Hippocampal Sclerosis

Hippocampal sclerosis (HS) is the most common pathology in adult focal epilepsy, particularly mesial temporal lobe epilepsy (MTLE). The pathology — selective neuronal loss in specific hippocampal subfields with reactive gliosis — produces a characteristic gross appearance and a clinical syndrome of refractory temporal lobe epilepsy with characteristic semiology, EEG pattern, and MRI findings. Resection of the affected hippocampus produces excellent seizure outcomes in many patients, making HS one of the most surgically treatable causes of refractory epilepsy. This page covers HS pathology and the broader epilepsy pathology landscape.

Hippocampal Sclerosis Pathology

Selective Vulnerability Pattern

  • CA1 (Sommer sector): most affected; severe neuronal loss with reactive astrogliosis. Most vulnerable to seizure-induced damage AND to anoxia.
  • CA3: variable; can be severely affected (especially with repeated status epilepticus).
  • CA4 (hilus of dentate): variable; severe loss in some patterns.
  • CA2: relatively spared (CA2 is more resistant).
  • Dentate granule cells: variable; sometimes granule cell dispersion (irregular widening of granule cell layer).

ILAE Classification of Hippocampal Sclerosis

  • HS ILAE Type 1: severe CA1 + CA4 loss; CA3 and CA2 variable. The classical pattern; majority of cases.
  • HS ILAE Type 2: predominantly CA1 loss; other regions less affected.
  • HS ILAE Type 3: predominantly CA4 loss; less CA1 involvement (rare; often associated with other pathology).
  • No HS: gliosis without neuronal loss; rare in temporal lobe epilepsy samples.

Associated Features

  • Granule cell dispersion: irregular widening of granule cell layer.
  • Mossy fiber sprouting: dentate granule cell axons sprout aberrantly into molecular layer.
  • Reactive astrogliosis throughout affected regions.
  • Subpial gliosis (Chaslin gliosis): astrocyte band in molecular layer.

Clinical Features (Mesial Temporal Lobe Epilepsy)

Seizure Semiology

  • Aura: epigastric rising sensation, déjà vu, jamais vu, olfactory hallucination, fear.
  • Automatisms: oroalimentary (lip-smacking, chewing), manual.
  • Impaired awareness (focal impaired awareness seizure).
  • Postictal confusion.
  • Postictal language difficulty if dominant.
  • Sometimes contralateral dystonic posturing.

Background

  • Often history of febrile seizures in childhood (especially prolonged or focal).
  • Family history of seizures in some.
  • Onset of recurrent seizures in adolescence or young adulthood.
  • Often becomes refractory over time.

Imaging

  • MRI features of HS:
    • Hippocampal atrophy (especially CA1).
    • T2/FLAIR hyperintensity.
    • Loss of internal hippocampal architecture (digitations).
    • Often unilateral (predominantly).
  • Specialized hippocampal protocols may be required.

EEG Findings

  • Anterior temporal interictal epileptiform discharges.
  • Often unilateral or with lateralized predominance.
  • Sphenoidal or other deep electrodes may reveal mesial sources.
  • Video-EEG monitoring: temporal lobe seizure onset with typical semiology.

Surgical Treatment

  • Anterior temporal lobectomy: standard procedure (anterior 4-5 cm of dominant; less if non-dominant).
  • Selective amygdalohippocampectomy: spares more neocortex.
  • Laser interstitial thermal therapy (LITT): emerging less invasive option.
  • 60-70% become seizure-free; another 20% have substantial reduction.
  • Risks: memory impairment if bilateral or dominant; field deficit; rarely psychosis.

Other Epilepsy-Associated Pathologies

Hippocampal Sclerosis of Aging (Limbic-Predominant Age-Related TDP-43 Encephalopathy — LATE)

  • Hippocampal sclerosis pattern in older patients without traditional MTLE.
  • TDP-43 inclusions (often coexists with AD pathology).
  • Contributes to amnestic dementia in elderly.

Focal Cortical Dysplasia (FCD)

Discussed in cortical malformations page. Second most common cause of refractory focal epilepsy after HS.

Tuberous Sclerosis Tubers

Cortical tubers similar to FCD type IIb. Discussed in phakomatoses.

Dysembryoplastic Neuroepithelial Tumor (DNET) and Ganglioglioma

Long-term epilepsy-associated tumors (LEATs). Discussed in circumscribed gliomas. Often present with childhood / adolescent epilepsy + temporal lobe location.

Encephalomalacia

Post-traumatic, post-infarct, post-infectious scar tissue → epileptic focus.

Sturge-Weber and Other Cortical Lesions

Discussed in respective pages.

Rasmussen Encephalitis

Chronic unilateral focal encephalitis in children:

  • T-cell mediated.
  • Progressive hemiatrophy.
  • Refractory focal seizures (often epilepsia partialis continua).
  • Pathology: chronic lymphocytic infiltration, microglial nodules, neuronal loss, gliosis.
  • Treatment: hemispherectomy is curative for seizures (with hemiparesis trade-off).

