Lennox-Gastaut syndrome (LGS) is a chronic, developmental epileptic encephalopathy whose definition has evolved over time, much like the syndrome itself. There are three elements of the syndrome that have remained consistent since it’s early description: childhood onset; multiple medication-resistant seizure types; and cognitive impairment. To review each element of the definition: first, the epilepsy onset is in early childhood with the highest incidence between 3 to 5 years, before 8 years of age, but certainly before 18 years of age.1,2 LGS consists of multiple types of medication-resistant seizure types with tonic, atonic and atypical absence seizures which are the core group of seizure types with a caveat that not all of the seizures may be present at onset.1,3

Secondly, although not initially described in the syndrome, cognitive impairment is now considered a mandatory diagnostic element but is also not necessarily present at the onset of the syndrome.1,2 Thirdly, there are specific EEG patterns diagnostic of the syndrome, namely, diffuse slow spike wave complexes (DSSW) and generalized paroxysmal fast activity (GPFA). Neither EEG pattern in isolation is pathognomonic of the disorder, and both are transient, waxing and waning over time.3,4

To clarify the diagnosis and establish consistent diagnostic criteria for research and clinical purposes, the current ILAE criteria specify the syndrome has onset before 18 years and include:5 multiple medication-resistant seizures types which must include tonic seizures,1 cognitive and behavioral impairments which may develop later in the course, and6 EEG findings of generalized slow spike wave discharges (DSSW) and generalized paroxysmal fast activity (GPFA).1 (Table 1). In all 3 of the required EEG elements, the temporal variation of the symptomatology is acknowledged. The seizure types, including the mandatory tonic seizures vary over the patient’s course and may not be present at the onset of the syndrome, and may dissipate over time.5,1,2

The path to development of LGS can take many roads. Sixty five to 80 % of patients with LGS have an identifiable cause.1,7,8. This number is expected to grow based on further genetic developments.9 LGS can develop from focal cortical lesions, such as those in Tuberous Sclerosis Complex (TSC), cortical dysplasia, asymmetric hemispheric injury as in neonatal hypoxic ischemic encephalopathy (HIE), neonatal infections, traumatic brain injury, and genetic syndromes.2,10 Given the early age of onset, the working theory is that susceptibility of the immature brain, whether disrupted by discrete lesions or widespread injury, leads to the development and reinforcement of aberrant cerebral networks between cortical structures, the midbrain, and the brainstem. The synchronization of these networks leads to the development of the characteristic EEG findings, and the subsequent development of the symptomatic generalized seizure types.11

The aim of this review is to focus on the EEG elements, both ictal and interictal, present in LGS as they develop and vary over time. The scope of the review will focus on clinically available EEG studies, so that clinicians can become more comfortable with recognizing the characteristic patterns and identify patients early in the course of the syndrome. Clinical treatment trials suggest that patients without an early improvement in EEG background or reduction in interictal epileptiform discharges are least likely to respond to treatment, highlighting the crucial role that early identification and proper treatment have on the patient’s epilepsy and neurocognitive development.12