of increased radiotracer deposition (hyperperfusion) - SISCOM may identify epileptogenic foci that are inconspicuous on visual assessment of ictal and interictal SPECT images
- F-18 FDG PET: Epileptogenic focus appears as area of hypometabolism (larger than epileptogenic focus) - Statistical parametric mapping can help detect and confirm foci of hypometabolism
- MR is ideal modality to identify structural causes of seizure/epilepsy - Mesial temporal sclerosis: Decreased volume, increased T2 signal in hippocampus - Focal cortical dysplasia (FCD): Focal cortical thickening and increased T2 signal, blurring of gray-white junction
Clinical Issues
- Most common cause of epilepsy - Young: FCD - Adolescent/young adult: Mesial temporal sclerosis
- Antiepileptic drugs are 1st line of therapy
- Surgical resection: Reserved for patients with intractable, focal epilepsy

TERMINOLOGY

Definitions
- Seizure: Clinical manifestation of aberrant neuronal electrical discharge(s) in brain
- Epilepsy: ≥ 2 unprovoked, afebrile seizures

IMAGING

General Features
- Imaging of isolated/acute seizures is generally structural (CT/MR) and reserved for patients with focal or complex seizures or focal neurologic signs
- Imaging is important component in management of epilepsy - Structural imaging (CT/MR) often performed at diagnosis to exclude structural cause - Multimodality imaging (including nuclear) plays substantial role in work-up of intractable epilepsy
- Work-up of intractable epilepsy is multidisciplinary/multimodality process - Clinical assessment: History, seizure semiology - Electroencephalography (EEG): Scalp and intracranial - MR: Structural, ± functional - Perfusion (SPECT) imaging - Metabolic (F-18 FDG PET) imaging - Magnetoencephalography
Nuclear Medicine Findings
- Nuclear imaging not part of routine evaluation of isolated seizures
- May identify structurally inconspicuous epileptogenic foci
- SPECT perfusion - Radiotracer deposition reflects regional cerebral blood flow - Tc-99m hexamethylpropyleneamine oxime (HMPAO) and Tc-99m ethyl cysteinate dimer (ECD) most commonly used tracers - Lipophilic, small molecules diffuse across blood-brain barrier - ECD clears more rapidly from blood pool, has more linear extraction at high blood flow rates; less nonspecific scalp and soft tissue uptake than HMPAO - High 1st-pass extraction, peak accumulation in ~ 2 min, no substantial redistribution - Can image for at least 2 hours after injection without substantial loss of fidelity - Interictal SPECT - Epileptogenic focus appears as area of decreased radiotracer deposition (hypoperfusion) - Ictal SPECT - Epileptogenic focus appears as area of increased radiotracer deposition (hyperperfusion) - Correspondence to epileptogenic focus depends on interval between seizure onset and injection; longer intervals allow more propagation to surrounding tissue - Beware of pseudonormalization where hyperperfusion during ictus makes baseline hypoperfused focus appear symmetric to normal side - Higher sensitivity than interictal SPECT for epileptogenic focus - Subtraction ictal SPECT coregistered to MR (SISCOM) - Means to compare ictal and interictal SPECT imaging and localize abnormalities to MR - Foci of ictal hyperperfusion that correspond with interictal hypoperfusion appear as foci of activity on SISCOM - May identify epileptogenic foci that are inconspicuous on visual assessment of ictal and interictal SPECT images
- F-18 FDG PET - Interictal exam due to prolonged (~ 30 min) uptake of F-18 FDG - If seizure occurs during uptake phase, may see hypermetabolism at epileptogenic focus and in surrounding brain - Indirect marker of neuronal activity - Epileptogenic focus appears as area of hypometabolism - Area of hypometabolism is generally larger than epileptogenic focus - Highest accuracy in temporal lobe epilepsy (TLE), less likely to identify epileptogenic focus in extratemporal epilepsy - Contributes most in cases of structurally inconspicuous TLE or cases of suspected cortical dysplasia in children with apparent negative MR - Higher sensitivity than interictal SPECT, especially in TLE (84% vs. 66% in one metaanalysis) - Postprocessing with statistical parametric mapping (SPM) can help detect and confirm foci of hypometabolism - SPM compares, on pixel by pixel basis, F-18 FDG uptake in patient to normal database to identify foci of abnormally decreased uptake - In addition to identifying epileptogenic focus, it may provide prognostic information - For example, in patients with unilateral temporal lobe epilepsy, presence of bitemporal glucose hypometabolism is associated with poor memory performance
- With both SPECT and PET imaging, may see corresponding downstream abnormality in cerebellar hemisphere opposite involved cerebral hemisphere - Decreased radiotracer uptake on interictal SPECT and F-18 FDG PET - Increased radiotracer uptake on ictal SPECT
- Hypometabolism of ipsilateral thalamus can be seen in cases of focal epilepsy (mostly in frontal and temporal cortex, medial temporal lobe epilepsy)
- Other tracers - C-11 flumazenil: Binds CNS gamma-aminobutyric acid (GABA) receptors; GABA receptors are decreased in epileptogenic foci - C-11 has short half-life (20 minutes), requiring onsite cyclotron (limiting its use)
MR Findings
- Mesial temporal sclerosis: Decreased volume and increased T2 signal in mesial temporal structures, including hippocampal formation - Resultant asymmetry in size of temporal horn of lateral ventricle
- Focal cortical dysplasia (FCD): Cortical thickening and increased T2 signal, extension of T2 signal to ventricle, blurring of gray-white junction

