As per estimate, 1% of the world population suffers from epilepsy, of which the most common adult type is focal onset seizure with impaired awareness (previously known as complex partial seizure) . Williamson PD, French JA, Thadani VM, Kim JH, and colleagues noted that more than 80% cause of TLE is mesial temporal lobe sclerosis (MTS) . But in our study, 23.21% showed features suggestive of MTS, seven had tumours, one had non-specific white matter T2 hyperintensity in extratemporal location, 12 had hippocampal atrophy without signal alteration, and 1 had T2 hyperintensity of hippocampus without volume loss.
The major tumours detected in our study were glioblastoma multiforme (n 2), dysembryoplastic neuroectodermal tumour (n 3), ganglioglioma (n 1), and glioma (n 1). As per study by Brooks and colleagues, tumours were detected in 22% subjects .
In our study, 22 subjects (39.28%) had no obvious abnormalities in imaging with 1.5 T MRI. As per Muhlhofer W, Tan Y-L, Mueller SG, and Knowlton R, up to 30% of TLE cases have normal (“non lesional” or negative) magnetic resonance imaging (MRI) .
Magnetic resonance imaging (MRI) features of hippocampal sclerosis (HS) by visual analysis of MRI as described by Cendes and colleagues are hippocampal atrophy, increased T2/FLAIR signal, loss of internal structure, asymmetry of the horns of the lateral ventricles, atrophy of the anterior temporal lobe, and atrophy of the ipsilateral fornix and mammillary bodies .
Classical imaging findings in hippocampal sclerosis include volume loss and increased signal intensity in T2/FLAIR images . Increased signal intensity of hippocampus with atrophy visually was found in 13 (24.53%) patients on the right side and 9(16.98%) in patients on the left side. Six out of 53 cases had bilateral hippocampal atrophy without corresponding increase in signal intensity in T2.
Atrophy is the most specific and reliable feature of hippocampal sclerosis (HS) .
The significance of extent of atrophy of hippocampus becomes important in assessing the prognosis after surgery for mesial temporal sclerosis. In a study by Kim y and colleagues, it was seen that an MR imaging finding of hippocampal atrophy is the most useful sole prognostic indicator .
Visual identification of abnormal hippocampus is straightforward if one side is clearly normal and the other is abnormal. In symmetric bilateral disease or mild unilateral disease, visual analysis may produce problems .
The secondary findings in the mammillary bodies and fornix on MR imaging help in the diagnosis and lateralization of MTS . In our study out of 53 cases, 13 cases showed atrophy of mammillary bodies/fornix with temporal horn dilation ipsilateral to the side of hippocampal atrophy.
In patients with subtle findings of unilateral MTS, the secondary imaging features may add to improve diagnostic confidence. Although the secondary MR imaging findings associated with MTS are not sensitive predictors of this entity by themselves, they may offer clues in subtle cases .
Quantitative hippocampal volumetry has been shown to predict postsurgical outcome in various studies; however, according to Kim y and colleagues, the interpretation of MR images by visual inspection alone has a similar prognostic value .
Quantitatively, 15 (28.30%) subjects had hippocampal atrophy volumetrically vs. 13 (24.53%) visually on the right side, whereas on the left side, only 9 (16.98%) subjects were detected visually, while 10 (18.87%) were detected volumetrically to have atrophy. In our study, by volumetry, we were able to detect more cases of atrophy quantitatively which was not evident on visual inspection alone.
Another important indicator of hippocampal sclerosis is increased hippocampal T2 signal which indicates gliosis . The degree and extent of hippocampal gliosis also correlate with the T2 signal in the hippocampus.
MR imaging studies by Bronen RA, Jackson GD, and colleagues have described a variable frequency of T2 signal change in the hippocampus: change was observed in 12 to 65% of patients with hippocampal sclerosis .
But in our study, T2 hyperintensity were found in 30.18%. In our study, there is a single case with T2 hyperintensity of hippocampus ipsilateral to the clinical and EEG localisation of the seizure focus (right sided), however with no atrophy.
As per study by Kim y and colleagues, it has been suggested that finding of hippocampal atrophy is more useful than one of high T2 signal in determining hippocampal sclerosis by histology. As per Kim and colleagues, hippocampal atrophy was much more common finding than high T2 signal (97% vs. 61%).
T2-weighted hyperintensity is one of the salient radiologic features of HS which can be objectively assessed by quantitative measurement of T2 relaxation (T2 mapping). It has got higher sensitivity over visual analysis. It is of great importance to note that T2 values may be elevated even in the absence of atrophy . Thus, the combination of hippocampal atrophy with an elevated T2 value is both sensitive and specific for HS. As a result, combining findings of hippocampal volumes and T2 values can increase the yield to 99% of visually detected HS, but also 28% of those considered visually normal .
In our study, comparisons of the right and left hippocampal T2 relaxometry values in subjects with normal and abnormal hippocampus were found to have a statistically significant difference.
Jackson G. D and colleagues performed hippocampal T2 relaxometry as routine MRI examination and concluded that abnormal T2 relaxometry is significantly associated with intractable epilepsy .