Tonic seizures are the most characteristic type (Roger et al., 1989; Chevrie and Aicardi, 1972), although various estimates have placed their frequency from 17% of the cases (Niedermeyer, 1969) to 55% (Chevrie and Aicardi, 1972) and up to 95% (Gastaut et al., 1973a). The higher incidence has been found in series in which sleep tracings were systematically obtained. Tonic seizures occur frequently during non-REM sleep and less frequently during wakefulness, and they do not occur during REM sleep. They are usually brief, lasting from a few seconds to 1 minute, with an average duration of about 10 seconds. Consciousness is lost or obscured during the seizures, although arousal from light stages of sleep may occur (Erba and Cavazzuti, 1981). The axial subtype consists of a brief, but sustained, bilateral symmetric contraction of the axial muscles that results in a flexor movement of the head and trunk with apnea that occasionally is preceded by a brief cry. The eyes open and often deviate upwards. Clouding of consciousness and autonomic manifestations are usually associated with this subtype. In axorhizomelic seizures, associated abduction and elevation of the arms occurs, whereas global tonic attacks involve most muscles and affect the distal parts of the limbs. When the child is standing, the flexion of the lower limbs and body axis may forcefully throw the patient to the ground. Patients who fall in a rigid posture, like a statue, are having a tonic seizure, whereas those who fall by collapsing may have either an atonic seizure or a global tonic seizure with triple flexion of the lower extremities (Erba and Browne, 1983). In all of the types, autonomic phenomena, including tachycardia, cyanosis, flushing of the face, salivation, and lacrimation, are common. Generally, eyelid retraction, staring, mydriasis, and cyanosis can be intense. Tonic seizures may be quite mild, especially in sleep, when they are often limited to a brief apnea, eye opening, an upwards deviation of the eyes, or a minimal stiffening, leading them to be easily mistaken for physiologic phenomena such as yawning or stretching. Axial spasms are very brief tonic seizures lasting 2 to 4 seconds that are a common cause of falls in patients with LGS (Egli et al., 1985; Ikeno et al., 1985). In infants, tonic seizures may tend to occur in clusters and to be followed by atonia, possibly making them difficult to distinguish from infantile spasms (Donat and Wright, 1991a; Roger et al., 1979). In older patients, episodes of automatic behavior may follow the tonic phase or may alternate with tonic contractions (Oller-Daurella, 1970). Such tonic automatic seizures have been found in 16% of patients (Roger et al., 1989).

The ictal EEG of tonic seizures may show a very fast discharge at a rhythm of 20 plus or minus 5 Hz that is usually of increasing amplitude or a discharge of 10 Hz with a high amplitude from onset; otherwise, it is similar to the “epileptic recruiting rhythm” (Brenner and Atkinson, 1982; Fariello et al., 1974; Gastaut et al., 1963, 1974a), lasting from 4 to 10 seconds in most cases. However, it may last from 30 to 60 seconds. Less commonly, a simple flattening of the tracing throughout the attack can be the only EEG manifestations (Fig. 4.1); the fast rhythm is followed by a brief discharge of spike-wave complexes or a burst of slow waves. The electromyographic (EMG) reading during tonic seizures resembles that of a normal tonic contraction.

Atypical absence seizures, which are observed in 13% to 100% of patients, are the second most common type of seizure in LGS (Aicardi and Gomes, 1992;
Gastaut et al., 1973a). They may closely resemble typical absences, although their onset and termination are said to be less abrupt or mild and they may be hardly detectable clinically because the loss of consciousness can be incomplete, allowing the child to continue his or her activities, albeit at a slower pace and imperfectly. The decreased consciousness is often associated with some loss of muscle tone, erratic myoclonic jerks, sialorrhea, or mild hypertonia of neck and back muscles. Relatively frequently, consciousness seems to be preserved. However, the execution of simple repetitive tasks is interfered with by the occurrence of the EEG discharge (Gastaut et al., 1974b; Porter et al., 1973; Goode et al., 1970). In some cases (Erba and Cavazzuti, 1977), a selective impairment of higher cortical functions in which responsiveness is maintained has been documented. The ictal EEG pattern of atypical absences is variable. The classic 3-Hz spike-waves are never observed (Beaumanoir and Dravet, 1992; Ohtahara, 1988; Porter and Penry, 1983; Goode et al., 1970), and this is often the only clear difference from typical absences. Often, a 10-Hz discharge that is similar to that recorded during tonic seizures is seen; in other cases, a burst of spike-waves, usually at 2.5 Hz or less, is recorded and is sometimes initiated by a brief phase of fast rhythm. A sudden voltage attenuation, sometimes with superimposed 20-Hz low-amplitude activity, may be present. Atypical absences are not precipitated by hyperventilation or photic stimulation. The duration of these discharges usually is 5 to 30 seconds. Identical discharges can be unassociated with clinical manifestations. The same applies to the discharges of fast rhythms that occur during slow sleep, although, in both situations, excluding the presence of minimal clinical phenomena can be difficult (Ohtsuka et al., 1982; Gastaut et al., 1963, 1974b).

Myoclonic seizures are considered less characteristic and less common in LGS than tonic seizures or atypical absences. Their frequency has been variably
estimated at 11% to 28% of cases (Beaumanoir and Dravet, 1992; Geoffroy et al., 1983; Dravet et al., 1982; Gastaut et al., 1973a; Chevrie and Aicardi, 1972). Myoclonic attacks are extremely brief, shocklike muscle contractions that may be isolated or repeated in a saccadic manner, usually for only a few seconds. The jerks are often bilateral and symmetric (massive myoclonus), and they preferentially involve the axial flexor muscles and the abductors of both arms. The involvement of the lower limbs can cause the patient to fall to the ground. No appreciable loss of consciousness occurs. However, myoclonias may be quite prominent in some patients with LGS (the so-called myoclonic variant). They are probably a common cause of the falls that constitute a major problem for children with LGS, although the mechanism of the fall may be impossible to determine without polygraphic recording. The EEG concomitants of most myoclonic seizures are bursts of generalized polyspike-waves that often have a frontal predominance. Recently, in a small group of patients, Bonanni et al. (2002) showed that the myoclonus in LGS is not really bilateral and synchronous, like that of idiopathic myoclonic epilepsies, but that it was constantly generated in one hemisphere with rapid generalization consistent with callosal transmission, which suggests that the origin of LGS may be focal in at least a proportion of the cases.