Description
Mesial temporal sclerosis (MTS) is a pathological condition characterised by loss of neurons, gliosis, and atrophy, particularly affecting the hippocampus, amygdala, and parahippocampal gyrus. It is a common cause of temporal lobe epilepsy (TLE).
Pathogenesis
While the exact pathogenesis remains unclear, it is generally accepted that an initial precipitating injury (such as febrile seizures, trauma, hypoxia, or inflammation) can cause hippocampal damage. Subsequent recurrent seizures may then contribute to the progression of hippocampal sclerosis.
Subtype
- Classical Mesial Temporal Sclerosis: Characterised by neuronal loss and gliosis in specific subfields of the hippocampus, predominantly in the CA1 and CA4 regions.
- Type 1 (classical or typical) MTS: Also known as hippocampal sclerosis with predominant neuronal loss and gliosis in the CA1 and CA4 (also known as the end folium) sectors of Ammon’s horn.
- Type 2 MTS: There’s more pronounced neuronal loss in the CA1 sector than the CA4 sector.
- Type 3 MTS: It involves selective neuronal loss and gliosis in the CA4 sector.
Epidemiology, Risk Factors & Associations
- Febrile seizures during childhood are a known risk factor for the development of MTS.
- Association with temporal lobe epilepsy (TLE), with up to 50-70% of patients with TLE demonstrating MTS.
Clinical Features
Patients with mesial temporal sclerosis often present with temporal lobe epilepsy, characterised by recurrent, unprovoked seizures. Clinically, seizures often have semiology suggestive of temporal lobe onset, such as déjà vu, fear, auditory or visual distortions, and automatisms like lip smacking and fumbling.
Complications
- Refractory seizures: Despite medical management, many patients with MTS continue to experience seizures.
Pathological Features
Histopathology
- Macroscopic: The affected structures may demonstrate atrophy with sclerosis and a firm consistency on gross examination.
- Microscopic: Histological findings typically include loss of neurons and gliosis.
Radiological Features
General Features
- Characteristically demonstrates atrophy and increased T2 signal of the hippocampus, amygdala and parahippocampal gyrus.
- Volume loss can often be appreciated in the ipsilateral fornix and mammillary body due to degeneration secondary to decreased input.
- Calcification is not a typical feature of MTS.
MRI
- T1WI: Atrophy of mesial temporal structures including the hippocampus, amygdala, and parahippocampal gyrus. Hippocampal signal is often increased, reflecting gliosis and decreased neuronal density.
- T2WI: High signal in the hippocampus, amygdala, and parahippocampal gyrus. This reflects gliosis and decreased neuronal density.
- T1 C+: No significant enhancement is observed in MTS.
- FLAIR: High signal in the hippocampus, amygdala, and parahippocampal gyrus, similar to T2WI.
- DWI/ADC: No restricted diffusion.
CT
- Non-contrast: Limited in the evaluation of MTS. May demonstrate volume loss of the mesial temporal structures but is less sensitive and specific than MRI.
Grading and Staging
No established grading or staging system for MTS exists.
Diagnosis
Diagnosis is made based on a combination of clinical features, EEG findings of temporal lobe epilepsy, and characteristic MRI findings. In refractory cases, invasive EEG with depth electrodes may be used.
Differential Diagnosis
- Hippocampal tumors or other lesions: These can also cause seizures and may mimic MTS on imaging. However, tumors would typically show enhancement post-contrast, which is not seen in MTS.
- Other causes of temporal lobe epilepsy: These can present similarly but would not show the characteristic MRI findings of MTS.
Management
Management usually involves antiepileptic drugs to control seizures. In patients with refractory epilepsy, surgical resection of the epileptogenic zone (usually anterior temporal lobectomy) may be performed. Post-surgery histopathological examination of the resected tissue confirms the diagnosis.