SLC6A1-NDD: Clinical Overview and Resources for Medical Professionals
SLC6A1-related neurodevelopmental disorder (SLC6A1-NDD) is a rare genetic neurodevelopmental disorder with a broad and variable clinical spectrum.
This page has been developed by SLC6A1 Europe, a patient-led organisation, to provide healthcare professionals with concise, evidence-based information on the clinical presentation, diagnosis, management and follow-up of individuals with SLC6A1-NDD.
The information on this page is drawn from published scientific literature and is intended to support, not replace, specialist clinical assessment, professional judgement and individualised care.
>97%
Developmental Delay
In a 2026 clinical study by Varnet et al., developmental delay was documented in 97% (61/63) of individuals with SLC6A1-NDD. Developmental regression was also reported in 41%, sometimes involving the loss of previously acquired language or motor skills.
Reference: [8] Varnet et al., 2026.
~85%
Epilepsy
According to Goodspeed et al. (2023), seizures affect approximately 85% of individuals with SLC6A1-NDD. Absence or atypical absence, myoclonic and atonic seizures are the most frequently reported types. Some individuals experience treatment-resistant epilepsy.
Frequently Affected
Language Impairment
Johannesen et al. (2018) identified language impairment as the most common developmental difficulty in their study of 34 individuals with SLC6A1 variants. Goodspeed et al. (2023) further reported that expressive language is often more severely affected than motor development, with abilities ranging from delayed speech to minimal or absent verbal communication.
References: [1] Goodspeed et al., 2023; [2] Johannesen et al., 2018.
78%
Behavioural Difficulties
Varnet et al. (2026) documented behavioural difficulties in 78% (46/59) of individuals with SLC6A1-NDD in their clinical study. Among those affected, 80% experienced disruptive emotional outbursts, often accompanied by low frustration tolerance and impulsivity.
Reference: [8] Varnet et al., 2026
41%
Aggression and Self-Injury
Varnet et al. (2026) reported aggression in 41% (26/63) of individuals with SLC6A1-NDD and self-injurious behaviour in 8 participants in their clinical study. Caregivers frequently described aggression as unpredictable and particularly challenging to manage in daily life.
Reference: [8] Varnet et al., 2026
60% ASD, 29% ADHD
Autism and ADHD
Varnet et al. (2026) reported autism spectrum disorder (ASD) in 60% (38/63) and attention-deficit/hyperactivity disorder (ADHD) in 29% (18/63) of individuals with SLC6A1-NDD in their clinical study.
References: [8] Varnet et al., 2026
Clinically Relevant
Psychiatric Features
Varnet et al. (2026) reported anxiety in 28% (13/46) and selective mutism in 7% (3/46) of individuals with documented behavioural difficulties in their clinical study. Rees et al. (2020) also identified an association between rare SLC6A1 variants and schizophrenia, although its relevance to SLC6A1-NDD remains uncertain.
References: [8] Varnet et al., 2026; [9] Rees et al., 2020.
65%
Movement and Coordination difficulties
Varnet et al. (2026) documented movement and coordination difficulties in 65% (41/63) of individuals with SLC6A1-NDD in their clinical study, including clumsiness, tremor, ataxia and gait abnormalities. Among 20 participants assessed prospectively, 90% screened positive for probable developmental coordination disorder.
Reference: [8] Varnet et al., 2026.
56%
Sleep Disturbances
Varnet et al. (2026) documented sleep disturbances in 56% (35/63) of individuals with SLC6A1-NDD in their clinical study. All 20 participants assessed prospectively showed clinically significant sleep difficulties on standardised questionnaires, suggesting that these problems may be underrecognised.
Reference: [8] Varnet et al., 2026.
