7 Epilepsy
7.1 DDx of Pediatric Epilepsy
7.1.0.1 1. Focal Cortical Dysplasia (FCD)
Epidemiology / relevance – most common substrate in children < 3 y needing surgery; ~80 % of surgically‑treated intractable cases.
MRI clues
- Focal cortical thickening.
- Blurring of the gray–white junction.
- Subcortical T2/FLAIR hyperintensity; when it forms a “transmantle” stripe to the ventricle, think type IIB with balloon cells.
Functional imaging
- Inter‑ictal FDG‑PET: focal hypometabolism larger than MRI lesion.
- Magnetic‑source imaging (MEG): clustered spikes localising epileptogenic zone.
7.1.0.2 2. Tuberous Sclerosis Complex
Key lesions – cortical/subcortical tubers, sub‑ependymal nodules, giant‑cell astrocytoma.
MRI
- Tubers: signal depends on myelination. In older infants/children, tubers are T1 hypo / T2 hyperintense.
PET & MEG
- PET: epileptogenic tuber = hypometabolic area larger than signal abnormality on MRI.
- MEG: spikes localise to active tubers.
7.1.0.3 3. Hemimegalencephaly
Pathology – hamartomatous over‑growth of one hemisphere.
MRI
- Unilateral cortical thickening ± pachygyria.
- Ipsilateral white‑matter T2 hypointensity from hypermyelination; enlarged ventricle.
Advanced techniques
- Diffusion‑tensor / tractography: increased anisotropy & fiber number on affected side.
- Ictal PET: focal cortical hypermetabolism during seizure.
7.1.0.4 4. Mesial Temporal (Hippocampal) Sclerosis
- Clinical pearl – frequent in adolescents/young adults with prior febrile‑seizure history.
- MRI: hippocampal atrophy on T1; T2/FLAIR hyperintensity in mesial temporal structures.
- PET: broader hypometabolic zone than hippocampal atrophy alone—high sensitivity for lateralisation.
7.1.0.5 5. Epilepsy‑Associated Developmental Tumors
(ganglioglioma, DNET, pleomorphic xantho‑astrocytoma, etc.)
MRI pattern
- Cortically‑based, T1 hypo‑ to iso‑, T2 hyperintense; minimal edema.
- May enhance or be partially cystic; often sits within FCD‑like cortex.
Treatment implication – gross‑total resection usually curative.
7.1.0.6 6. Hypothalamic Hamartoma
- Classic presentation – gelastic (laughing) seizures ± precocious puberty.
- MRI – sessile or pedunculated mass isointense to gray matter; non‑enhancing.
- Epileptogenic hallmark – always involves mamillary bodies; intra‑hypothalamic types linked to early severe epilepsy.
7.1.0.7 7. Rasmussen Encephalitis
- Course – progressive unilateral cortical inflammation, drug‑resistant focal seizures, hemiparesis.
- MRI – evolving unilateral cortical/subcortical T2 hyperintensity → progressive hemispheric atrophy.
- PET – inter‑ictal unilateral cortical hypometabolism extends beyond visible MRI changes.
7.1.0.8 8. Perinatal Arterial Infarction
- Presentation – neonatal seizures, later spastic hemiparesis.
- MRI – territory‑based encephalomalacia, gliosis; often MCA distribution.
- PET – concordant hypometabolism ± remote thalamic deafferentation.
7.1.0.9 9. Sturge–Weber Syndrome
Triad – facial port‑wine stain, leptomeningeal angioma, epilepsy.
Imaging
- CT: gyriform “tram‑track” calcifications, choroid‑plexus enlargement.
- MRI: leptomeningeal enhancement post‑gadolinium, cortical/subcortical T2 changes, ipsilateral atrophy.