Status Epilepticus Pathology

  • Selective neuronal damage in:
    • CA1, CA3, CA4, dentate hilus of hippocampus.
    • Cortical layers III, V.
    • Cerebellar Purkinje cells.
    • Thalamus.
  • Reflects combination of intrinsic vulnerability + excitotoxic / hypoxic / metabolic injury from prolonged seizures.

SUDEP (Sudden Unexpected Death in Epilepsy)

Death in patients with epilepsy without other apparent cause. Mechanism likely combination of seizure-induced cardiac arrhythmia, central apnea, and autonomic dysregulation. Pathology often shows no specific findings; the diagnosis is by exclusion.

🔍 Did You Know?

The pattern of selective hippocampal subfield vulnerability in mesial temporal sclerosis — severe CA1 + CA3/CA4 loss with sparing of CA2 and dentate granule cells — has fascinated neurologists since Sommer described it in 1880. The CA1 vulnerability is shared with hypoxic-ischemic injury (where it produces “Sommer sector necrosis” in cardiac arrest survivors), reflecting CA1’s unusually high glutamate receptor density, low calcium buffering, and metabolic demand. But the additional CA3/CA4 involvement in HS is distinctive of epileptic injury and likely reflects seizure-induced excitotoxic damage, particularly to mossy fiber-receiving CA3 pyramidal cells and to hilar interneurons in CA4. The relative sparing of CA2 has intrigued researchers — CA2 has different transcriptional programs, different glutamate receptor profiles, and different intrinsic membrane properties that confer resistance to seizure-induced excitotoxicity. The mossy fiber sprouting (dentate granule cell axons growing aberrantly into molecular layer) is thought to create reverberant excitatory loops that may sustain hippocampal seizures. The translation of these molecular insights into therapy has been challenging — surgery remains the most effective treatment for refractory mesial temporal lobe epilepsy. But the principle that understanding why specific neurons are vulnerable might enable targeted neuroprotection is driving research, and trials of compounds that modify glutamate or calcium handling continue. The 140+ years between Sommer’s description and our molecular understanding illustrate how the foundations of clinical neuropathology continue to inform basic neuroscience.

Pitfalls and Pearls

  • Hippocampal sclerosis: most common adult focal epilepsy pathology.
  • Selective subfield vulnerability: CA1 + CA3 + CA4 + hilus; sparing of CA2.
  • ILAE HS Type 1: classical pattern; most common.
  • Granule cell dispersion + mossy fiber sprouting: associated features.
  • Mesial temporal lobe epilepsy: aura + automatisms + impaired awareness + postictal confusion.
  • MRI HS features: hippocampal atrophy + T2/FLAIR hyperintensity + loss of digitations.
  • Anterior temporal lobectomy: 60-70% seizure-free; selective amygdalohippocampectomy alternative.
  • LITT: emerging less invasive option.
  • LATE (limbic-predominant TDP-43 encephalopathy): HS-like pattern in elderly; TDP-43.
  • FCD type II + balloon cells + mTOR pathway: second most common refractory epilepsy pathology.
  • Long-term epilepsy-associated tumors (LEATs): DNET, ganglioglioma; BRAF V600E common.
  • Rasmussen encephalitis: chronic unilateral hemiatrophy; refractory seizures; hemispherectomy curative.
  • Status epilepticus damage: CA1 + CA3 + cortex + cerebellum.
  • SUDEP: sudden death in epilepsy; mechanism multifactorial.
  • Childhood febrile seizures: associated with later MTLE (especially prolonged febrile seizures).

References

  1. Love S, Budka H, Ironside JW, Perry A, eds. Greenfield’s Neuropathology. 9th ed. CRC Press; 2015.
  2. Blümcke I, Thom M, Aronica E, et al. International consensus classification of hippocampal sclerosis in temporal lobe epilepsy. Epilepsia. 2013;54(7):1315-1329.
  3. Engel J Jr, McDermott MP, Wiebe S, et al. Early surgical therapy for drug-resistant temporal lobe epilepsy. JAMA. 2012;307(9):922-930.
  4. Nelson PT, Dickson DW, Trojanowski JQ, et al. Limbic-predominant age-related TDP-43 encephalopathy (LATE): consensus working group report. Brain. 2019;142(6):1503-1527.
  5. Bien CG, Granata T, Antozzi C, et al. Pathogenesis, diagnosis and treatment of Rasmussen encephalitis. Brain. 2005;128(3):454-471.
  6. Surges R, Thijs RD, Tan HL, Sander JW. Sudden unexpected death in epilepsy: risk factors and potential pathomechanisms. Nat Rev Neurol. 2009;5(9):492-504.