Clinical Characteristics

STXBP1 mutations are characterized by a variety of signs and symptoms. The condition caused by STXBP1 mutations is often referred to as STXBP1-encephalopathy (STXBP1-E).


Epilepsy is reported in approximately 95% of patients with STXBP1-E. The onset tends to be early in life, with about half of the patients discovered in the neonatal period. However, there is an enormous variability and examples of epilepsy starting up to 13 years of age have been reported as well. There’s also variability in the type of seizures, that can be distinguished based on clinical signs and EEG (electroencephalography) patterns. Some patients are diagnosed with an epilepsy syndrome such as Ohtahara or West syndrome. Newly diagnosed patients often encounter difficulty with seizure control (see section Parents – Management).

Developmental delay/ intellectual disability

All described patients with STXBP1 mutations have some degree of developmental delay. Both motor and speech development can be severely affected and are not only limited to the presence or severity of seizures. A minority of patients are reported to have developmental regression. This means that these few patients lose some of their previously acquired abilities. This is not clearly related to the seizure activity.

Behavioral disorders

The behavioral symptoms of STXBP1-E patients may consist of an autism spectrum disorder (ASD) or autistic features, such as stereotypical behavior. Other behavioral difficulties reported in a smaller number of patients include hyperactivity and aggressive behavior.

Movement disorder

Ataxia, a lack of muscle coordination, is the most frequent movement disorder in STXBP1-E. Other possible movement issues that may occur include tremor, spasticity, dyskinesia (uncontrolled, involuntary movement) and dystonia (involuntary muscle contractions).

Other features

Hypotonia (muscle weakness) can be found in some STXBP1-E patients. Feeding difficulties are also relatively common, sometimes accompanied by growth issues. Structural brain abnormalities can be present in roughly half of the patients, but are not specific to the diagnosis. A small number of patients present with a small head circumference.