seizures, but while some authors report that tonic seizures become rare to absent, for others, tonic seizures remain the major problem (Roger et al 1989, Dulac and Engel ILAE link). Electroencephalogram (EEG) and magnetoencephalogram (MEG) features Slow ( 2.5 Hz) SW, ﬁrst described by Gibbs et al (1939), is the most prominent feature in interictal awake EEG. The average age of onset of slow SW is 8.2 years, with a mean duration of 8.6 years. However, EEG criteria should also include 10–20 Hz epileptic recruiting or fast rhythm, appearing principally in sleep (Genton et al 2000). Generalized polySW and lower voltage of short duration fast activities are usual during slow sleep. Photic stimulation has no impact on the EEG while hyperpnea may exacerbate slow SW. Focal paroxysmal discharges or slowing may be recorded in symptomatic LGS, related to the underlying pathology. The ictal EEG pattern depends on the seizure type. Atypical absences are recorded during slow SW but they are sometimes difﬁcult to distinguish from the background from a clinical and EEG point of view. Myoclonic seizures are associated with bilateral and symmetrical SW and polySW whereas fast activities are typical of tonic seizures. With age and disease evolution, EEG changes and multifocal spikes may be seen as well as diffuse or localized slowing. Diffuse slow SW tend to be “frontalized”. MEG records the magnetic signal generated by the intraneuronal current. This signal is not attenuated or distorted as is the electric signal. MEG, with up to more than 100 channels, is more powerful in dipole localization and in evaluating transcallosal spread in generalized epileptiform discharges due to secondary bisynchrony. In most cases, MEG information is combined with MRI images and can provide anatomic localization of interictal epileptiform activities (Technology Evaluation Center 2003). Anecdotal case reports (Sakurai et al 2006) offer some support for the use of MEG in “atypical” LGS, particularly if surgical treatment is being considered. Imaging (positon emission tomography (PET) scan, magnetic resonance imaging (MRI) features The MRI is by deﬁnition, normal in cryptogenic cases, ie, in about 30% of cases. However, high MRI resolution may show small amounts of cortical dysplasia in a previously normal MRI. Occasionally, a brain tumor (Quarato et al 2002) or broad dysplasia (You and Lee 2007b) can be found with Neuropsychiatric Disease and Treatment 2008:4(6) Lennox-Gastaut treatment MRI and consequently surgery may be offered as a curative treatment. Usually, symptomatic cases have a worse prognosis than cryptogenic LGS. Goldsmith et al (2000) did not ﬁnd (in a retrospective study of 107 conﬁrmed cases of LGS from 245 presumed cases) any prognostic value of MRI on seizure outcome; 74 of the 107 patients were followed-up for 3 years. However, these investigators divided their LGS patients into three groups: cryptogenic, symptomatic and indeterminate. According to the ILAE, only cryptogenic and symptomatic cases are recognized. Based on the descriptions given by Goldsmith et al the indeterminate cases belong to the cryptogenic group and this multiple subdivision gives less weight to the statistics. PET scan may show focal hypometabolism even in cryptogenic cases (Gur RC et al 1982; Ferrie et al 1996). Therefore, this imaging technique is recommended for presurgical evaluation, even if the MRI is normal or the EEG shows multifocal abnormalities. Differential diagnosis Clinically, the differential diagnosis of drop attacks is the major concern. The slow SW pattern represents another challenge for diagnosis on awake EEG. Some epileptic syndromes or clinical situations must be excluded: – Atypical cases of continuous SW during slow sleep with akinetic absence status and/or negative myoclonia – Doose syndrome – Dravet syndrome – Toxic encephalopathy – Metabolic diseases, such as Glut-1 deﬁciency syndrome (De Vivo et al 1991; Wang et al 2005) and pyruvate dehydrogenase deﬁciency (Wexler et al 1997), – Focal or multifocal epilepsy with secondary bisynchrony on EEG – Late onset epileptic spasms It is, therefore, mandatory to record a sleep EEG and, if necessary, video-EEG monitoring or polygraph recording, before making a diagnosis of LGS. It is not rare (up to 40% of secondary generalized epilepsies in childhood) (Camﬁeld and Camﬁeld 2007) for children with mental handicap, slow SW and several seizure types to remain with an undetermined diagnosis despite full investigation. Evolution and prognosis Overall prognosis remains poor in spite of new therapeutic strategies. About 90% of patients become mentally 1003