Impaired consciousness in epilepsy

Summary

Consciousness is essential to normal human life. In epileptic seizures consciousness is often transiently lost, which makes it impossible for the individual to experience or respond. These effects have huge consequences for safety, productivity, emotional health, and quality of life. To prevent impaired consciousness in epilepsy, it is necessary to understand the mechanisms that lead to brain dysfunction during seizures. Normally the consciousness system—a specialised set of cortical-subcortical structures—maintains alertness, attention, and awareness. Advances in neuroimaging, electrophysiology, and prospective behavioural testing have shed light on how epileptic seizures disrupt the consciousness system. Diverse seizure types, including absence, generalised tonic-clonic, and complex partial seizures, converge on the same set of anatomical structures through different mechanisms to disrupt consciousness. Understanding of these mechanisms could lead to improved treatment strategies to prevent impairment of consciousness and improve the quality of life of people with epilepsy.

Introduction

Imagine that at any moment you could suddenly become unconscious and be unable to control your behaviour. This burden is carried by many people with epilepsy who face their lives each day not knowing when all their plans and activities will be devastated by seizures. Uncontrolled seizures comprise the most important factor that impairs quality of life in people with epilepsy,1, 2 particularly seizures that disrupt consciousness. When consciousness is lost, patients might be injured, lose productivity at work or school, be exposed to social stigmatisation, or even lose their lives. Because of the enormous importance of impaired consciousness in epilepsy, interest has been growing in direct investigation of this feature of the disorder. Understanding of the mechanisms that underlie impairment of consciousness could lead to improved treatments for epilepsy and other disorders of consciousness.

Not all seizures cause impaired consciousness. During some localised focal motor, sensory, or autonomic seizures patients may retain the ability to interact with their surroundings, answer questions, respond to commands, and recall events. Seizures that do impair consciousness seem to be diverse in terms of behaviour, physiology, and patients' demographics. They include absence seizures, which are brief staring episodes seen most commonly in young children, generalised tonic-clonic seizures, which are dramatic convulsive attacks with severe unresponsiveness, and complex partial seizures, which involve staring and automatic repetitive movements (table). What these three seizure types have in common to explain the impairment of consciousness had been unclear, but advances in neuroimaging technologies and physiological studies in human patients and animal models have provided fundamental new insights. Despite differences, all three seizure types converge on a common set of structures, including the frontoparietal association cortex and the subcortical arousal systems in the thalamus and upper brainstem (table). Involvement of these anatomical structures seems to be crucial for impaired consciousness in epilepsy and other disorders of consciousness.3, 4

This Review places epilepsy in the context of other consciousness disorders. General mechanisms of consciousness are discussed, along with how they relate to absence, generalised tonic-clonic, and complex partial seizures. For each seizure type I present findings from behavioural, electrophysiological, and neuroimaging studies and insights gained from experimental animal models. Finally, I discuss implications for patients' quality of life, future directions for additional investigation, and potential therapies to reduce impairment of consciousness in people with epilepsy.