Hwr Powell

Preoperative fMRI predicts memory decline following anterior temporal lobe resection

Background: Anterior temporal lobe resection (ATLR) benefits many patients with refractory temporal lobe epilepsy (TLE) but may be complicated by material specific memory impairments, typically of verbal memory following left ATLR, and non-verbal memory following right ATLR. Preoperative memory functional MRI (fMRI) may help in the prediction of these deficits. Objective: To assess the value of preoperative fMRI in the prediction of material specific memory deficits following both left- and right-sided ATLR. Methods: We report 15 patients with unilateral TLE undergoing ATLR; eight underwent dominant hemisphere ATLR and seven non-dominant ATLR. Patients performed an fMRI memory paradigm which examined the encoding of words, pictures and faces. Results: Individual patients with relatively greater ipsilateral compared with contralateral medial temporal lobe activation had greater memory decline following ATLR. This was the case for both verbal memory decline following dominant ATLR and for non-verbal memory decline following non-dominant ATLR. For verbal memory decline, activation within the dominant hippocampus was predictive of postoperative memory change whereas activation in the non-dominant hippocampus was not. Conclusion: These findings suggest that preoperative memory fMRI may be a useful non-invasive predictor of postoperative memory change following ATLR and provide support for the functional adequacy theory of hippocampal function. They also suggest that fMRI may provide additional information, over that provided by neuropsychology, for use in the prediction of postoperative memory decline.

MR tractography predicts visual field defects following temporal lobe resection

A superior homonymous quadrantanopia is a well recognized complication of anterior temporal lobe resection and occurs because of disruption of the Meyer loop, the anterior part of the optic radiation. The authors used diffusion tensor imaging tractography to visualize the optic radiation before and after surgery, demonstrating the disruption of Meyer loop in a patient who developed a quadrantanopia. Preoperative imaging of the optic radiation will be useful in predicting visual field defects following temporal lobe resection.

Noninvasive in vivo demonstration of the connections of the human parahippocampal gyrus

MR tractography techniques provide a method for noninvasively studying white matter pathways in vivo. In this study we have used diffusion tensor imaging (DTI) and the fast marching tractography (FMT) algorithm to plot the structural connectivity of the human parahippocampal gyrus (PHG) in 10 healthy subjects, using seed points selected in the anterior parahippocampal gyrus. Our results demonstrate connectivity between the parahippocampal gyrus and the anterior temporal lobe, orbitofrontal areas, posterior temporal lobe and extrastriate occipital lobe via the lingual and fusiform gyri. We also demonstrate for the first time noninvasively direct connectivity between the parahippocampal gyrus and the hippocampus itself. These results agree with previous histological tract-tracing studies in animals. The connections demonstrated between neocortical areas and the hippocampus via the parahippocampal gyrus may provide the structural basis for theoretical models of memory and higher visual processing.

Material-specific lateralization of memory encoding in the medial temporal lobe: Blocked versus event-related design

Lesion-deficit studies have provided evidence for a functional dissociation between the left medial temporal lobe (MTL) mediating verbal memory encoding and right MTL mediating non-verbal memory encoding. While a small number of functional MRI studies have demonstrated similar findings, none has looked specifically for material-specific lateralization using subsequent memory effects. In addition, in many fMRI studies, encoding activity has been located in posterior MTL structures, at odds with lesion-deficit and positron emission tomography (PET) evidence. In this study, we used an event-related fMRI memory encoding paradigm to demonstrate a material-specific lateralization of encoding in the medial temporal lobes of ten healthy control subjects. Activation was left-lateralized for word encoding, bilateral for picture encoding, and right-lateralized for face encoding. Secondly, we demonstrated the locations of activations revealed using an event-related analysis to be more anterior than those revealed using a blocked analysis of the same data. This suggests that anterior MTL structures underlie memory encoding as judged by subsequent memory effects, and that more posterior activity detected in other fMRI studies is related to deficiencies of blocked designs in the analysis of memory encoding.

Imaging language pathways predicts postoperative naming deficits

Naming difficulties are a well recognised, but difficult to predict, complication of anterior temporal lobe resection (ATLR) for refractory epilepsy. We used MR tractography preoperatively to demonstrate the structural connectivity of language areas in patients undergoing dominant hemisphere ATLR. Greater lateralisation of tracts to the dominant hemisphere was associated with greater decline in naming function. We suggest that this method has the potential to predict language deficits in patients undergoing ATLR.

Long term monitoring in refractory epilepsy: the Gowers Unit experience

Introduction: Guidelines from the National Institute for Health and Clinical Excellence (NICE) and the International League Against Epilepsy recommend long term EEG monitoring (LTM) in patients for whom seizure or syndrome type is unclear, and in patients for whom it is proving difficult to differentiate between epilepsy and non-epileptic attack disorder (NEAD). The purpose of this study was to evaluate this recommended use of LTM in the setting of an epilepsy tertiary referral unit. Methods: This study reviewed the case notes of all admissions to the Sir William Gowers Unit at the National Society for Epilepsy in the years 2004 and 2005. A record was made of the type, duration and result of all LTM performed both prior to and during the admission. Pre- and post-admission diagnoses were compared, and patients were divided according to whether LTM had resulted in a change in diagnosis, refinement in diagnosis or no change in diagnosis. The distinction between change and a refinement in the diagnosis was made on the basis of whether or not this alteration resulted in a change in management. Results: 612 patients were admitted during 2004 and 2005, 230 of whom were referred for diagnostic clarification. Of these, LTM was primarily responsible for a change in diagnosis in 133 (58%) and a refinement of diagnosis in 29 (13%). In 65 (29%) patients the diagnosis remained the same after LTM. In those patients in whom there was a change in diagnosis, the most common change was in distinguishing epilepsy from NEAD in 73 (55%) and in distinguishing between focal and generalised epilepsy in 47 (35%). LTM was particularly helpful in differentiating frontal lobe seizures from generalised seizures and non-epileptic attacks. Inpatient ambulatory EEG proved as effective as video telemetry in helping to distinguish between NEAD, focal and generalised epilepsy. Discussion: The study revealed that LTM led to an alteration in the diagnosis of 71% of patients referred to a tertiary centre for diagnostic clarification of possible epilepsy. Although LTM is relatively expensive, time consuming and of limited availability, this needs to be balanced against the considerable financial and social cost of misdiagnosed and uncontrolled seizures. This service evaluation supports the use of performing LTM (either video or ambulatory) in a specialist setting in patients who present diagnostic difficulty.