From OMIM
Developmental and epileptic encephalopathy-4 (DEE4) is a neurologic disorder characterized by the onset of tonic seizures in early infancy (usually in first months of life). In most cases, seizures increase in frequency and become refractory. Affected individuals have profoundly impaired psychomotor development with poor head control, limited or no ability to walk, spastic quadriplegia, and poor or absent speech. Brain imaging may show cortical atrophy and hypomyelination. EEG studies in the more severe cases show a burst-suppression pattern, consistent with a clinical diagnosis of Ohtahara syndrome, and/or hypsarrhythmia, consistent with a clinical diagnosis of West syndrome. Less severely affected individuals have later onset of seizures (summary by Saitsu et al., 2008; Hamdan et al., 2009). For a general phenotypic description and a discussion of genetic heterogeneity of developmental and epileptic encephalopathy, see 308350.  http://www.omim.org/entry/612164

From MedlinePlus Genetics
STXBP1 encephalopathy is a condition characterized by abnormal brain function (encephalopathy) and intellectual disability. Most affected individuals also have recurrent seizures (epilepsy). The signs and symptoms of this condition typically begin in infancy but can start later in childhood or early adulthood. In many affected individuals who have epilepsy, the seizures stop after a few years, and the other neurological problems continue throughout life. However, some people with STXBP1 encephalopathy have seizures that persist.

In people with STXBP1 encephalopathy, intellectual disability is often severe to profound. In addition, speech and motor skills, such as sitting, crawling, and walking, can be delayed. Though they may acquire the skill late, many children with the condition can walk independently by age 5. Affected individuals usually learn their first words later than their peers, sometimes not until late childhood. Some can communicate verbally using simple sentences, while others never develop the skill.  

About 85 percent of people with STXBP1 encephalopathy develop epilepsy. The most common seizures in this condition are infantile spasms, which occur before age 1 and consist of involuntary muscle contractions. Other seizure types that can occur in people with this condition include uncontrolled muscle twitches (myoclonic seizures), sudden episodes of weak muscle tone (atonic seizures), partial or complete loss of consciousness (absence seizures), or loss of consciousness with muscle rigidity and convulsions (tonic-clonic seizures). Most people who have STXBP1 encephalopathy have more than one type of seizure. In about one-quarter of affected individuals, the seizures are described as refractory because they do not respond to therapy with anti-epileptic medications.

Other neurological problems that occur in people with STXBP1 encephalopathy include features of autism spectrum disorder; weak muscle tone (hypotonia); and movement problems, such as difficulty coordinating movements (ataxia), involuntary trembling (tremors), and muscle stiffness (spasticity). In some cases, areas of brain tissue loss (atrophy) have been found on medical imaging.  https://medlineplus.gov/genetics/condition/stxbp1-encephalopathy