Lennox-Gastaut syndrome (LGS) has a genetic component, but it is not a straightforward inherited condition. Most cases are sporadic, meaning they appear in children with no family history of the disorder. When a genetic change is involved, it is usually a new (de novo) mutation that occurred randomly during early development rather than one passed down from a parent. Between 3 and 30 percent of people with LGS do have a family history of some form of epilepsy, suggesting inherited factors play a role in a subset of cases.
Why Most Cases Are Not Inherited
LGS is best understood as a syndrome with many possible triggers rather than a single genetic disease. It can result from structural brain differences, infections, metabolic problems, immune-related causes, or genetic changes. In many children, no clear cause is ever identified. This wide range of potential origins is part of what makes the genetics of LGS so difficult to pin down.
When gene mutations are found in a child with LGS, they are almost always de novo. That means the mutation happened spontaneously in the parent’s egg or sperm, or very early in embryonic development. Neither parent carries the mutation in their own cells. This is an important distinction for families: having one child with LGS caused by a de novo mutation does not typically raise the risk for future children.
Genes Linked to LGS
Researchers have identified variants in several genes that appear in small numbers of people with LGS. These include SCN1A, SCN8A, STXBP1, CHD2, FOXG1, DNM1, GABRB3, and ALG13, among others. Each of these genes plays a role in how nerve cells in the brain communicate, but exactly how a change in any one of them leads to LGS is still not fully understood.
Some of these genes control sodium channels, which are gateways that let electrical signals pass between brain cells. Others affect receptors for GABA, the brain’s main calming chemical, or are involved in how brain cells release signaling molecules. A mutation in any of these areas can disrupt the normal balance of electrical activity in the brain, but each gene accounts for only a small fraction of all LGS cases. There is no single “LGS gene.”
LGS can also develop as part of a broader genetic condition. Tuberous sclerosis complex (TSC) is one well-known example. In TSC, mutations in the TSC1 or TSC2 genes cause abnormal growths in the brain and other organs, and the resulting brain changes can trigger seizures that evolve into an LGS pattern. Children with TSC who develop LGS have a clear genetic cause, but the genetic change is specific to TSC rather than to LGS itself.
What LGS Looks Like
LGS typically begins in early childhood, usually between ages 1 and 7. It is defined by a combination of multiple seizure types, a distinctive brain wave pattern on EEG, and, in most cases, intellectual disability or developmental delays. The seizure types that raise suspicion for LGS include tonic seizures (sudden stiffening of the body), atonic seizures (sudden loss of muscle tone causing falls), and atypical absence seizures (brief episodes of staring or reduced awareness that start and stop less abruptly than typical absence seizures).
On an EEG, LGS produces a characteristic slow spike-and-wave pattern at 2.5 Hz or less, most prominent over the frontal region of the brain. During sleep, the EEG also shows bursts of fast rhythmic activity between 10 and 30 Hz. Both of these EEG features are required for a formal diagnosis.
Why Genetic Testing Matters
Even though no single gene causes most cases of LGS, genetic testing has become an increasingly important part of evaluation. Exome sequencing, which reads the protein-coding portions of a child’s DNA, can sometimes identify a specific mutation that changes how the condition is managed.
The reason is precision. Different genetic causes respond to different treatments, and some mutations make certain medications harmful rather than helpful. For instance, children with SCN1A loss-of-function variants (the same gene involved in Dravet syndrome) can worsen on sodium channel-blocking seizure medications, which are standard treatments for other forms of epilepsy. Knowing the genetic cause helps doctors avoid that mistake. Conversely, children with SCN8A gain-of-function variants may benefit from those same sodium channel blockers.
For children whose LGS stems from tuberous sclerosis complex, medications that target the overactive growth-signaling pathway (the mTOR pathway) can act as disease-modifying therapy rather than just suppressing seizures. The ketogenic diet, a high-fat and very low-carbohydrate eating plan used to control seizures, is considered a precision treatment for children with glucose transporter type 1 deficiency, a metabolic condition that can produce an LGS pattern. In each of these scenarios, identifying the underlying genetic or metabolic cause directly shapes the treatment plan.
The Role of Family History
The 3 to 30 percent range for family history of epilepsy in LGS families is broad, and it reflects how variable this syndrome is. Having a relative with epilepsy does not mean LGS itself runs in the family. It may mean that a general susceptibility to seizures exists in the family’s genetic background, and that susceptibility interacted with other factors (brain injury, infection, a second genetic hit) to produce LGS in one individual.
For parents of a child with LGS who are considering having more children, genetic counseling can help clarify the specific situation. If a de novo mutation has been identified, the recurrence risk is generally very low. If a heritable condition like tuberous sclerosis complex is the cause, the risk depends on whether the parent also carries the TSC gene variant. And in the many cases where no genetic cause is found, counselors can discuss what the available evidence suggests about recurrence based on the family’s overall history.