Dravet Syndrome

Dravet Syndrome

Overview

Dravet syndrome (DS), formerly known as severe myoclonic epilepsy of infancy (SMEI), is a rare, devastating developmental and epileptic encephalopathy (DEE) with an estimated prevalence of 1 in 15,700–40,000 live births. The disorder typically manifests in the first year of life with prolonged, fever-triggered (febrile) seizures, followed by the emergence of multiple seizure types including myoclonic, atypical absence, and focal seizures. Beyond seizures, patients experience progressive developmental regression, intellectual disability, gait abnormalities, and high mortality (estimated 10–15% cumulative mortality by age 20).

The vast majority of Dravet syndrome cases are caused by heterozygous loss-of-function variants in [SCN1A](/entities/scn1a) (sodium channel neuronal type 1 alpha subunit), a gene encoding a voltage-gated sodium channel critical for neuronal excitability, particularly in GABAergic interneurons[@dravet2018].

Genetics and Molecular Basis

SCN1A Gene

[SCN1A](/entities/scn1a) is located on chromosome 2q24.3 and encodes Nav1.1, a voltage-gated sodium channel alpha subunit predominantly expressed in inhibitory GABAergic interneurons throughout the brain. The gene spans approximately 190 kb and contains 26 exons. Over 1,000 pathogenic variants have been described, including:

Dravet Syndrome

Overview

Dravet syndrome (DS), formerly known as severe myoclonic epilepsy of infancy (SMEI), is a rare, devastating developmental and epileptic encephalopathy (DEE) with an estimated prevalence of 1 in 15,700–40,000 live births. The disorder typically manifests in the first year of life with prolonged, fever-triggered (febrile) seizures, followed by the emergence of multiple seizure types including myoclonic, atypical absence, and focal seizures. Beyond seizures, patients experience progressive developmental regression, intellectual disability, gait abnormalities, and high mortality (estimated 10–15% cumulative mortality by age 20).

The vast majority of Dravet syndrome cases are caused by heterozygous loss-of-function variants in [SCN1A](/entities/scn1a) (sodium channel neuronal type 1 alpha subunit), a gene encoding a voltage-gated sodium channel critical for neuronal excitability, particularly in GABAergic interneurons[@dravet2018].

Genetics and Molecular Basis

SCN1A Gene

[SCN1A](/entities/scn1a) is located on chromosome 2q24.3 and encodes Nav1.1, a voltage-gated sodium channel alpha subunit predominantly expressed in inhibitory GABAergic interneurons throughout the brain. The gene spans approximately 190 kb and contains 26 exons. Over 1,000 pathogenic variants have been described, including:

• Missense variants (~40%): amino acid substitutions that disrupt channel function
• Nonsense/frameshift variants (~30%): premature stop codons leading to truncated proteins
• Splice site variants (~15%): aberrant mRNA processing
• Duplications/deletions (~5%): copy number variants affecting gene dosage
• Large genomic rearrangements (~5%): encompassing whole exons or the entire gene[@dravet2018]

Pathophysiology

Nav1.1 is critical for the proper function of GABAergic inhibitory interneurons. Loss of functional Nav1.1 channels leads to:

This "interneuronopathy" model explains why DS is distinct from other channelopathies: the primary defect is loss of inhibition, not gain of excitability[@dravet2018].

Epidemiology

| Metric | Value |
|--------|-------|
| Prevalence | 1:15,700–40,000 live births |
| Incidence | ~1:20,000–50,000 (varies by region) |
| Sex ratio | Slight male predominance (1.5:1) |
| Family recurrence | <5% (mostly de novo variants) |
| Mutational origin | ~80% de novo, ~20% inherited from affected or mosaic parent |

Prevalence estimates have improved with broader genetic testing; some studies suggest prevalence may be as high as 1:5,000 given underdiagnosis in resource-limited settings[@dravet2018].

Clinical Presentation

Early Infancy (0–12 months)

The hallmark presentation is onset of febrile seizures (temperature >38°C) between 3–18 months, typically 6–9 months. These initial seizures are often prolonged (>10 minutes, qualifying as status epilepticus) and may be generalized or hemiclonic. Importantly, initial development is typically normal before seizure onset.

Childhood (1–5 years)

Multiple seizure types emerge, including:

• Myoclonic seizures — sudden, brief jerks of limbs or face
• Atypical absence seizures — staring spells with mild automatisms
• Focal seizures — with or without secondary generalization
• Tonic-clonic seizures — generalized convulsions
• Status epilepticus — prolonged seizures (>5 min) requiring emergency intervention

Adolescence and Beyond

Seizures may become less frequent in some patients, but the burden of cognitive impairment, behavioral disorders (autism spectrum features, ADHD), and motor dysfunction (ataxia, gait instability) persists. Patients remain dependent on caregivers throughout life.

