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PubMed This is a summary of 21 peer-reviewed journal articles Updated
Neurology

The LGS Diagnostic Triad: Symptoms and EEG Findings

At a Glance

Lennox-Gastaut Syndrome (LGS) is diagnosed using a 'triad' of three specific features: multiple types of hard-to-control seizures (especially tonic and atonic drop attacks), distinct EEG patterns like slow spike-and-wave and GPFA, and cognitive or developmental delays.

Diagnosing Lennox-Gastaut Syndrome (LGS) is like putting together a complex puzzle. Doctors look for a specific “triad”—three distinct features that must all be present to confirm the diagnosis [1][2]. Understanding these symptoms and the technical language on an EEG (electroencephalogram) report can help you advocate for the most accurate diagnosis.

1. Multiple Drug-Resistant Seizures

In LGS, patients do not just have one type of seizure; they have several different kinds that are often refractory (difficult to control with medicine) [3][4].

  • Tonic Seizures: These are the “hallmark” of LGS [5]. The muscles suddenly tighten or stiffen. These often happen during sleep (nocturnal tonic seizures) and may be brief enough to go unnoticed [5][6].
  • Atonic Seizures (Drop Attacks): The person suddenly loses muscle tone and goes limp, often leading to a fall [7]. In LGS, these “drops” can also be caused by a sudden stiffening (tonic-atonic) [8].
  • Atypical Absence Seizures: These are “staring spells” that are more gradual and last longer than a typical absence seizure. The person may be partially responsive or move their mouth or hands slightly [9][10].

2. Specific EEG Patterns

An EEG records the brain’s electrical activity. To confirm LGS, the report usually needs to mention two specific patterns:

  • Slow Spike-and-Wave (SSW): This is a pattern of electrical discharges that occurs at a frequency of less than 2.5 times per second (Hertz/Hz) [11][2]. This pattern often becomes more active or “louder” during sleep [12].
  • Generalized Paroxysmal Fast Activity (GPFA): This is a “fingerprint” of LGS, consisting of rapid bursts of electrical activity (often 10–20 Hz), most commonly seen during sleep [13][14].

EEG Report Checklist for Parents

When reading the EEG report, look for these specific terms:

  1. Slow spike-and-wave (< 2.5 Hz) [11].
  2. GPFA or Generalized Paroxysmal Fast Activity [13].
  3. Activation during sleep (the EEG patterns get worse during non-REM sleep) [12].

3. Cognitive and Developmental Impact

LGS is an encephalopathy, meaning the constant electrical “noise” in the brain interferes with normal function [15]. Most patients will have some degree of intellectual disability or developmental delay [10][1]. These challenges often become more apparent as the child grows into adulthood [16].

Differentiating LGS from Similar Syndromes

Because LGS shares symptoms with other rare epilepsies, it can sometimes be misdiagnosed.

Syndrome Key Difference from LGS
West Syndrome Occurs in infants; characterized by “infantile spasms” and an EEG pattern called hypsarrhythmia. It can evolve into LGS later [17][5].
Dravet Syndrome Often caused by a specific gene mutation (SCN1A). Seizures are frequently triggered by fever or heat [18][19].
Doose Syndrome Also called MAE; characterized by “myoclonic-atonic” seizures (a jerk followed by a drop). Unlike LGS, it usually does not include tonic stiffening seizures [8][12].
Pseudo-LGS A term used for syndromes that look like LGS on an EEG but may not have tonic seizures or the same severe developmental outlook [20][1].

If the diagnosis is unclear, a referral to a Level 4 Epilepsy Center for a high-resolution evaluation may be necessary to distinguish between these conditions [2][21].

Common questions in this guide

What is the diagnostic triad for Lennox-Gastaut Syndrome?
The LGS diagnostic triad includes three key features that doctors look for to confirm the condition. These are multiple types of drug-resistant seizures, specific abnormal brain wave patterns on an EEG, and some degree of cognitive or developmental delay.
What does slow spike-and-wave mean on my child's EEG report?
Slow spike-and-wave is a specific pattern of electrical activity in the brain that occurs at a frequency of less than 2.5 times per second. It is one of the primary EEG findings doctors use to diagnose LGS, and it often becomes more pronounced during sleep.
What are the most common types of seizures in LGS?
Tonic seizures, which involve sudden muscle stiffening, are the hallmark of LGS and frequently happen during sleep. Atonic seizures (also known as drop attacks) and atypical absence staring spells are also very common in children with LGS.
What is GPFA on an epilepsy sleep study?
GPFA stands for Generalized Paroxysmal Fast Activity. It is a rapid burst of brain electrical activity, usually between 10 and 20 Hertz, that frequently appears during non-REM sleep and acts as a strong indicator of Lennox-Gastaut Syndrome.
How is LGS different from Doose Syndrome?
While both conditions can involve drop attacks, Doose Syndrome is characterized by a muscle jerk followed by a drop. Unlike LGS, Doose Syndrome typically does not include the tonic stiffening seizures that are characteristic of Lennox-Gastaut Syndrome.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Does the EEG show 'slow spike-and-wave' discharges, and what is the frequency in Hertz (Hz)?
  2. 2.Was a sleep EEG performed, and did it show 'Generalized Paroxysmal Fast Activity' (GPFA)?
  3. 3.Are the 'drop attacks' caused by tonic stiffening or atonic limpness, and how does that affect treatment?
  4. 4.Does the clinical picture perfectly match LGS, or is there a chance this is another syndrome like Doose or Dravet?
  5. 5.How do the seizures and EEG results compare to 'Pseudo-LGS,' and what does that mean for long-term development?

