Thalamotemporal alteration and postoperative seizures in temporal lobe epilepsy

Pharmacoresistant epileptic seizures are strongly associated with mesial temporal lobe epilepsy (mTLE) due to hippocampal sclerosis (HS).1, 2 mTLE is the epilepsy disorder for which operations are the most frequently performed.3 Although surgical treatment of intractable mTLE is very effective, producing a 7‐fold increased likelihood of seizure freedom and significantly improved quality of life 1 year after surgery compared to an unoperated control group,4 30 to 40% of patients will continue to experience disabling postoperative seizures 2 years after surgery.4, 5 The proportion of patients continuing to experience any seizure‐related symptom (eg, aura, nondisabling seizures) is greater, particularly after longer periods after surgery.6 It is currently unknown why a large subgroup of patients continues to experience seizures despite surgical intervention. Several studies have reported associations between various clinical factors and postoperative outcome in mTLE,3, 6 whereas other studies have reported no associations with the same clinical variables.7, 8 Furthermore, given that different clinical associations with seizure outcome have been reported at different postoperative time points in the same patients,3 and given the clinical heterogeneity of mTLE, preoperative clinical data do not allow reliable prediction of likely outcome for individual patients. Patients with mTLE and evidence of a focal lesion, such as HS, are known to have improved postoperative outcome compared to patients with mTLE and no lesion.7, 9 However, up to 40% of patients with mTLE and HS will continue to experience persistent postoperative seizures.9 There has therefore been an attempt in some studies to identify brain alterations, most typically increased atrophy from T1‐weighted MRI, in patients with continued seizures relative to those rendered seizure free. There are, however, inconsistent findings. Some studies have indicated that hippocampal volume contralateral to the seizure focus may be abnormal in patients with persistent postoperative seizures,10, 11 whereas others have not.12, 13, 14 More morphometric MRI research on postoperative prognosis is required. Given the recent modifications in the classification of epilepsy disorders to consider the importance of brain networks involved in seizure onset, including focal epilepsies,15 there has been a new direction of research in mTLE to model neuroimaging data in terms of connected networks.16 An increasing volume of research is indicating that mTLE is a systems network disorder.16, 17, 18 It is conceivable that structural and functional network alterations are not uniform in mTLE, and specific subtypes of network alterations may be resistant to pharmacological treatment and surgical intervention. Investigation of the relationship between preoperative brain connectivity and postoperative seizure outcome in patients with mTLE using diffusion tensor imaging (DTI) is lacking, and may provide important insights into the contribution of brain alterations to persistent seizures that cannot be visualized or quantified using conventional MRI data routinely acquired in the context of preoperative evaluation. In the present study, we performed a series of volumetric, morphometric, and DTI investigations assessing the relationship between preoperative quantitative imaging and postoperative seizure outcome. We additionally investigated the relationship between the extent of mesial temporal lobe resection and postoperative outcome by quantitatively analyzing postoperative MRI data.