Current themes in neuroimaging of epilepsy: Brain networks, dynamic phenomena, and clinical relevance

Abstract

Brain scanning methods were first applied in patients with epilepsy more than 30 years ago. A very substantial literature now exists in this field, which is exponentially increasing. Contemporary neuroimaging studies in epilepsy reflect new concepts in the epilepsies, as well as current methodological developments. In particular, this area is emphasising the role of networks in epileptogenicity, the existence of dynamic phenomena which can be captured by imaging, and is beginning to validate the implementation of neuroimaging in the clinic. Here, recent studies of the last 5 years are reviewed, covering the full range of neuroimaging methods with SPECT, PET and MRI in epilepsy.

Introduction

Early conceptualisations of epilepsy as either “focal” or “generalised” have been subjected to increasing challenge over decades by the identification of widespread bilateral phenomena in focal epilepsies (for example, the network changes reviewed by Spencer (2002)) and the finding of focal features of supposedly generalised epilepsy syndromes, exemplified by the finding of a localised cortical focus which drives absence seizures in a rat model (Meeren et al., 2002). Current neuroimaging studies of human epilepsies are increasingly revealing networks of interconnected brain regions which characterise specific epilepsy disorders. These findings further challenge the concept of “localisation-related” epilepsy, and also challenge assumptions about specific focal origins of associated cognitive deficits in epilepsy disorders.

An image may be considered a merely static representation of an object. In the context of a highly dynamic entity such as the brain, imaging might be rather limited as a means to describe relevant phenomena, especially in contrast with the temporally rich data produced using neurophysiological methods. On the contrary, from the earliest era of detailed human brain imaging, dynamically changing phenomena have been central to neuroimaging investigation of epilepsy – for example, imaging seizure dynamics using ictal single photon emission computed tomography (SPECT). Many imaging modalities have been developed to capture dynamic phenomena which may change rapidly or more slowly (such as blood flow, glucose metabolism, and aspects of neurochemistry and neurotransmitter systems). Even imaging modalities capable only of capturing static (structural) phenomena can be repeated serially over time, and capture dynamic and evolving changes related to natural history or treatment interventions.

Neuroimaging in epilepsy has not been immune to a broad criticism of biomedical research, that scientific advances are not translated into benefit for patients sufficiently quickly. Despite more than three decades of neuroimaging research involving complex brain scanning, relatively few imaging tools are in routine use, and very few are supported by the kind of substantial evidence-base which would be required to support the implementation of a new treatment.

In this review, I attempt to assemble neuroimaging publications of the last 5 years into these three themes: the growing evidence for network abnormalities in epilepsies; explorations of dynamic phenomena; and attempts to develop the evidence required to move new imaging modalities into clinical routine. I include here only typical imaging methods, and do not discuss methods which rely only on EEG or MEG source localisation. Where necessary, a few older studies are described to set the context, but the emphasis is entirely on recent studies, and the near future. The intention is to bring these studies to the attention of non-experts in neuroimaging, in the hope that productive cross-fertilisation between fields may be facilitated.