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|Year : 2016 | Volume
| Issue : 6 | Page : 1243-1244
Neuropsychological implications of white matter microstructural lesions determined by diffusion tensor imaging in magnetic resonance imaging-negative temporal lobe epilepsy
Manas K Panigrahi1, V Sudhindra2, Sita Jayalakshmi2
1 Department of Neurosurgery, Krishna Institute of Medical Sciences, Secunderabad, Telangana, India
2 Department of Neurology, Krishna Institute of Medical Sciences, Secunderabad, Telangana, India
|Date of Web Publication||11-Nov-2016|
Manas K Panigrahi
Department of Neurosurgery, Krishna Institute of Medical Sciences, Secunderabad, Telangana
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Panigrahi MK, Sudhindra V, Jayalakshmi S. Neuropsychological implications of white matter microstructural lesions determined by diffusion tensor imaging in magnetic resonance imaging-negative temporal lobe epilepsy. Neurol India 2016;64:1243-4
|How to cite this URL:|
Panigrahi MK, Sudhindra V, Jayalakshmi S. Neuropsychological implications of white matter microstructural lesions determined by diffusion tensor imaging in magnetic resonance imaging-negative temporal lobe epilepsy. Neurol India [serial online] 2016 [cited 2019 May 22];64:1243-4. Available from: http://www.neurologyindia.com/text.asp?2016/64/6/1243/193764
Advances in neuroimaging are helpful to identify focal lesions that are responsible for epilepsy, with the hope that they can be surgically resected. Temporal lobe epilepsy (TLE), the most common form of focal epilepsy, is often accompanied by mesial temporal lobe sclerosis (MTS), which is characterized by specific patterns [identified on routine magnetic resonance imaging (MRI) as hippocampal atrophy, T2 signal hyperintensity, or both] of neuronal loss and gliosis. However, this specific pattern is reported in only 25% of TLE patients in an outpatient-based setting, and in around 70% of TLE patients in a tertiary care center  probably owing to improved imaging facilities in tertiary care centers.
Rodríguez-Cruces et al., in a literature review on white matter lesions in TLE, reported that, with the advent of diffusion-weighted MRI (dMRI) in epilepsy, TLE ceased to be a focal disorder with a clear unilateral lesion. TLE is now truly a brain network disorder, and the classical MTS lesion is accompanied by microarchitectural abnormalities of white matter that interconnect remote brain areas. In voxels occupied by a single fiber population, diffusion corresponds to the orientation of fibers, which is used to reconstruct the trajectories of macroscopic fiber tracts using tractography. Thus, dMRI simultaneously provides information about brain connectivity and features of the underlying tissue. There are numerous methods to analyze the behavior of water diffusion, however, diffusion tensor imaging (DTI) has been, by far, the most widely used for the study of TLE because its relatively low demands in terms of data acquisition.
Neuropsychological implications of white matter abnormalities
Cognitive functions rely on multimodal information support conveyed by connections that are provided by white matter fascicles., White matter microstructural abnormalities have previously been associated with cognitive disabilities in several domains in patients with TLE. The frontoparietal network (superior longitudinal fasciculus, cingulum, and contralateral temporal lobe) that supports working memory is disrupted in patients with TLE, with specific diffusion abnormalities in the interconnecting white matter. Memory, both delayed and immediate, has been shown to correlate with fractional anisotropy (FA) of the anterior and medial portions of the temporal lobe, respectively. Moreover, patients with left TLE have been shown to have more pronounced deficits of working memory because of extensive white matter abnormalities. In addition, patients with blurring of the gray/white matter interface show more neuropsychological deficiencies than patients without blurring.
There is considerable evidence of language impairment in patients with TLE, perhaps due to alterations of white matter bundles involved in its perception and production. The spatial organization of the functional cortical language networks are disrupted in TLE patients, particularly with left hemispheric onset. These differences in brain connectivity are likely explained by the altered diffusion parameters in the uncinate, arcuate, and inferior fronto-occipital fascicles, as well as the inferior portion of the cingulum. Importantly, the extent of surgical resection of the fiber bundles involved in language predicts the postsurgical naming deficits. As dMRI and tractography become more widely utilized during surgical evaluation of TLE patients, more information regarding the association between white matter pathology with cognitive and language decline will be available.
In the current issue of Neurology India, Narenmandula et al., extend the implications of diffusivity in TLE. The study evaluated the association between functional anisotropy and mean diffusivity values measured on 3T MRI scanner with the neuropsychological score of language and memory, in 26 patients with TLE with subtle MRI findings (using a well-defined inclusion criteria), and a group of 17 age-matched healthy controls. The authors report that an increased mean diffusivity (MD) value suggests an increase in the extracellular space of the nerve fibers, suggesting the existence of lesions. Furthermore, the authors suggest that the number of fibers involved and the cognitive functions impaired are related. It is further suggested that changes in the structural integrity of left uncinate fasciculus are associated with changes in language function. Importantly, in DTI measurements, the authors observe that a change in the functional anisotropy is more sensitive than MD in the prediction of cognitive function and memory and language functions. Another important observation that could be of diagnostic value is that verbal memory function declines as fiber bundle axonal lesions and extracellular space increase. However, the study has a few drawbacks and limitations. The correlation values for diffusivity results and neuropsychological scores are not furnished. It will be interesting to know the strength of such correlations (if any). The authors speculate that a longitudinal study may reveal the consistency of their findings since decreased functional anisotropy is also observed in demyelination and cross-bundle fibers. It is possible their findings indicate that seizure occurrence itself affects the FA and MD values.
An increase in white matter diffusivity indicates dysfunctional and prematurely aging white matter because of the presence of seizures in patients with TLE. The reported findings suggest a useful role of the white matter diffusivity values in planning surgical treatment for patients with TLE. The current study hints at further research evaluating the role of advancements in neuroimaging techniques in predicting cognitive, language, and seizure prognosis in patient with TLE. Further research is also required if the reported observations vary with semiology, duration, and etiology of TLE. All these factors will eventually be of use in estimating the appropriate time of surgery.
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Conflicts of interest
There are no conflicts of interest.
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