Diagnosis and Genetic Confirmation
The clinical manifestations associated with SLC6A1-NDD overlap with many other genetic neurodevelopmental disorders. For this reason, SLC6A1-NDD cannot usually be identified on clinical features alone and diagnosis relies on molecular genetic testing. [1]
Genetic Confirmation
A molecular diagnosis of SLC6A1-NDD is established when a heterozygous pathogenic or likely pathogenic variant in SLC6A1 is identified in an individual with a compatible clinical phenotype. [1]
Testing should adequately cover the SLC6A1 coding regions and canonical splice-site regions. Pathogenic variants include missense, nonsense, small insertion/deletion and splice-site variants. [1]
A variant of uncertain significance (VUS) does not by itself establish or exclude the diagnosis and should be interpreted in the context of the clinical phenotype, inheritance and available functional evidence. [1,2]
Parental testing may help determine whether the variant occurred de novo or was inherited and can contribute to variant interpretation and genetic counselling.
EpiPred – Missense Variant Interpretation
EpiPred is a publicly accessible, gene-specific machine learning tool developed by Dr. Gemma Carvill and collaborators as part of the NIH-funded EpiMVP initiative.
The tool predicts the potential pathogenicity of missense variants in five epilepsy-related genes, including SLC6A1, and is particularly relevant for investigating variants of uncertain significance (VUS).
EpiPred can assess all possible missense substitutions in the supported genes, providing additional information to support variant interpretation. However, predictions do not establish a clinical diagnosis and must be considered alongside clinical, genetic and functional evidence.
Published experimental validation has focused on STXBP1; predictions for SLC6A1 should therefore be regarded as investigational rather than definitive clinical classifications. [7]
For a broader overview of the clinical spectrum and underlying biology of SLC6A1-NDD, see What is SLC6A1-NDD?
Epilepsy and EEG
Epilepsy is one of the most common manifestations of SLC6A1-NDD. The epilepsy phenotype is predominantly generalised, although focal seizures and focal epilepsy have also been reported. Age at seizure onset and epilepsy severity vary considerably between individuals. [1–3]
Seizure Types and Epilepsy Phenotypes
The electroclinical spectrum of SLC6A1-NDD is broad. Reported seizure types include:
-
typical and atypical absence seizures
-
absence seizures with eyelid myoclonia
-
myoclonic seizures
-
atonic seizures and drop attacks
-
myoclonic-atonic seizures
-
generalised tonic-clonic seizures
-
focal seizures, less commonly [1–4]
Different seizure types frequently coexist in the same individual.
Recognised epilepsy phenotypes include epilepsy with myoclonic-atonic seizures (EMAtS), early-onset and childhood absence epilepsy, epilepsy with eyelid myoclonia, genetic or unclassified generalised epilepsy, and developmental and epileptic encephalopathy. Rarely, Lennox–Gastaut syndrome and focal epilepsy phenotypes have also been reported. [1–4]
Absence seizures, particularly atypical absences, and EMAtS are among the most characteristic epilepsy presentations associated with SLC6A1-NDD, although the phenotype is not restricted to these syndromes. [1–3]
Status Epilepticus
Status epilepticus has been reported in SLC6A1-NDD.
Documented presentations include:
-
absence status epilepticus / non-convulsive status epilepticus (NCSE)
-
generalised convulsive status epilepticus, reported in individual cases [2,4]
Because absence seizures may be frequent, subtle or prolonged, an unexplained change in responsiveness, awareness or behaviour should prompt consideration of an ictal state and EEG evaluation when clinically appropriate.
Separately, EEG patterns resembling electrical status epilepticus during sleep (ESES/EE-SWAS) have been reported in a small number of individuals with SLC6A1-NDD. This represents a sleep-activated EEG pattern and should be distinguished from clinical status epilepticus. [2]
EEG Findings
The most characteristic EEG finding is generalised 2–4 Hz spike-and-wave or polyspike-and-wave activity, often irregular and sometimes with an occipital predominance or gradual posterior onset. [1–3]
Importantly, EEG recordings may be normal in young children, particularly before the age of 2 years. A normal early EEG therefore does not exclude SLC6A1-NDD or the later development of epilepsy. [2,4]
Other reported EEG findings include:
-
exacerbation of generalised epileptiform discharges with hyperventilation
-
occipital intermittent rhythmic delta activity (OIRDA)
-
generalised background slowing
-
focal epileptiform abnormalities in a minority of individuals
-
marked activation of epileptiform discharges during sleep [1–3]
Marked spike-and-wave activation in sleep (SWAS), historically described as an ESES-like pattern, has been reported in a small number of individuals. [2]
EEG findings should always be interpreted together with the clinical presentation, as seizure types and electroclinical patterns can evolve over time.