Questions For You

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References

References (21)
  1. 1

    Applying the ILAE diagnostic criteria for Lennox-Gastaut syndrome in the real-world setting: A multicenter retrospective cohort study.

    Nightscales R, Chen Z, Barnard S, et al.

    Epilepsia open 2024; (9(2)):602-612 doi:10.1002/epi4.12894.

    PMID: 38135919
  2. 2

    The epilepsy phenotype in adult patients with intellectual disability and pathogenic copy number variants.

    d'Orsi G, Martino T, Palumbo O, et al.

    Seizure 2017; (53()):86-93 doi:10.1016/j.seizure.2017.11.009.

    PMID: 29156220
  3. 3

    Understanding the Burden of Lennox-Gastaut Syndrome: Implications for Patients, Caregivers, and Society in High and Low Resource Settings: A Narrative Review.

    Mourid MR, Oduoye MO

    Health science reports 2024; (7(12)):e70169 doi:10.1002/hsr2.70169.

    PMID: 39669191
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    Lennox-Gastaut syndrome: a comprehensive review.

    Asadi-Pooya AA

    Neurological sciences : official journal of the Italian Neurological Society and of the Italian Society of Clinical Neurophysiology 2018; (39(3)):403-414 doi:10.1007/s10072-017-3188-y.

    PMID: 29124439
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    Evolution of Infantile Spasms to Lennox-Gastaut Syndrome: What Is There to Know?

    Nelson JA, Demarest S, Thomas J, et al.

    Journal of child neurology 2021; (36(9)):752-759 doi:10.1177/08830738211000514.

    PMID: 33764203
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    Comparative Analysis of Lennox-Gastaut Syndrome With Different Subtypes of Tonic Seizures: A Single-Center Retrospective Cohort Study.

    Wang S, Zhao X, Li T, et al.

    Pediatric neurology 2024; (161()):132-138 doi:10.1016/j.pediatrneurol.2024.07.018.

    PMID: 39378606
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    Stereotactic laser anterior corpus callosotomy for Lennox-Gastaut syndrome.

    Tao JX, Satzer D, Issa NP, et al.

    Epilepsia 2020; (61(6)):1190-1200 doi:10.1111/epi.16535.

    PMID: 32401350
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    Epilepsy with myoclonic-atonic seizures, also known as Doose syndrome: Modification of the diagnostic criteria.

    Oguni H

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    Absence-Atonic Seizures: A Rare Generalized Seizure Type.

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    Journal of child neurology 2026; (41(3)):407-409 doi:10.1177/08830738251359388.

    PMID: 40696998
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    Evolution and course of early life developmental encephalopathic epilepsies: Focus on Lennox-Gastaut syndrome.

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    Epilepsia 2018; (59(11)):2096-2105 doi:10.1111/epi.14569.

    PMID: 30255934
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    Discovering EEG biomarkers of Lennox-Gastaut syndrome through unsupervised time-frequency analysis.

    Hu DK, Pinto-Orellana MA, Rana M, et al.

    Epilepsia 2025; (66(2)):541-553 doi:10.1111/epi.18211.

    PMID: 39666270
  12. 12

    Long-term outcome of developmental and epileptic encephalopathies.

    Van Bogaert P

    Revue neurologique 2022; (178(7)):659-665 doi:10.1016/j.neurol.2022.01.009.

    PMID: 35489823
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    Paroxysmal fast activity is a biomarker of treatment response in deep brain stimulation for Lennox-Gastaut syndrome.

    Dalic LJ, Warren AEL, Spiegel C, et al.

    Epilepsia 2022; (63(12)):3134-3147 doi:10.1111/epi.17414.

    PMID: 36114808
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    Automatic detection of generalized paroxysmal fast activity in interictal EEG using time-frequency analysis.

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    History of Lennox-Gastaut Syndrome: An electro-clinical voyage in search of an epileptic syndrome.

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    Seizure 2025; (133()):251-260 doi:10.1016/j.seizure.2025.11.001.

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    Lennox-Gastaut syndrome in adulthood: Long-term clinical follow-up of 38 patients and analysis of their recorded seizures.

    Vignoli A, Oggioni G, De Maria G, et al.

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    Pediatric Epileptic Encephalopathies: Pathophysiology and Animal Models.

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    Seminars in pediatric neurology 2016; (23(2)):98-107.

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    Characteristics of Genetic Variations Associated With Lennox-Gastaut Syndrome in Korean Families.

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    SCN1A Mutation-Beyond Dravet Syndrome: A Systematic Review and Narrative Synthesis.

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    [Why the diagnosis of Lennox-Gastaut syndrome is a rare one?]

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This page explains the diagnostic criteria and EEG findings for Lennox-Gastaut Syndrome for educational purposes. Always consult a pediatric neurologist or epileptologist to interpret your child's specific medical tests and symptoms.

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