Theories of impaired consciousness in epilepsy

Abstract

Although the precise mechanisms for control of consciousness are not fully understood, emerging data show that conscious information processing depends on the activation of certain networks in the brain and that the impairment of consciousness is related to abnormal activity in these systems. Epilepsy can lead to transient impairment of consciousness, providing a window into the mechanisms necessary for normal consciousness. Thus, despite differences in behavioral manifestations, cause, and electrophysiology, generalized tonic-clonic, absence, and partial seizures engage similar anatomical structures and pathways. We review prior concepts of impaired consciousness in epilepsy, focusing especially on temporal lobe complex partial seizures, which are a common and debilitating form of epileptic unconsciousness. We discuss a "network inhibition hypothesis" in which focal temporal lobe seizure activity disrupts normal cortical-subcortical interactions, leading to depressed neocortical function and impaired consciousness. This review of the major prior theories of impaired consciousness in epilepsy allows us to put more recent data into context and to reach a better understanding of the mechanisms important for normal consciousness.

Figure 1

Network inhibition hypothesis for loss of consciousness in complex partial seizures. (A) Under normal conditions, the upper brain stem–diencephalic activating systems interact with the cerebral cortex to maintain normal consciousness (yellow represents normal activity). (B) A focal seizure (red) involving the mesial temporal lobe unilaterally. (C) Propagation of seizure activity from the mesial temporal lobe to midline subcortical structures. (D) Disruption of the normal activating functions of the midline subcortical structures leads to depressed activity (blue) in bilateral regions of the fronto–parietal association cortex, leading to loss of consciousness. Reproduced with permission from Ref. . (In color in Annals online.)