Recommended Surveillance
GeneReviews recommends longitudinal surveillance for individuals with SLC6A1-NDD, with particular attention to: [1]
-
developmental progress and educational needs
-
seizure control and the emergence of new seizure types
-
changes in tone and movement disorders
-
ASD, ADHD, aggression, self-injury, anxiety and sleep disturbances
-
constipation and diarrhoea
-
changing family support and care-coordination needs
Monitoring should be adapted to the individual's clinical presentation and needs.
Management
There are currently no approved disease-specific treatments for SLC6A1-NDD. Management is supportive and should be individualised according to the patient's clinical manifestations and functional needs. [1]
Care is typically multidisciplinary and may involve neurology, developmental medicine, rehabilitation, speech and language therapy, behavioural or psychiatric support, nutrition and other specialties as required.
Epilepsy should be managed by an experienced neurologist. Several anti-seizure medications may be effective, but no medication has been demonstrated to be specifically effective for SLC6A1-NDD. [1]
For a detailed overview of symptomatic treatments, available evidence and emerging therapeutic approaches, see Treatments & Therapeutic Development.
Resources
The following resources provide further clinical and scientific information on SLC6A1-NDD.
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References
1. Goodspeed K, Demarest S, Johannesen K, Kang J, Lal D, Angione K.
SLC6A1-Related Neurodevelopmental Disorder.
GeneReviews®. University of Washington, Seattle. 2023.
PMID: 36780407.
2. Johannesen KM, Gardella E, Linnankivi T, et al.
Defining the phenotypic spectrum of SLC6A1 mutations.
Epilepsia. 2018;59(2):389–402.
doi: 10.1111/epi.13986.
3. Samanta D.
SLC6A1-Related Neurodevelopmental Disorder: A Scoping Review of Clinical Features and Emerging Therapeutic Strategies.
Pediatric Neurology. 2026;180:155–170.
doi: 10.1016/j.pediatrneurol.2026.04.014.
4. Caraballo RH, Chacón S, Touzon MS, Loos M, Juanes M.
De novo absence status epilepticus associated with the SLC6A1 gene in a pediatric patient.
Epileptic Disorders. 2024;26:233–235.
doi: 10.1002/epd2.20175.
5. Stefanski A, Pérez-Palma E, Brünger T, et al.
SLC6A1 variant pathogenicity, molecular function and phenotype: a genetic and clinical analysis.
Brain. 2023;146(12):5198–5208.
doi: 10.1093/brain/awad292.
6. Li Z, Han C, Zhao H.
Clinical and genetic analysis of epilepsy with myoclonic-atonic seizures caused by SLC6A1 gene variant.
Frontiers in Pediatrics. 2025;12:1492062.
doi: 10.3389/fped.2024.1492062.
7. Calhoun JD, Wang C, Biar CG, et al.
Gene-specific machine learning model EpiPred identifies likely pathogenic variants in the epilepsy-related gene STXBP1.
Journal of Clinical Investigation. 2026.
doi: 10.1172/JCI207160.
8. Varnet ME, Dahshi H, Goodspeed K, McDonnell PP, Stripe LL, Armstrong DM.
Beyond Seizure Management in a Developmental Epileptic Encephalopathy: Expanding the SLC6A1 Neurodevelopmental Disorder Phenotype.
Neurology Genetics. 2026;12(5).
doi: 10.1212/NXG.0000000000200436.
9. Rees E, Han J, Morgan J, et al.
De novo mutations identified by exome sequencing implicate rare missense variants in SLC6A1 in schizophrenia.
Nature Neuroscience. 2020;23(2):179–184.
doi: 10.1038/s41593-019-0565-2.