7.1.1 One‑Glance Comparison Table
| Condition | Signature MRI Findings | Typical FDG‑PET / MEG Clue | Extra Tips |
|---|---|---|---|
| Focal cortical dysplasia | Cortical thickening, blurred GM–WM border, subcortical T2 stripe | Focal hypometabolism larger than lesion; MEG spikes | “Transmantle” sign → type IIB |
| Tuberous sclerosis | Multiple cortical tubers (age‑dependent signal) | Hypometabolic tuber bigger than MRI lesion | Look for sub‑ependymal nodules/SEGA |
| Hemimegalencephaly | Unilateral cortical thickening + T2 hypo WM, ventricular enlargement | Ictal hypermetabolism; DTI hyper‑anisotropy | Early hemispherotomy often required |
| Mesial temporal sclerosis | Hippocampal atrophy + T2/FLAIR hyperintensity | Lateralising hypometabolism wider than hippocampus | Surgical cure rates ≈ 90 % |
| Developmental tumors | Cortical, T2 bright, little edema ± cyst/enhancement | Variable; focus on structural lesion | Often associated FCD |
| Hypothalamic hamartoma | Iso‑gray, non‑enhancing hypothalamic mass | — (not typically PET‑localised) | Involves mamillary bodies |
| Rasmussen encephalitis | Progressive unilateral T2 hyperintensity → atrophy | Unilateral cortical hypometabolism | Hemispherectomy considered |
| Perinatal infarct | Territory encephalomalacia, gliosis | Matching hypometabolism, ± thalamus | Consider perinatal history |
| Sturge‑Weber | Leptomeningeal enhancement, gyriform calcification, atrophy | — (if performed, hypometabolism under angioma) | Gradient‑echo sensitive for Ca²⁺ |
GM = gray matter, WM = white matter
7.2 Hippocampal Sclerosis
Explaining Hippocampal Sclerosis
The user asks for a comprehensive explanation of hippocampal sclerosis including its definition, imaging findings, and clinical implications. I’ll cover key aspects like pathophysiology, imaging with MRI, CT, PET, SPECT, and DTI, typical findings (e.g., increased T2 signal, hippocampal atrophy), differential diagnoses, and its relevance for surgical planning. I’ll mention anatomical structures, MR sequences (T1, T2, FLAIR), gray matter loss, and associated changes like fornix atrophy. I’ll clarify that MRI is the gold standard for diagnosing this condition.
Thought for 12 seconds
7.2.1 Hippocampal Sclerosis (Mesial Temporal Sclerosis, MTS)
7.2.1.1 Clinical & Pathologic Snapshot
| Feature | Key Points |
|---|---|
| Definition | Neuron loss + gliosis—chiefly in the CA1 (Sommer sector) and CA3 regions of the hippocampus; often extends to dentate gyrus and parahippocampal gyrus. |
| Epidemiology | Most common structural cause of drug‑resistant focal epilepsy in children → young adults; slight left‑side predominance. |
| Etiologic associations | Prolonged febrile seizures, perinatal hypoxia/ischemia, trauma, CNS infection, genetic predisposition (e.g., SCN1A), dual pathology with cortical dysplasia or low‑grade tumors. |
| Clinical hallmark | Medically refractory mesial temporal lobe epilepsy (MTLE) manifesting as déjà‑vu, epigastric aura, automatisms, and post‑ictal dysphasia when dominant hemisphere is involved. |
7.2.1.2 Why Imaging Matters
- Diagnosis & lateralisation of the epileptogenic zone.
- Surgical planning (anterior temporal lobectomy / selective amygdalohippocampectomy).
- Outcome prediction—complete resection of the sclerotic hippocampus yields ~70–90 % seizure‑free rates.
7.2.1.3 MRI Protocol Essentials
| Sequence | Plane / Parameters | Rationale |
|---|---|---|
| 3D T1 MPRAGE or SPGR | Isotropic ≤ 1 mm, oblique coronal perpendicular to long axis of hippocampus | Precise volumetry & internal architecture |
| T2‑weighted fast SE / TSE | Oblique coronal, 2–3 mm | Detect hyperintensity, loss of internal lamination |
| FLAIR | Same plane | Highlights mesial temporal signal change even when T2 is subtle |
| High‑res T2 (e.g., TSE DRIVE/CUBE/VISTA) | 0.5–1 mm in‑plane, 2‑mm slice | Visualise internal stratification (“digitations”) |
| Diffusion tensor imaging (DTI) | 32+ directions | Reduced fractional anisotropy along perforant path & fornix |
| MR spectroscopy (single‑voxel PRESS) | TE = 30–35 ms | ↓N‑acetyl‑aspartate / Cr & NAA/Cho ratios ipsilaterally |
7.2.1.4 Core MRI Findings
| Finding | Typical Appearance & Tips |
|---|---|
| Hippocampal atrophy | Shrinkage on T1 ± widened choroidal fissure; compare to contralateral side and age‑matched norms. |
| T2/FLAIR hyperintensity | Reflects gliosis; often greatest in CA1 & subiculum; becomes subtler as sclerosis “matures.” |
| Loss of internal architecture | Disappearance of normal stratified lamination/“digitations” on high‑res T2. |
| Associated changes | Ipsilateral temporal‑horn dilatation, amygdala atrophy, fornix and mammillary‑body shrinkage, contralateral hippocampal subtle mirror changes (“crossed MTS”) in long‑standing cases. |
Quantitative criteria (commonly used cut‑offs)
- Volume ↓ > 10–15 % vs contralateral side or normative database.
