Consciousness Lost: The lightning storm of seizures

Epilepsy is brain disorder characterized by spontaneous, repeated seizures. During seizures, nerve cells fire in massive, synchronized bursts. One might think of it as a massive electrical storm, with high peaks and valleys of lightning spreading throughout the brain. About one in 200 adults have recurrent epilepsy. It can be devastating, but most victims can be helped with medication or surgery.

Many epileptic attacks leave people unconscious — sometimes looking “frozen in place,” sitting or standing for minutes, but without consciousness. The basis of this loss of consciousness has been the subject of intense debate. Loss of consciousness is one of the key signs of the “generalised seizures,” as opposed to “partial seizures,” in which only a few areas of the brain show epileptic lightning storms, and where the subject does not lose consciousness.

Recent studies have identified new brain mechanisms that lead to unconsciousness during generalised seizures.

Until the 1940s it was thought that epilepsy was a purely cortical phenomenon. However, this changed when pioneering brain scientists Wilder Penfield and Herbert Jasper proposed their “centrencephalic theory” — meaning the centre of the encephalon (brain). They claimed that the central brain, the thalamus and upper brain stem, serve as the origin of cortical seizures, and then spread to cortex. (See Figure 1). The thalamus and upper brain stem serve as the origin of epileptic seizures Since then, lower areas, like the cerebellum and brainstem, have been shown to be involved as well. Epileptic seizures can recruit vast areas of the brain, with tens of billions of neurons.

Recent research has tended to emphasize the interaction of cortex and subcortical networks. Today, most researchers agree that both cortex and subcortex are involved. But controversy still remains. What happens deep underneath the cortex is very hard to assess in humans. Medical EEG only reaches the upper parts of the brain. Even advanced brain scan methods have difficulty revealing local activity in the lower brainstem. As a result, most evidence comes from animal studies. The human picture is still unclear.

In a recent article, Andrew D. Norden and Hal Blumenfeld from the Yale University School of Medicine propose what they call a “network inhibition hypothesis”. They propose that focal cortical seizures cause disrupted activity in subcortical structures such as the diencephalon, a collection of deep structures in the brain. (Figure 1) One of the main structures here is the thalamus, the great relay station to and from the cortex. The researchers believe that the thalamus plays a particularly important role.

Why does it make sense to include the thalamus? Because thalamus “mirrors” cortex. All regions of cortex are closely connected to large nuclei in the thalamus, massive clumps of millions of neurons, which act as great relay stations for cortical processes. Often the easiest way a signal from cortex can flow is to “bounce down” to the thalamus, and then spread upward to cortex again, and elsewhere. It is not unlike a major road intersection that allows neuronal signals to spread.

According to Norden and Blumenfeld, the disruption of diencephalic activity causes a widespread inhibition of many areas, especially in the cortex. This last step may be responsible for the loss of consciousness. Disruption of diencephalic activity causes a widespread inhibition of many areas, especially in the cortex Focal seizure activity — the lightning in the storm — in cortex propagates to the midline thalamus and upper brain stem, disrupting their normal activating function. This, in turn, may lead to the inactivation of widespread regions of forward areas of cortex, causing loss of consciousness. This entire process is also called “ictal diaschisis” (“ictus” means seizure, and “diaschisis” is a term for “a depression of local neural activity caused by dysfunction in a separate but functionally related brain region”).

The new approach is a thalamocortical network theory of consciousness. Norden and Blumenfeld’s ideas are closely related to other general theories about consciousness, such as those proposed by Llinás et al and Newman (see Suggested Readings below). The “network inhibition hypothesis” seems in harmony with other evidence about the brain basis of consciousness. Consciousness is now widely thought to require cooperation between the thalamus and the cortex. However, as the authors write, their hypothesis needs further testing.

From many sources of evidence, science seems to be converging on a single story about consciousness and the brain.

The development of a Generalised Seizure

TM, an epileptic patient at a neuropsychological rehab facility, suffers from a kind of generalised epilepsy called tonic-clonic seizures. His seizure follows a typical development: First, TM might claim to notice a vague feeling that something is “going to happen”. Judging by his experience, he is well aware that a seizure is building up, and he is therefore able to take countermeasures, such as lying down on the floor.

Without further notice, TM will emit a short cry (often called the “epileptic cry”, due to air being pressed out of the lounges), and, if standing, fall to the floor. His muscles stiffen (tonic phase) and the extremities jerk and twitch (clonic phase). At this time, TM does not respond to questions, and is not able to recall anything presented to him (for example words like “red” or “chair”). The seizure usually lasts for about 1 minute.

TM slowly regains consciousness after a few minutes, but often falls asleep due to fatigue and a state of confusion.

It is possible to study the seizures with the electroencephalogram (EEG), a measure of the brain’s electric activity, especially in the cortex. Following the course of TMs seizure, the early phase is paralleled by disrupted activity in the left temporal lobe, then the bilateral temporal lobes, and finally a global disruption, spanning the entire brain.

© 2003 TZ Ramsøy

Author Information

Thomas Z. Ramsøy – Neuropsychologist
The Lions Collegium
Tuborgvej 181
2400 Copenhagen NV
Denmark
Homepage: www.ramsoy.dk

References

1. The role of subcortical structures in human epilepsy; Andrew D. Norden and Hal Blumenfeld; Epilepsy & Behavior, Volume 3, Issue 3, June 2002, Pages 219-231.

Abstract

Like normal cerebral function, epileptic seizures involve widespread network interactions between cortical and subcortical structures. Although the cortex is often emphasized as the site of seizure origin, accumulating evidence points to a crucial role for subcortical structures in behavioral manifestations, propagation, and, in some cases, initiation of epileptic seizures. Extensive previous studies have shown the importance of subcortical structures in animal seizure models, but corresponding human studies have been relatively few. We review the existing evidence supporting the importance of the thalamus, basal ganglia, hypothalamus, cerebellum, and brain stem in human epilepsy. We also propose a “network inhibition hypothesis” through which focal cortical seizures disrupt function in subcortical structures (such as the medial diencephalon and pontomesencephalic reticular formation), leading secondarily to widespread inhibition of nonseizing cortical regions, which may in turn be responsible for behavioral manifestations such as loss of consciousness during complex partial seizures.