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Abstract
Temporal lobe epilepsy (TLE) is the most common cause of both focal and refractory seizures. Hippocampal sclerosis is the most common histological findings in temporal lobe epilepsy.
A variety of imaging modalities have been used to evaluate seizure patients specialty those with temporal lobe origin, among these are magnetic resonance imaging (MRI), positron emission tomography (PET) and recently magnetic resonance spectroscopy (MRS).
Proper localization of seizure focus is the main goal to properly manage these patients and determining whether subjected to medical or surgical intervention. Surgical removal of the focus eliminates or greatly reduces seizures in about 50 to 90 % of patients with TLE who have concordant hippocampal atrophy. Mesial temporal sclerosis is the sole pathologic finding in 65% of temporal lobectomy specimens from adult patients with TLE.
On magnetic resonance imaging, mesial temporal sclerosis displays hippocampal atrophy, signal intensity and structural distortion. However, even after that, a category of patients with TLE appear to have negative MR images and these patients with negative MRI may have difficulty in proper focus localization and if candid for surgery, may have poorer seizure control after hippocampectomy.
Because it is thought that in temporal lobe epilepsy the seizure focus is usually localized in the Hippocampus, most MRS studies thus far were performed in the Hippocampal and or the medial temporal lobe regions.
In vivo MRS is increasingly used for the clinical study in seizure patients, especially in those with temporal lobe epilepsy. MRS is considered to be complementary to MRI in evaluating seizure patients, both studies can be performed as a single integrated examination.
Magnetic resonance spectroscopy (MRS) can detect in vivo cerebral metabolites at cellular level, non-invasively. Principles underlying MRS are basically the same as those of magnetic resonance imaging (MRI), with the results of MRS measurements are presented in terms of qualitative or quantitative intensities or ratio of metabolites. Proton MRS can be performed together with MRI during the same examination session, providing biochemical data in addition to structural information.
In patients with temporal lobe epilepsy, including those with negative MR images, lateralization of seizure focus can be determined on the basis of metabolite changes non-invasively using MRS technique. These metabolite changes include changes in NAA or Creatine (Cr) & Choline (Cho) intensity level as well as changes in the ratio between NAA / Cr + Cho.
The most important metabolite in use in MRS concerning seizure patients is NAA, the important neuronal marker where decrease its level reveals neuronal loss or dysfunction. The reduced intensity of N-acetyl aspartate (NAA), on proton MRS is consistently identified in the mesial temporal lobe or Hippocampus of patients with temporal lobe epilepsy (TLE), reflecting underlying neuronal loss or dysfunction.
NAA appears to reflect a high degree of cellular integration. It is considered as an important neuronal marker, reflecting good neuronal density and viability. Thus destructive or infiltrative diseases that selectively cause neuronal loss will result in decrease NAA level relative to other metabolites. In MRS, the decrease level of NAA has been interpreted as an indication to loss of neurons or loss of neuronal activity.
The Choline is a constituent of the phospholipids metabolism of cell membranes and reflects cell membrane turn over. The increased Cho signal is associated with increased cell membrane build up or higher cellular density. Tissue necrosis has been characterized as showing decreased level of NAA as well as Cho resonance.