What is mesial temporal sclerosis?

Mesial Temporal Sclerosis (MTS)

Mesial temporal sclerosis is a pathological condition characterized by hippocampal atrophy with specific neuronal loss in the dentate, CA1, and CA3/CA4 regions accompanied by gliosis, and is the most common cause of medically intractable temporal lobe epilepsy. 1

Pathophysiology and Characteristics

• MTS involves selective neuronal loss and axonal reorganization in the hippocampal formation, indicating vulnerability to excitotoxic processes that contribute to the pathogenesis of epilepsy 1
• The condition represents an active pathological process where neurons in the temporal lobe remain physiologically and biochemically active, facilitating seizure recurrence rather than being a static lesion 1
• Histologically, MTS presents as hippocampal atrophy with neuronal loss in specific regions (dentate, CA1, and CA3/CA4) accompanied by gliosis 2
• The excitotoxic mechanism involves excessive glutamate release, leading to prolonged neuronal depolarization and cytotoxic calcium influx 3

Clinical Presentation and Natural History

• Patients with MTS typically have a characteristic clinical history including:
  • Early brain insult (often febrile convulsions in infancy) 1, 4
  • A seizure-free interval of variable duration 1
  • Development of stereotypic complex partial seizures 1
• Not all patients with MTS are medically intractable; studies show approximately 25% achieve complete seizure control with medication 4
• Poor prognostic factors for medical management include early age of seizure onset, history of febrile convulsions, and epileptiform discharges on EEG 4

Diagnostic Evaluation

• MRI is the initial imaging investigation of choice, with thin coronal slices through the suspected seizure origin area, showing 84% sensitivity compared to CT's 62% 5
• FDG-PET demonstrates glucose hypometabolism in MTS cases and provides additional information when MRI is normal or shows nonspecific findings (sensitivity 63-67%) 5
• Ictal SPECT can demonstrate hyperperfusion of the epileptogenic region during seizures, while interictal SPECT typically shows hypoperfusion 5
• Subtraction ictal SPECT co-registered to MRI (SISCOM) improves sensitivity and specificity of seizure-related perfusion networks 5

Management Approaches

• Medical management is typically attempted for approximately 2 years before considering surgical intervention, though earlier surgery may be considered when multiple appropriate antiepileptic drugs fail 5
• Complete surgical resection of the epileptogenic region is the treatment of choice for medically refractory MTS 5
• Extended resection has shown higher success rates and low morbidity compared to lesionectomy alone 5
• Temporal lobectomy demonstrates better seizure outcomes than lesionectomy alone in multiple studies 5
• Gamma knife radiotherapy has been explored as a less invasive alternative to surgery, but may not always adequately control seizures and can lead to radiation-induced changes in brain tissue 2

Prognostic Factors

• Concordance between multiple diagnostic modalities (MRI, EEG, PET, SPECT) is associated with better surgical outcomes 5
• Complete removal of the epileptogenic zone is critical for optimal seizure control 5
• Duration of epilepsy prior to resection does not appear to significantly affect epilepsy outcome 5
• Contralateral thalamic hypometabolism on imaging is associated with poorer surgical outcomes compared to ipsilateral thalamic hypometabolism 6

Important Considerations and Pitfalls

• Incomplete surgical resection significantly increases the risk of seizure recurrence 5
• Functional neuroimaging (PET/SPECT) should be considered when MRI is normal or shows nonspecific findings 5
• Cognitive impairment correlates with extratemporal hypometabolism, involving the mesial frontoparietal networks implicated in the default mode network 6
• Bitemporal glucose hypometabolism may reflect memory deficit with higher risk of postoperative memory decline 6