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Brain mechanisms of altered conscious states during epileptic seizures

Nature Reviews Neurology volume 5, pages 267–276 (2009)Cite this article

Abstract

Impaired consciousness has long been considered the hallmark of epileptic seizures. Both generalized seizures and complex partial seizures are characterized by a multifaceted spectrum of altered conscious states, in terms of the general level of awareness and the subjective contents of consciousness. Complete loss of consciousness occurs when epileptic activity involves both cortical and subcortical structures, as in tonic–clonic seizures and absence seizures. Medial temporal lobe discharges can selectively impair experience in complex partial seizures (with affected responsiveness) and certain simple partial seizures (with unaffected responsiveness). Electrical stimulation of temporal lobe structures has been shown to evoke similar subjective experiences. Findings from neurophysiological and brain-imaging studies in epilepsy have now demonstrated that involvement of the bilateral thalamus and upper brainstem leads to selective impairment of frontoparietal association cortices and midline 'default mode' networks, which results in ictal loss of consciousness. The spread of epileptic discharges from the medial temporal lobe to the same subcortical structures can ultimately cause impairment in the level of consciousness in the late ictal and immediate postictal phase of complex partial seizures. This paper reviews novel insights into the brain mechanisms that underlie alterations of consciousness during epileptic seizures and the implications for clinical practice in terms of diagnosis and management.

Key Points

  • A patient's level of general awareness and subjective contents of consciousness can both be altered to some degree during epileptic seizures

  • Generalized seizures (tonic–clonic seizures and absence seizures) are characterized by complete loss of consciousness—that is, unresponsiveness in the absence of any ictal experience

  • Complex partial seizures (especially those with a medial temporal lobe focus) are associated with variable degrees of responsiveness and specific alterations in the subjective ictal experience

  • Neurophysiological and functional neuroimaging studies suggest that, in generalized and complex partial seizures, bilateral thalamus and upper brainstem involvement causes selective disruption of frontoparietal associative networks, which results in impaired consciousness

  • Ictal impairment of the general level of awareness seems related to transient disruption of frontoparietal and midline associative networks, which subserve 'default mode' brain function during the conscious resting state

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Figure 1: EEG patterns for different seizure types.
Figure 2: Patterns of brain activity associated with impaired consciousness during generalized tonic–clonic seizures.
Figure 3: Thalamic and cortical distribution of changes to generalized spike–wave discharges in 46 patients with idiopathic generalized epilepsy.
Figure 4: Patterns of brain activity associated with impaired consciousness during absence seizures.
Figure 5: Patterns of brain activity associated with impaired consciousness during complex partial seizures with medial temporal lobe focus.

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Acknowledgements

The authors wish to thank Dr Hugh Rickards for his insightful comments on the manuscript.

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  1. Department of Neuropsychiatry, University of Birmingham, and Birmingham and Solihull Mental Health NHS Foundation Trust, Birmingham, UK

    Andrea Eugenio Cavanna

  2. Department of Neurology, Amedeo Avogadro University, Novara, Italy

    Francesco Monaco

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Cavanna, A., Monaco, F. Brain mechanisms of altered conscious states during epileptic seizures. Nat Rev Neurol 5, 267–276 (2009). https://doi.org/10.1038/nrneurol.2009.38

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