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KEY FACTS

Terminology
- Seizure-associated neuronal loss & gliosis in hippocampus & adjacent structures
Imaging
- Primary features: Abnormal T2 hyperintensity, hippocampal volume loss/atrophy, obscuration of internal architecture
- Secondary signs: Ipsilateral fornix & mammillary body atrophy, enlarged ipsilateral temporal horn, & choroidal fissure
- ↑ hyperintensity on DWI (T2 shine-through)
- ↓ NAA in hippocampus, temporal lobe
Top Differential Diagnoses
- Status epilepticus
- Low-grade astrocytoma
- Choroidal fissure cyst
- Hippocampal sulcus remnant
- Focal cortical dysplasia
Pathology
- Prolonged febrile seizures may produce acute hippocampal injury → subsequent atrophy
- Coexistent 2nd developmental lesion in 15% of patients with mesial temporal sclerosis (MTS)
Clinical Issues
- Partial complex seizures
- Often history of childhood febrile or medically intractable seizures
- Surgical temporal lobectomy reserved for medically intractable seizures, intolerable drug side effects
Diagnostic Checklist
- Most common cause of partial complex epilepsy in adult age group
- Low-grade neoplasms & focal cortical dysplasia more common causes of partial complex epilepsy than MTS in pediatric age group

TERMINOLOGY

Abbreviations
- Mesial temporal sclerosis (MTS)
Synonyms
- Ammon horn sclerosis, hippocampal sclerosis (HS)
Definitions
- Seizure-associated neuronal loss & gliosis in hippocampus & adjacent structures

IMAGING

General Features

Best diagnostic clue

- Primary features: Abnormal T2 hyperintensity, hippocampal volume loss/atrophy, obscuration of internal architecture
- Secondary signs: Ipsilateral fornix & mammillary body atrophy, enlarged ipsilateral temporal horn, & choroidal fissure
- Additional findings: Loss of ipsilateral hippocampal head (pes) digitations, parahippocampal gyrus white matter (WM) atrophy, ↑ T2 signal in anterior temporal WM

Location

- Mesial temporal lobe(s), 15-20% bilateral but usually asymmetric
- Hippocampus > amygdala > fornix > mammillary bodies

Size

- Slight to marked ↓ in hippocampal volume

Morphology

- Abnormal shape, size of affected hippocampus

CT Findings

NECT

- Usually normal; CT insensitive to MTS

MR Findings

T1WI

- ↓ hippocampal size
- Loss of normal hippocampal gray-white differentiation
- ± ipsilateral fornix, mammillary body atrophy
- Quantitative hippocampal volumetry: ↑ sensitivity of MTS detection (particularly bilateral MTS)

T2WI

- Hippocampal atrophy
- Obscuration of normal internal architecture
- ↑ hippocampal signal intensity
- ± ipsilateral fornix, mammillary body atrophy, dilatation of ipsilateral temporal horn
- ± abnormal hyperintensity, volume loss in ipsilateral anterior temporal lobe

FLAIR

- Hyperintense signal in abnormal hippocampus

DWI

- ↑ hyperintensity on DWI (T2 shine-through)
- ↑ diffusivity on ADC
- DTI: ↓ fractional anisotropy affecting widespread WM tracts, extensive areas with ↑ radial diffusivities independent of disease side

T1WI C+
```
- No enhancement
```

MRS

- ↓ NAA in hippocampus, temporal lobe
- ↓ NAA/Cho & ↓ NAA/Cho+Cr suggests MTS
- ± lactate/lipid peaks after 24 hours of continual seizure

Angiographic Findings
- Presurgical Wada testing: Neuropsychologic testing after intracarotid amobarbital (Amytal) injection - Lateralize memory & language functions - Predict postoperative memory loss, feasibility of surgery - May help lateralize seizure onset
- fMRI mapping replacing Wada testing
Nuclear Medicine Findings
- FDG PET: Hypometabolism in abnormal mesial temporal lobe
- SPECT: Hypoperfusion (interictal) or hyperperfusion (ictal) in epileptogenic zone (EZ) - Sensitivity of ictal > interictal

Imaging Recommendations

Best imaging tool

- High-resolution MR imaging
- MRS, quantitative volumetry may help lateralize MTS in difficult cases

Protocol advice

- Thin-section coronal T2WI & FLAIR (3 mm), coronal 3D SPGR (1-2 mm), angled perpendicular to long axis of hippocampus