DIFFERENTIAL DIAGNOSIS

CNS Tumor
- May or may not be cause of seizures
- May appear as focus of hypo- or hyperperfusion on SPECT
- May appear as focus of hypometabolism on F-18 FDG PET if low grade
Congenital Anomalies
- Structural abnormalities: Heterotopic gray matter, abnormal sulcation, schizencephaly
- May appear as focus of hypometabolism on FDG PET, even if not epileptogenic
Tuberous Sclerosis
- Can be difficult to identify epileptogenic focus as majority of tubers are hypoperfused and hypometabolic
- Epileptogenic tubers generally show perfusion/metabolic abnormalities larger than area of structural abnormality
Rasmussen Encephalitis
- Progressive inflammatory process involving unilateral cerebral hemisphere, generally with progressive cerebral atrophy
- Appears as large areas (lobar or hemispheric) of perfusion or metabolic abnormality

PATHOLOGY

Gross Pathologic & Surgical Features
- Mesial temporal sclerosis: Hard, shrunken hippocampus
- FCD: Firm, rubbery cortical focus
Microscopic Features
- Mesial temporal sclerosis - Variable distribution of pyramidal neuronal loss - Gliosis
- FCD: Abnormalities in neuronal migration resulting in cortical dyslamination - Type I: Abnormal cortical layering - Ia: Abnormal radial migration of neurons with abundant microcolumns (> 8 neurons vertically arranged), most conspicuous in layer 3 - May see immature small neurons or hypertrophic pyramidal neurons outside of layer 5 - Ib: Abnormal tangential cortical layering - May see no layering at all or abnormal layering of layers 2 &/or 4 - May see immature small neurons, hypertrophic pyramidal neurons outside of layer 5, or normal neurons with disordered dendrites - Ic: Abnormal radial and tangential lamination - Type II: Abnormal cortical layering andcytologic abnormalities - IIa: No identifiable cortical layering except layer 1 - Dysmorphic neurons (enlarged cell body and nucleus, neurofilament accumulation in cytoplasm) - IIb: No identifiable cortical layering except layer 1 - Dysmorphic neurons and balloon cells (large cell body, multiple nuclei, eosinophilic cytoplasm) - Type III: Abnormal cortical layering associated with primary brain lesion (adjacent or in same lobe) - IIIa: FCD associated with hippocampal sclerosis - IIIb: FCD associated with tumors - IIIc: FCD associated with vascular malformation - IIId: FCD associated with any other lesion acquired in early life

CLINICAL ISSUES

Demographics

Epidemiology

- Seizure
        - Most common causes of acute seizures: Fever, infection, head injury
        - Febrile seizures
                  - Usually occur between 6 months and 5 years of age
                  - Occur in 3-8% of children < 5 years
                  - 60% risk of recurrence
- Epilepsy
        - Highest incidence in 1st year of life (~ 90-200 per 100,000)
        - Prevalence higher in rural areas
        - Risk factors
                  - Family history
                  - Prior febrile seizure: 2-7% develop epilepsy
        - Most common causes
                  - Young: FCD
                  - Adolescent/young adult: Mesial temporal sclerosis
        - Intractable epilepsy occurs in 20-30%

Natural History & Prognosis
- Epilepsy subtypes: Current terminology - Genetic abnormality without discrete structural abnormality - Structural or metabolic - Discrete structural epileptogenic lesion - Many of these are genetic in etiology - Metabolic condition leading to propensity for seizures - Unknown cause
- FCD: Malformation of cortical development - Secondary to insult (genetic, infectious, ischemic) during development
- Mesial temporal sclerosis - Often secondary to insult (infection, trauma, febrile seizures) early in life
Treatment
- Antiepileptic drugs - 1st line of therapy - Managing clinician will often try multiple agents and combinations of agents to achieve seizure reduction (or freedom) with minimum of side effects
- Ketogenic diet
- Vagal nerve stimulator - Used in patients with drug-resistant epilepsy who are not candidates for resection
- Deep brain stimulator - Used in patients with drug-resistant epilepsy who are not candidates for resection
- Surgical resection: Reserved for patients with drug-resistant, focal epilepsy - Outcomes better if resection includes sites identified on SISCOM - Outcome of surgery for FCD is better if lesion visible by MR

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References

Selected References

Images

Selected Images

Coronal interictal F-18 FDG PET in a 15-year-old with focal left temporal seizures on EEG and findings of left hippocampal sclerosis on MR shows diffuse hypometabolism of the left temporal lobe , indicative of a seizure focus. Interpretation of functional brain imaging is aided by the fact that perfusion (SPECT) and metabolism (F-18 FDG) should be symmetric between the cerebral hemispheres and between the cerebellar hemispheres.

