Interictal manifestations

Cognitive and behavioral manifestations

Prior to syndrome onset, 20–30% of children with Lennox-Gastaut syndrome (LGS) are free from neurologic and neuropsychological deficits. These problems inevitably appear during the evolution of LGS. The frequency and severity of mental retardation are increased in association with the following factors:20,22,27,28,40

• symptomatic LGS
• a history of West syndrome (infantile spasms)
• onset of symptoms before 12 to 24 months of age
• higher seizure frequency

In young children, the psychiatric symptoms consist of mood instability and personality disturbances. Slowing or arrest of psychomotor development and educational progress characterize the neuropsychological symptoms.

In older children, character problems predominate, and acute psychotic episodes or chronic forms of psychosis with aggressiveness, irritability, or social isolation may occur.28 Reaction time and information processing are the most impaired cognitive functions.20 Kaminska and colleagues26 found that the main characteristics of mental deterioration were:

• apathy
• memory disorders
• impaired visuomotor speed
• perseverance

The interictal EEG

The interictal EEG is characterized by a slow background, which can be constant or transient. Permanent slowing of the background is associated with poor cognitive prognosis.28

The hallmark of the awake interictal EEG is the diffuse slow spike and wave. This pattern consists of bursts of irregular, generalized spikes or sharp waves followed by a sinusoidal 350- to 400-millisecond slow wave20 with an amplitude ranging from 200 to 800 microvolts,40 which can be symmetric or asymmetric. The amplitude very often is higher in the anterior region, in the frontal or frontocentral areas. In some cases, however, the activity may dominate in the posterior head regions.40 The frequency of the slow spike and wave activity commonly is between 1.5 and 2.5 Hz (Figure 1).40

Slow spike and wave pattern in a 24-year-old awake man with Lennox-Gastaut syndrome. The slow posterior background rhythm displays frequent periods of 2- to 2.5-Hz discharges that are maximal in the bifrontocentral areas and occur in trains of up to 8 seconds without any associated clinical manifestations.

Slow spikes and waves are usually not activated by photic stimulation or hyperventilation.20,40 The characteristic diffuse slow spike and wave pattern of LGS gradually disappears with age and is replaced by focal epileptic discharges, especially multiple independent spikes.27

During non-REM sleep, discharges are more generalized and more frequent and consist of polyspikes and slow waves. In REM sleep, spike and waves decrease.20 During periods of frequent seizures, the total duration of REM sleep diminishes.20

Ictal manifestations

Types of seizures in LGS include tonic, atonic, myoclonic, and atypical absence seizures, often associated with other, less common types.

Tonic seizures

Tonic seizures probably are the most characteristic type. The reported frequency ranges from 17% to 95%.18,25 Tonic seizures are more frequent during non-REM sleep. They occur as one of three types:20,24,28

• axial tonic, involving the head and the trunk with head and neck flexion, contraction of masticatory muscles, and eventual vocalizations
• axorhizomelic tonic, in which there is tonic involvement of the proximal upper limbs with elevation of the shoulders and abduction of the arm
• global tonic, with contraction of the distal part of the extremities, sometimes leading to a sudden fall and other times mimicking infantile spasms

The EEG is characterized by a diffuse, rapid (10- to 13-Hz), low-amplitude activity, mainly in the anterior and vertex areas (“recruiting rhythm”), The activity progressively decreases in frequency and increases in amplitude (Figure 2). A brief, generalized discharge of slow spike and waves or flattening of the recording may precede this pattern. Diffuse slow waves and slow spike and waves may follow it. These rapid discharges are common during non-REM sleep. Unlike tonic-clonic seizures, no postictal flattening occurs.

Tonic seizure with arm extension in a 24-year-old man with Lennox-Gastaut syndrome. Prior to the seizure, the patient was watching television. He then experienced a sudden body jerk and leaned forward, with his arms extended and fixed at shoulder level. The EEG shows a diffuse burst of high-voltage polyspike and slow wave activity followed by 1 second of relative attenuation and then paroxysmal rapid activity that is maximal in the bifrontocentral head regions.

Clinical manifestations appear 0.5–1.0 seconds after the onset of EEG manifestations and last several seconds longer than the discharge.20,28

Atonic, myoclonic, and myoclonic-atonic seizures

Atonic, massive myoclonic, and myoclonic-atonic seizures can be difficult to differentiate by clinical observation alone. Considerable discrepancies involve the application of these terms. The reported frequency ranges from 10% to 56%.18,20,22,23,27,39

Ikeno and colleagues41 found that pure atonic seizures are exceptional and that most involved a tonic or myoclonic component. In 95% of patients, atonic, tonic, and myoclonic seizures coexist.28

Such seizures incite sudden falling, producing injuries (so-called drop attacks, Sturzanfälle). Such falling sometimes is limited to the head, so that the head falls on the chest (head drop, head nod, nictatio capitis).7,28,41

The EEG is characterized by slow spike and waves, polyspike and waves, or diffuse rapid rhythms.28 Simultaneous video EEG recording and polygraphy allows a more precise diagnosis.

Atypical absence seizures

The reported frequency of atypical absence seizures ranges from 17% to 100%.18,24 In most studies, the frequency of the different types of seizures reported is based on parental counting of seizures or reviews of clinic charts or is not specifically stated. Unfortunately, parental ability to recognize and identify atypical absence seizures correctly is poor. In one study using video EEG monitoring in a cohort of children with LGS, parental recognition was 27% for atypical absence seizures, whereas the sensitivity was as high as 80% for myoclonic seizures and 100% for tonic, atonic, tonic-clonic, clonic, and complex partial seizures.42

Diagnosis of atypical absence seizures may be difficult because their onset may be gradual20,24,28 and loss of consciousness may be incomplete, allowing the patient to continue activities to some degree. Automatisms may be observed.20 The seizure may end either gradually or abruptly.20,24,28

The EEG is characterized by diffuse, slow (2- to 2.5-Hz) and irregular spike and waves, which may be difficult to differentiate from interictal bursts.20 Discharges of rapid rhythms sometimes may be seen, preceded by flattening of the record for 1 to 2 seconds and followed by a progressive development of irregular fast rhythm in the anterior and central regions, ending with brief spike and waves.24,28

Other types of seizures

Generalized tonic-clonic seizures are reported in 15% of LGS patients, and complex partial seizures occur in 5%.20

Status epilepticus of different types (absence status epilepticus, tonic status epilepticus, nonconvulsive status epilepticus) can occur18,20 and often is prolonged and resistant to treatment.

The EEG during absence status epilepticus reveals continuous spike and wave discharges, usually at a lower frequency than at baseline. Rapid rhythms occur during tonic status epilepticus.28

Adapted from: Glauser TA and Morita DA. Lennox-Gastaut syndrome. In: Devinsky O and Westbrook LE, eds. Epilepsy and Developmental Disabilities. Boston: Butterworth-Heinemann; 2001;65–78.
With permission from Elsevier (www.elsevier.com).
Reviewed and revised May 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.

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