DIFFERENTIAL DIAGNOSIS

Status Epilepticus
- Clinical history of multiple seizures or status epilepticus
- Temporary T2 hyperintensity ± gyriform enhancement in affected cortex, hippocampus
Low-Grade Astrocytoma
- Hyperintense temporal lobe WM mass (usually nonenhancing)
- ± seizures, young adults typical
Choroidal Fissure Cyst
- Asymptomatic CSF signal cyst in choroidal fissure distorts normal hippocampus - Round on axial, coronal images - Oval, parallels temporal lobe long axis on sagittal imaging
- No abnormal T2 hyperintensity in mesial temporal lobe
Hippocampal Sulcus Remnant
- Failure of normal hippocampal sulcus involution → asymptomatic cyst between dentate gyrus, cornu ammonis (CA)
- Common normal variant (10-15%)
Cavernous Malformation
- Heterogeneous hyperintense "popcorn" lesion with dark complete hemosiderin rim
- ± seizures
Dysembryoplastic Neuroepithelial Tumor
- Demarcated "bubbly," variably enhancing cortical mass ± regional cortical dysplasia
- Partial complex seizures
Focal Cortical Dysplasia
- Most common dual pathology associated with MTS
- T2 hyperintensity in anterior temporal WM

PATHOLOGY

General Features

Etiology

- Controversial whether acquired or developmental
        - Acquired: Follows complicated febrile seizures (FS), status epilepticus, encephalitis
        - Developmental: 2nd developmental lesion identified in 15%
        - 2-hit hypothesis: (1) Initial precipitating injury (like complicated seizures), (2) ↑ vulnerability (such as genetic predisposition or developmental anomaly)
        - Most likely MTS represents common outcome of both acquired & developmental processes
- FS most common childhood seizure disorder (2-5%)
        - Prolonged FS may produce acute hippocampal injury → subsequent atrophy

Genetics

- Familial cases of mesial temporal lobe epilepsy (TLE), FS reported
- Recent studies suggest relationship between FS & later epilepsy development may be genetic
- Syndrome-specific genes for FS (channelopathies) account for small proportion of FS cases

Associated abnormalities

- Coexistent 2nd developmental lesion (15%)

Gross Pathologic & Surgical Features
- Normal hippocampus divided anatomically into head (pes), body, tail - Subdivision into Ammon horn, dentate gyrus, hippocampal sulcus, fimbria, alveus, subiculum, parahippocampal gyrus, collateral sulcus
- Mesial temporal lobe atrophy: Hippocampal body (85-90%), tail (60%), head (50%), amygdala (10%)
- Absence of hemorrhage or necrosis
- HS described by gross pathologists as shrunken & hardened hippocampus with characteristic histologic neuronal loss, glial proliferation
Microscopic Features
- Chronic astrogliosis with fine fibrillary background of bland astrocytic nuclei & ↓ residual neurons - Selective loss of inhibitory interneurons, abnormal axonal sprouting, reorganization of neural transmitter receptors, alterations in 2nd messenger systems, & hyperexcitability of granule cells
- Ammon horn, CA, contains 4 zones of granular cells: CA1, CA2, CA3, CA4 - CA1, CA4 pyramidal cell layers most susceptible to ischemia - All hippocampal regions may show varying neuronal cell loss

CLINICAL ISSUES

Presentation

Most common signs/symptoms

- Partial complex seizures, automatisms
        - Simple at younger ages, increasingly complex & discrete with age
- Motor manifestations ↓ with ↑ age; less abundant in adults

Other signs/symptoms

- May progress to generalized tonic-clonic seizures

Clinical profile

- Often history of childhood febrile or medically intractable seizures
        - History of complex or prolonged FS ↑ risk for development of hippocampal injury, MTS
- Surface electro- (EEG) or magneto- (MEG) encephalogram helpful for localization (60-90%)
- Intracranial EEG (subdural or depth electrodes) may be indicated if noninvasive studies discordant

Demographics

Age

- Disease of older children, young adults

Sex
```
- No sex predominance
```

Epidemiology

- MTS accounts for majority of epilepsy patients undergoing temporal lobe seizure surgery

Natural History & Prognosis
- Anterior temporal lobectomy 70-90% successful in MTS, 40-55% if MR normal
- ↓ surgical success when amygdala involved
Treatment
- Clinical management based on phenotypic features of initial febrile & subsequent seizures
- Medical treatment initial approach
- Surgical temporal lobectomy reserved for medically intractable seizures, intolerable drug side effects - Resection includes anterior temporal lobe, majority of hippocampus, variable portions of amygdala - Surgical resection of EZ
- MR-guided laser interstitial thermal therapy (MRgLITT) less invasive option for stereotactic ablation of EZ

DIAGNOSTIC CHECKLIST

Consider
- Most common cause of partial complex epilepsy in adults
- Bilateral in 15-20%; difficult to detect without quantitative volumetry unless severe
- MTS imaging findings not found in normal seizure-free patients (controversial)
Image Interpretation Pearls
- Coronal high-resolution T2WI, FLAIR MR most sensitive for MTS; dual pathology in 15%
- In pediatric age group, low-grade neoplasms & cortical dysplasia more common causes of partial complex epilepsy than MTS

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References

Selected References

Images

Selected Images

Coronal graphic depicts the characteristic appearance of mesial temporal sclerosis (MTS). The right hippocampus is small (atrophic) with loss of normal internal architecture, reflecting neuronal loss and gliosis. Note concordant atrophy of the ipsilateral fornix and widening of the ipsilateral temporal horn and choroidal fissure.

