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Introduction

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Tonic seizures have long been recognized as a seizure type that often occurs in the setting of symptomatic generalized epilepsy, more specifically, as part of the Lennox-Gastaut syndrome. Although this may be the most common clinical scenario where tonic seizures are seen, they occur in other epilepsy syndromes and can also be a manifestation of focal epilepsy with secondary generalization. The prominent bilateral involvement of muscle groups led to classifying tonic seizures as a “generalized” seizure type, and though true in many patients, a variety of electroencephalographic (EEG) patterns and clinical phenomena have been described. This chapter will address tonic seizures across the age spectrum, with particular attention paid to specific clinical and neurophysiologic correlates.

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Overview

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Historically, tonic seizures were recognized as a distinct seizure type by Gastaut and colleagues in 1963, when they described in detail the different types of tonic seizures based on clinical and electrophysiological observations.1 This observational study described distinct subtypes of tonic seizures based on the involvement of specific muscle groups. A common feature of all types of tonic seizures is sustained tonic contraction of different muscle groups without evolution into a clonic phase. Clinically, five different types were described: tonic axial, tonic axorhizomelic, global tonic, asymmetrical and unilateral tonic, and tonic seizures terminating with a brief clonic phase.1 Gastaut et al. also noted that these seizures often occurred during sleep and were brief, lasting between 10 and 50 seconds. Nearly 50 years after this initial description, the underlying pathophysiology of tonic seizures remains poorly understood, despite the fact that they are often refractory to medical management and associated with significant cognitive impairment.

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The International League Against Epilepsy (ILAE) describes a tonic seizure as a rigid, violent muscular contraction, fixing the limbs in some strange position. There is usually deviation of the eyes and the head towards one side, and this may amount to rotation involving the whole body (sometimes actually causing the patient to turn around, even two or three times). The features are distorted; the colour of the face, unchanged at first, rapidly becomes pale and then flushed and often livid as the fixation of the chest by these spasms stops the movements of respiration. The eyes are open or closed; the conjuctiva is insensitive; the pupils dilate widely as cyanosis comes on. As the spasm continues it commonly changes in its relative intensity and different part causing slight alterations in position of the limbs.2

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This description highlights the focal and autonomic features that may be present in tonic seizures. Bilateral, symmetric (or asymmetric) tonic seizures may be generalized or focal in onset, whereas unilateral tonic seizures are most likely focal in onset, originating from the contralateral hemispheric. Chatrian and colleagues used the term tonic-autonomic seizures to emphasize the autonomic alterations often present during tonic seizures.3 The most common alteration was arrest of respiration, followed by a gasp ...

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