John Libbey Eurotext

Temporal pole and mesiotemporal epilepsy: introductory remarks Volume 4, supplément 1, Supplement 1, September 2002

It is now half a century since temporal lobe epilepsy was first identified and recognized as the most common type of refractory, focal epilepsy. The neuronal systems responsible for the seizures which characterize this form of epilepsy fail to respond to currently available anti-epileptic drugs in fully one third of cases although the exact reasons for this non-responsiveness are not yet understood. In contrast, the discharge responsible for propagation and secondary generalization is usually controllable with drugs. While certain issues related to the anatomical background of temporal lobe epilepsy remain controversial, modern imaging modalities ­ notably MRI ­ are proving to be powerful tools in this field, making it possible to visualize and make accurate measurements of all the various structures of the temporal lobe, and to correlate the results with clinical data and surgical outcomes. On the basis of the region in which the seizures originate, it has been proposed that two distinct forms of temporal lobe epilepsy exist, namely medial and lateral. However, distinguishing these two forms on purely clinical criteria remains difficult. Earlier analyses carried out by Murray Falconer and his team at King's Maudsley Hospital working with the neuropathological data of Nicholas Corsellis and Clive Bruton were already hinting at the existence of different types of temporal lobe epilepsy, the surgical outcomes of which are very different. This led Falconer to propose that only patients with evidence of hippocampal sclerosis were suitable candidates for surgery. The same work revealed the causative role of prior, prolonged febrile convulsions in the pathogenesis of mesial temporal sclerosis. Ever since, it has been clear that temporal lobe epilepsy is not a single, uniform pathological entity.

Later, the role of the amygdala in the onset of temporal seizures (and the type of ictal symptoms which are associated with this structure) was discovered by William Feindel, Wilder Penfield and Theodore Rasmussen. Speculation continues as to the role of other temporal lobe structures such as the entorhinal cortex and the hippocampal gyrus, although no comparable, systematic studies have yet been conducted.

The first surgical modalities to manage temporal lobe epilepsy involved resection of anterior temporal structures without damaging the language areas in the dominant hemisphere. Despite the large scope of this type of procedure, outcomes were mixed, largely because of the diversity of the underlying problems which were being thus tackled. When the importance of prolonged febrile convulsions and mesial temporal sclerosis was discovered and the volumes of the hippocampus and the amygdaloid nucleus could be accurately estimated using MRI, it became possible to distinguish a sub-population of patients with unilateral hippocampal atrophy among whom the post-surgical success rate was over 80%. On the other hand, the corresponding success rate in patients with either bilateral atrophy or no detectable hippocampal volume loss was only 50%. The definition of this second group ­ which luckily represents a minority of patients ­ led to a number of different projects aimed at elucidating the pathological processes in this special form of temporal lobe epilepsy. Dual pathology in which mesial temporal atrophy was combined with some other lesion was recognized as accounting for poor surgical outcome in patients whose MRI examination showed either uni- or bi-lateral involvement. The process of secondary epileptogenesis as conceived by Frank Morrell is probably another contributing factor in some surgical failures but it is more important when considering patients with lesional temporal lobe epilepsy. More recently, the idea of pseudo-temporal lobe epilepsy has been proposed in which temporal lobe EEG discharges may be induced by an epileptogenic lesion elsewhere, i.e. in frontal, occipito-temporal, parietal or insular structures as has been documented by Murray Falconer, Fish et al. and Palmini et al., among others. Finally, modern imaging techniques have made it possible to detect congenital malformations such as periventricular nodular heterotopia, double cortex syndrome and hypothalamic hamartomas. With all these underlying problems, epileptic discharges may be detected in the temporal lobe although, of course, temporal lobectomy is not helpful in most of these cases. Thus, progress in the diagnosis of special forms of temporal epilepsy and of pseudotemporal epilepsy means that suitable candidates for surgical treatment can be more reliably selected; for some of these patients today, the chance of post-operative cessation of seizures is very high.

The traditional surgical approach used to involve resection of anterior temporal structures including the pole and the lateral neocortex although this is now considered by some as being unnecessarily extensive. A less radical and more elegant strategy has been proposed in which only those structures in which abnormalities can be detected by MRI are ablated. One such operation is selective amydalo-hippocampectomy which was pioneered by Paulo Niemeyer and developed by Yasargil, Olivier and others. Outcomes seem to be comparable for both approaches and most of the discussion of their relative merits centers on the benefit in terms of cognition and other functional aspects afforded by the sparing of tissue in the selective strategy. Currently, there is no evidence of any particular benefit according to Jones-Gotman's neuropsychological data. In parallel to these developments, the field of functional examination of the temporal lobe and related structures was launched by Bancaud and Talairach, and developed by the late, lamented, Claudio Munari. This type of examination is now routinely practiced in a number of epilepsy surgery units. Increasing interest in the surgical management of epilepsy has lead these centers ­ including our own ­ to develop this tool which yields vital information about the physiology of various cerebral structures and can be used to localize the epileptogenic zone in patients in whom diagnosis is proving difficult.

A number of articles have focussed on the role of temporal structures other than the hippocampus and the amygdala, notably the entorhinal cortex and the parahippocampal gyrus. On the other hand, the temporal pole ­ which is routinely ablated in anterior temporal resection ­ has not been specifically investigated, partly because resection of this region alone (apart from for the treatment of specific, confined lesions) is rare. In order to probe the role of these other structures in temporal lobe epilepsy, Munari carried out a systematic examination of the temporal pole. In this work, T2 signal abnormalities from the white matter in the anterior part of the temporal lobe were observed which incited the various neuropsychological and neuropathological investigations focusing on the region of the temporal pole which constitute the basis of this Symposium. These studies will certainly contribute to our understanding of the functions and problems associated with the temporal pole, and its place in the epileptogenic process. Many issues remain to be resolved, notably the significance of an increased MRI T2 signal in patients without any mesial temporal sclerosis, and of the temporal pole atrophy which is often associated with the abnormal signal. At this time, no particular symptoms appear to differentiate patients with temporal pole lesions from those with other forms of temporal lobe epilepsy but investigations focusing on this aspect might conceivably reveal specific clinical patterns.

One important question is whether simple isolation of epileptogenic structures ­ rather than their radical ablation ­ would give a satisfactory outcome. This perennial question might finally find an answer as a result of work on epileptogenic activity in the temporal pole. With the current state of knowledge, no definitive answer can be given about any benefit accruing from ablation of the pole in extended amydalo-hippocampectomy; not even whether such a more radical procedure wight give a higher success rate or not. The articles in this volume represent a contribution to clarifying our understanding of the temporal pole abnormalities detected by MRI, and of the role of the temporal pole in temporo-limbic epilepsy.