Coronal FLAIR MR in a 16-year-old boy with intractable epilepsy shows an area of gliosis in the left parietal lobe related to remote trauma .

Coronal F-18 FDG PET in the same patient shows hypometabolism corresponding to the left parietal gliosis , as well as hypometabolism (relative to the right) of adjacent parietal lobe . This corroborates SPECT findings and suggests a seizure focus.

Interictal Tc-99m ECD SPECT in the same patient shows hypoperfusion corresponding to the left parietal gliosis and involving the adjacent parietal lobe .

Ictal Tc-99m ECD SPECT in the same patient shows hypoperfusion corresponding to the left parietal gliosis . The adjacent parietal lobe , however, is hyperperfused (relative to the right), reflecting seizure activity.

Sagittal T1 C+ MR in a 17-year-old girl shows a frontal dysembryoplastic neuroepithelial tumor (DNET) as a nonenhancing, low-signal lesion .

Sagittal F-18 FDG PET in the same patient shows focal hypometabolism associated with the DNET . In this case, the hypometabolism is reflective of both the low grade of this tumor and its epileptogenicity.

Axial T2 MR in a 12-year-old girl with epilepsy shows a right temporal focal cortical dysplasia (FCD), apparent as cortical and subcortical increased signal .

Axial F-18 FDG PET in the same patient shows hypometabolism associated with the FCD , supporting the epileptogenic nature of this lesion.

Coronal F-18 FDG PET in the same patient shows global hypometabolism of the right temporal lobe with more conspicuous hypometabolism of the hippocampus . These findings support a right temporal localization of the seizure focus.

Interictal Tc-99m ECD SPECT in a 2-year-old with intractable epilepsy shows enlargement of the left lateral ventricle and hypoperfusion of the left temporal lobe .

Ictal Tc-99m ECD SPECT in the same patient shows an area of relative hyperperfusion in the overall hypoperfused left temporal lobe, suggesting a seizure focus.

Coronal SISCOM derived from the interictal and ictal SPECT and T1 MR in the same patient confirms an area of abnormal perfusion in the left temporal lobe , suggestive of a seizure focus. The left lateral ventricle is dilated due to extensive white matter loss.

Axial FLAIR MR in a 12-year-old with tuberous sclerosis and increasing seizure frequency shows multiple cortical tubers .

Axial interictal F-18 FDG PET in the same patient at the same level shows hypometabolism associated with the cortical tubers .

Axial interictal Tc-99m ECD SPECT in the same patient shows hypoperfusion corresponding to the cortical tubers .

Fused axial F-18 FDG PET and FLAIR MR in the same patient shows correspondence between areas of hypometabolism and the cortical tubers . Hypometabolism is similar in extent to the tubers. Areas of hypometabolism larger than underlying lesions can suggest an epileptogenic focus.

Coronal interictal F-18 FDG PET in a 7-year-old with intractable seizures shows hypometabolism in the right temporal lobe , particularly mesially in the hippocampal formation .

Coronal T2 FLAIR MR at the same level in the same patient shows abnormally increased signal in the right hippocampal formation in this child with mesial temporal sclerosis.

Additional Images

Fused FDG PET MR in a 9-year-old boy with left lower extremity clonic movements shows a focal region of hypometabolism within the right paracentral lobule (left-foot motor control).

FDG PET MR postprocessing demonstrates same area of hypometabolism localized within the right paracentral lobule.

Coronal T2 MR in the same patient shows focal area of blurring of the gray matter-white matter junction and abnormal gyration in the right paracentral lobule, suggesting FCD (initially missed on MR).

Left ankle flexion fMRI activated the right superior frontal gyrus anterior to the expected location of the left paracentral lobule, suggesting motor remapping distant to FCD . Normal right foot motor activation centered about the left paracentral lobule .

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KEY FACTS

Imaging

Clinical Issues

TERMINOLOGY

Definitions

IMAGING

General Features

Nuclear Medicine Findings

MR Findings

DIFFERENTIAL DIAGNOSIS

CNS Tumor

Congenital Anomalies

Tuberous Sclerosis

Rasmussen Encephalitis