CDKL5 deficiency disorder (CDD) is an X-linked developmental disorder caused by pathogenic variants in CDKL5. It is characterised by very early-onset epilepsy together with severe developmental impairment and frequent visual, motor, sleep, gastrointestinal and autonomic problems.
Early seizure onset
Seizures usually begin within the first months of life and often occur many times per day.
Epileptic spasms, tonic, focal, myoclonic and bilateral tonic-clonic seizures can all occur, and seizure type commonly changes with age.
EEG can be normal early
A normal early EEG does not exclude CDD.
As the disorder evolves, multifocal or generalised epileptiform abnormalities and other severe EEG patterns may appear.
Development
Motor, language and cognitive development are substantially affected.
Some individuals learn to sit or walk independently and some develop spoken words, but ability varies widely.
Cerebral visual impairment
Cerebral visual impairment is common and can include poor visual attention, abnormal fixation, strabismus and nystagmus.
Visual impairment can further affect communication and development.
Movement disorders
Chorea, dystonia, stereotyped movements and other movement abnormalities can occur.
These need to be distinguished from epileptic seizures.
Sleep, feeding and autonomic problems
Sleep disturbance, gastrointestinal problems, feeding difficulty and breathing abnormalities are common.
Management therefore requires multidisciplinary care.
Genetics and sex
CDD is X-linked and is diagnosed more often in females, but males can also be severely affected.
Severity can be influenced by variant type, X-chromosome inactivation and mosaicism.
How the diagnosis is made
Diagnosis requires a compatible clinical phenotype and a pathogenic or likely pathogenic CDKL5 variant.
Broad epilepsy or neurodevelopmental gene panels, exome sequencing or genome sequencing can identify the cause.
Treatment difficulty
Epilepsy is often drug resistant. Multiple antiseizure medicines may be tried over time.
Ketogenic dietary therapy and other non-drug treatments can be considered in selected people.
Ganaxolone
Ganaxolone is a neuroactive steroid modulator developed specifically for CDD-associated seizures and has regulatory approval in some countries.
Availability, age indications and reimbursement vary internationally, so it should not be presented as universally accessible.
Other therapies
Standard broad-spectrum antiseizure medicines remain widely used, and treatment often changes as seizure types evolve.
No current therapy eliminates the broader developmental disorder.
Research
CDD is an active area of disease-modifying research, including gene replacement and gene-regulation strategies.
These approaches remain investigational.
Why CDD is no longer considered a Rett variant
CDD was historically described as an early-seizure variant of Rett syndrome.
It is now recognised as a distinct genetic disorder with its own natural history, molecular cause and treatment development programme.
Worldwide access
Molecular diagnosis is essential but not universally available.
Access to ganaxolone, ketogenic therapy, communication support and cerebral-visual-impairment services is also highly unequal across countries.
The central point
CDD is a developmental disorder with epilepsy, not epilepsy alone. Seizure management is essential, but vision, movement, communication, sleep and feeding are equally important parts of care.
Related reading
Early Infantile DEE →
Genetic Testing in Epilepsy →
Sources
Medical note: medicine licensing and access for CDD differ internationally and change over time.
Evidence checked: September 2026.