Diagnosis

Clinical Diagnostic Criteria

Based on the International League Against Epilepsy (ILAE) classification, Dravet syndrome is diagnosed clinically by:

Genetic Confirmation

Genetic testing is the gold standard for diagnosis:

• Seizure-onset panel (first-line): tests SCN1A and related genes
• Whole exome sequencing (if panel negative): identifies SCN1A and other causes
• Chromosomal microarray (if panel/WES negative): detects deletions/duplications
• Targeted SCN1A sequencing (if family history): confirm variant in proband

EEG and Neuroimaging

• EEG: Initially normal, later showing generalized or focal abnormalities, photosensitivity (>40% of patients), and slowing of background activity
• MRI: Often normal early, may show cerebellar or cerebral atrophy later
• FDG-PET: May show regional hypometabolism in thalamus, cortex

Treatment

Anti-Seizure Medications (ASMs)

Dravet syndrome is notably resistant to many conventional ASMs. Evidence-based treatments include:

| Drug | Evidence Level | Mechanism | Notes |
|------|---------------|-----------|-------|
| Fenfluramine (Fintepla) | Strong (Phase 3 RCT) | 5-HT agonist, sigma-1 agonist | Only FDA-approved DS-specific therapy; significant seizure reduction[@dravet2020] |
| Cannabidiol (Epidiolex) | Strong (Phase 3 RCT) | CB1/CB2 modulation, GPR55 | FDA-approved for Dravet; reduces seizure frequency[@dravet2018] |
| Stiripentol (Diacomit) | Strong (EMA-approved) | GABA-A potentiation | Often used in combination with clobazam + valproate |
| Clobazam | Moderate | Benzodiazepine, GABA-A modulation | Tachyphylaxis common; useful as adjunct |
| Valproic acid | Moderate | Multiple mechanisms | Broad-spectrum ASM; often first-line |
| Topiramate | Low-moderate | Multiple mechanisms | May worsen cognitive effects |
| Levetiracetam | Low | SV2A modulation | Often ineffective; may worsen myoclonus |

AVOID: Carbamazepine, lamotrigine, vigabatrin, phenytoin — these can exacerbate seizures in DS.

Fenfluramine (Fintepla) — DS-Specific Therapy

Fenfluramine received FDA approval for Dravet syndrome in 2020 based on two Phase 3 placebo-controlled trials demonstrating a median seizure reduction of ~50-70%[@dravet2020]. Its mechanism involves:

• Serotonin (5-HT2B/2C) receptor agonism
• Sigma-1 receptor agonism
• Possible modulation of sodium and calcium channels

Non-Pharmacologic Therapies

• Ketogenic diet: 50-70% seizure reduction in some patients; challenging to maintain long-term
• Vagus nerve stimulation (VNS): May reduce seizure frequency by 30-50% in some patients
• Responsive neurostimulation (RNS): Limited data in DS specifically
• Corpus callosotomy: May help drop attacks/atonic seizures

Gene Therapy Approaches

Dravet syndrome is a prime target for gene therapy approaches given its monogenic cause and early-onset, progressive nature. Multiple approaches are in development:

Antisense Oligonucleotides (ASOs)

STK-001 (Stoke Therapeutics) — allele-specific ASO that increases SCN1A mRNA and Nav1.1 protein expression from the wild-type allele. Currently in Phase 1/2 clinical trials (NCT04442152)[@dravet2018]. See [clinical trial page for STK-001](/clinical-trials/stk001-dravet-syndrome-phase-1-2).

STK-002 (Stoke Therapeutics) — higher-dose formulation for adults in Phase 1.

Gene Activation (CRISPRa)

ETX101 (Encoded Therapeutics) — AAV9-delivered CRISPR-activation construct targeting the SCN1A promoter to increase endogenous expression. Preclinical IND-enabling studies. See [clinical trial page for ETX101](/clinical-trials/etx101-encoded-therapeutics-dravet-syndrome).

Gene Replacement

Full-length SCN1A (~6kb coding sequence) approaches AAV packaging limits (~4.7kb with regulatory elements). Strategies include:

• Mini-gene constructs (truncated but functional)
• Dual-vector split approaches (intein-mediated trans-splicing)
• Intracerebroventricular (ICV) delivery to maximize CNS tropism

Base/Prime Editing

With ~40% of Dravet patients having missense variants, precision editing approaches could address the underlying cause. Beam Therapeutics and academic groups are developing base editing strategies for SCN1A missense mutations.

Prognosis

| Outcome | Details |
|---------|---------|
| Seizure trajectory | Often improves after adolescence but rarely seizure-free |
| Cognitive outcome | 100% have intellectual disability (IQ typically 30-70) |
| Motor outcome | Progressive gait instability, ataxia in >50% |
| Behavioral | Autism spectrum features in 50-70%; ADHD, anxiety common |
| Sleep | Disrupted sleep architecture in >70% |
| Mortality | 10-15% by age 20; SUDEP (sudden unexpected death in epilepsy) is the leading cause |

Research Landscape

Key open questions for Dravet syndrome research:

References