Coronal T2 MR at 3.0T demonstrates normal bilateral hippocampal anatomy , size, and signal intensity. Note distinct layers of gray and white matter in the hippocampus.

Coronal T1 true inversion recovery MR at 3.0T shows asymmetric right hippocampal volume loss and obscuration of normal internal gray-white differentiation. The ipsilateral fornix is smaller than the normal left fornix.

Coronal T2 MR at 3.0T in the same patient with right hippocampal sclerosis (HS) shows hippocampal volume loss and obscuration of normal internal architecture but normal T2 signal intensity. FLAIR better shows the increase in signal intensity.

Coronal T2 MR in a patient with epilepsy shows a vertical left collateral sulcus and a globular hippocampus . This common variation is due to failure of hippocampal inversion.

Coronal T2 MR in a patient with temporal lobe epilepsy demonstrates primary and secondary imaging features of MTS. There is severe left hippocampal atrophy and hyperintensity . Secondary features are atrophy of the left fornix and mammillary body as well as dilation of temporal horn .

Coronal T2-weighted MR at 3.0T in a patient with prolonged febrile seizure shows abnormal enlargement and T2 hyperintensity in the right hippocampus . DWI (not shown) revealed reduced diffusion. The patient later developed HS.

Coronal FLAIR MR in a patient with longstanding partial complex epilepsy shows bilateral hyperintense hippocampi with loss of normal architecture, volume loss, R > L, consistent with bilateral mesial temporal sclerosis.

Coronal T2WI MR in a patient with chronic seizures with large right temporal lobe cavernous malformation demonstrates all 3 primary determinants of right HS (volume loss, T2 hyperintensity, and loss of internal architecture).

Coronal FLAIR MR in the same patient with right temporal lobe cavernous malformation better shows HS . Hyperintensity is usually more conspicuous on FLAIR, while T2 is better for depicting internal structure.

Additional Images

Coronal T2WI MR shows classic left hippocampal sclerosis with abnormal T2 hyperintensity, ipsilateral atrophy, and loss of internal architecture.

Coronal T1WI 3D SPGR demonstrates classic left HS with ipsilateral atrophy and loss of normal internal gray-white differentiation.

Coronal FLAIR MR in a patient with complex partial epilepsy with left hippocampal sclerosis shows ipsilateral volume loss and conspicuous hyperintensity resulting from gliosis.

Coronal T2WI MR in a 23 year old with complex partial seizures reveals abnormal hyperintensity and atrophy of the right hippocampus . Loss of internal architecture is also seen, typical of MTS.

Coronal T2WI MR shows abnormal hyperintense signal and atrophy of the right anterior temporal lobe related to prior injury.

Coronal T2WI MR in a patient with acute complex partial seizures shows abnormal enlargement and hyperintensity of the right hippocampus . Follow-up imaging 9 months later (not shown) confirmed subsequent development of right MTS.

Coronal STIR MR at 3.0T in a normal nonepileptic patient shows a prominent left collateral sulcus that changes the morphology of the adjacent normal hippocampus. This common anatomical variant can be mistaken for HS.

Coronal STIR MR at 3.0T in an individual who had been born prematurely with developmental delay shows diffuse white matter volume loss (L > R) and concordant left hippocampal volume loss with the abnormal T2 hyperintensity of HS.

Coronal T2WI FS MR in a patient with longstanding partial complex epilepsy shows bilateral shrunken, hyperintense hippocampi associated with diminished white matter volume in temporal lobes, consistent with bilateral HS.

Coronal STIR MR at 3.0 T in a normal nonepileptic patient imaged for headaches demonstrates normal bilateral hippocampal anatomy , size, and signal intensity.

Coronal T2 MR in a patient with epilepsy shows a vertical left collateral sulcus and a globular hippocampus . This common variation can be mistaken for HS.

Coronal T2 MR in a patient with longstanding partial complex epilepsy shows bilateral hyperintense hippocampi with volume loss, R > L, consistent with bilateral MTS.

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KEY FACTS

Terminology

Imaging

Top Differential Diagnoses

Pathology

Clinical Issues

Diagnostic Checklist

TERMINOLOGY

Abbreviations

Synonyms

Definitions

IMAGING

General Features

Best diagnostic clue

Location

Size

Morphology

CT Findings

NECT

MR Findings

T1WI

T2WI

FLAIR

DWI

T1WI C+

MRS

Angiographic Findings

Nuclear Medicine Findings

Imaging Recommendations

Best imaging tool

Protocol advice

DIFFERENTIAL DIAGNOSIS

Low-Grade Astrocytoma

Dysembryoplastic Neuroepithelial Tumor

PATHOLOGY

General Features

Etiology

Genetics

Associated abnormalities

Gross Pathologic & Surgical Features

Microscopic Features

CLINICAL ISSUES

Presentation

Most common signs/symptoms

Other signs/symptoms

Clinical profile

Demographics

Age

Sex

Epidemiology

Natural History & Prognosis

Treatment

DIAGNOSTIC CHECKLIST

Consider

Image Interpretation Pearls

References

Selected References

Images

Selected Images

Additional Images

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