Controversy exists as to the causative mechanism: is mesial temporal sclerosis a result of temporal lobe epilepsy or viceversa.
In children with newly diagnosed epilepsy, only approximately 1% have evidence of MTS on imaging.
Furthermore, in adults 3-10% of cases of mesial temporal sclerosis demonstrate bilateral changes even though symptoms may be unilateral.
Most patients present with complex partial temporal lobe epilepsy.
The relationship, if any, of mesial temporal sclerosis with febrile seizures is controversial, made all the more difficult due to the relative insensitivity of imaging and the difficulty in establishing whether a particular seizure was truly febrile. Up to a third of patients with established refractory temporal lobe epilepsy have a history of seizures in childhood at the time of fever.
Follow up of children with febrile seizures does not demonstrate significant increased incidence of temporal lobe epilepsy.
MRI is the modality of choice to evaluate the hippocampus, however dedicated TLE protocol needs to be performed if good sensitivity and specificity is to be achieved.
Thin section angled coronal sequences at right angles to the longitudinal axis of the hippocampus are required, to minimize volume averaging.
Coronal volume and coronal high resolution T2WI/FLAIR are best to diagnose MTS.
reduced hippocampal volume: hippocampal atrophy
increased T2 signal
abnormal morphology: loss of internal architecture (interdigitations of hippocampus)
Although comparing left to right side is easiest, it must be remembered that up to 10% of cases are bilateral, and thus if symmetry is the only feature being evaluated, many cases may be misinterpreted as normal.
Often mentioned, but probably one of the least specific findings, is enlargement of the temporal horn of the lateral ventricle.
If anything, care must be taken to not allow an enlarged horn to trick you into thinking the hippocampus is reduced in size.
When severe and long standing, additional associated findings include:
atrophy of the ipsilateral fornix and mamillary body
increased signal and or atrophy of the anterior thalamic nucleus
atrophy of the cingulate gyrus
increased signal and/or reduction in volume of the amygdala
reduction in volume of the subiculum
dilatation of temporal horn and temporal lobe atrophy
collateral white matter and entorhinal cortex atrophy
thalamic and caudate atrophy
ipsilateral cerebral hypertrophy
contralateral cerebellar hemiatrophy
loss of grey-white matter interface in the anterior temporal lobe
reduced white matter volume in the parahippocampal gyrus
Additional 3D volumetric studies can be performed, and although time consuming to post-process may be more sensitive to subtle hippocampal volume loss. Gadolinium is not required.
T2 relaxometry may also be useful in detecting cases of hippocampal sclerosis.
As a result of neuronal loss, the extra cellular space is enlarged and thus diffusion of water molecules is greater on the affected side, resulting in increased values on the affected side (higher signal on ADC).
Conversely, due to neuronal dysfunction and swelling, diffusion is restricted following a seizure, and thus vales are lower.
MR spectroscopy findings typically represent neuronal dysfunction
decreased NAA and decreased NAA/Cho and NAA/Cr ratios
decreased MI in ipsilateral temporal lobe
increased lipid and lactate soon after as seizure
MR perfusion demonstrates similar changes to SPECT (see below) with blood perfusion depending on when the scan is obtained.
During the peri-ictal phases, perfusion is increased, not only in the mesial temporal lobe but often in large parts of temporal lobe and hemisphere. In interictal periods, in contrast, perfusion is reduced 1).
The incidence of temporal lobe epilepsy (TLE) due to mesial temporal sclerosis (MTS) can be high in developing countries. Current diagnosis of MTS relies on structural MRI, which is generally unavailable in developing world settings.
The study of Mandell et al.shows strong evidence that temporal lobe and brain volume can be predictive of seizure outcome following temporal lobe resection, and that volumetric CT analysis of the temporal lobe may be feasible in lieu of structural MRI when the latter is unavailable. Furthermore, since the authors' methods are modality independent, these findings suggest that temporal lobe and normative brain volumes may further be useful in the selection of patients for temporal lobe resection when structural MRI is available 2).
SPECT and PET imaging are also a useful adjuncts, with both ictal and inter-ictal scans demonstrating abnormalities
ictal scan: hyper perfusion
interictal scan: hypo perfusion
Other causes of temporal lobe epilepsy (TLE) should be considered, especially as small temproal lobe cortical tumours can have similar appearances.
Resection of the hippocampus is the standard of care for medically intractable epilepsy in patients with mesial temporal sclerosis. Although temporal craniotomy in this setting is highly successful, the procedure carries certain immutable risks and may be associated with cognitive deficits related to cortical and white matter disruption.