- T2 signal intensity ↑ > 2 SD above control mean.
- Asymmetry index: \(\frac{(Right-Left)}{(Right+Left)/2} \times100\;\% > 10\%\).
7.2.1.5 Other Imaging Modalities
| Modality | Typical Finding | Utility |
|---|---|---|
| FDG‑PET (inter‑ictal) | Unilateral/bilateral temporal hypometabolism broader than MRI lesion | Highly sensitive lateraliser when MRI equivocal/normal |
| Ictal SPECT (HMPAO) | Focal mesial temporal hyper‑perfusion | Useful if PET & MRI discordant |
| CT | May show temporal‑horn enlargement & hippocampal low attenuation, but low sensitivity—MRI mandatory | |
| MEG / EEG‑fMRI | Spikes cluster over mesial temporal contacts | Complements MRI for surgical mapping |
7.2.1.6 Differential Diagnoses to Keep in Mind
| Mimic | Clues that Favour the Mimic |
|---|---|
| Low‑grade glioma (e.g., DNET, ganglioglioma) | Focal mass, cortical involvement, cystic change, contrast enhancement |
| Limbic encephalitis (paraneoplastic, autoimmune, viral) | Bilateral medial temporal T2/FLAIR hyperintensity without volume loss; clinical subacute memory loss |
| Post‑ictal edema (“pseudo‑MTS”) | Transient, symmetric FLAIR/T2 hyperintensity after status epilepticus; resolves on follow‑up |
| Developmental malformation (e.g., FCD type IIIa) | Adjacent cortical dysplasia contiguous with hippocampal changes |
7.2.1.7 Reporting Checklist
- Side (right, left, bilateral) and degree of hippocampal atrophy.
- Presence/absence of T2/FLAIR signal change.
- Temporal‑horn size and extrahippocampal atrophy (amygdala, fornix, mammillary bodies).
- Signs of dual pathology (cortical dysplasia, tumors).
- Contralateral hippocampus—normal vs subtle involvement.
- Note artifacts or limitations (motion, dental hardware).
- Correlate with EEG/PET if available; suggest surgical candidacy when appropriate.
7.2.1.8 Teaching Pearls
- T1‑only pitfall: Early childhood MTS may have minimal atrophy—always scrutinise T2/FLAIR for subtle hyperintensity.
- FLAIR superiority: In older children/adults, FLAIR often conspicuously bright even when T2 is ambiguous.
- Symmetry trap: Progressive contralateral degeneration can make both hippocampi look “equal”—volumetric analysis or normative database comparison avoids under‑calling.
- Dual pathology vigilance: Up to 15–30 % of MTS cases harbour a coexisting lesion, especially focal cortical dysplasia—missing it may lead to surgical failure.
7.2.1.9 Management Implications
- Positive MRI for unilateral MTS + concordant EEG/PET → high surgical success; early referral decreases cognitive decline.
- MRI‑negative MTLE—consider 7 T MRI, PET, invasive EEG.
- Bilateral MTS—palliative options (neuromodulation) or resection of dominant focus first.
7.2.2 Quick Reference Table—Modalities vs Hallmark Signs
| Modality / Parameter | Characteristic Finding in MTS |
|---|---|
| 3 T MRI T1 volume | Atrophied hippocampal head & body |
| Coronal T2 / FLAIR | Hyperintense mesial temporal signal |
| High‑res T2 | Loss of internal laminar “digitations” |
| DTI | Reduced fractional anisotropy in hippocampus & fimbria |
| MR Spectroscopy | ↓NAA, ↓NAA/Cr, ↓NAA/Cho |
| FDG‑PET (inter‑ictal) | Temporal lobe hypometabolism |
| Ictal SPECT | Hyper‑perfusion “hot spot” |
| CT | Temporal‑horn prominence (low sensitivity) |
Bottom line: Hippocampal sclerosis is the quintessential substrate of medically‑refractory mesial temporal lobe epilepsy. High‑resolution oblique‑coronal MRI remains the diagnostic gold standard—look for hippocampal atrophy, T2/FLAIR hyperintensity, and architectural effacement. Integrating structural MRI with functional/metabolic imaging refines lateralisation and optimises surgical outcomes.