From OMIM
Developmental and epileptic encephalopathy-13 (DEE13) is a neurologic disorder characterized by the onset of intractable seizures in the first year of life. Some patients may present with seizures in the first days, whereas others present later (between 2 and 7 months of age) after normal or only mild developmental delay. Affected individuals have profoundly impaired development or developmental regression after the onset of seizures, and show severe intellectual disability, poor or absent language, hypotonia, and are usually unable to walk. EEG shows variable abnormalities, including multifocal and generalized spike-wave discharges, sometimes with status epilepticus or hypsarrhythmia. Brain imaging may show cerebral atrophy (summary by Ohba et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.  http://www.omim.org/entry/614558

From MedlinePlus Genetics
SCN8A-related epilepsy with encephalopathy is a condition characterized by recurrent seizures (epilepsy), abnormal brain function (encephalopathy), and intellectual disability. The signs and symptoms of this condition typically begin in infancy.

The seizures in SCN8A-related epilepsy with encephalopathy include involuntary muscle contractions that occur before age 1 (infantile spasms), partial or complete loss of consciousness (absence seizures), involuntary muscle twitches (myoclonic seizures), or loss of consciousness with muscle rigidity and convulsions (tonic-clonic seizures). Most people with SCN8A-related epilepsy with encephalopathy have more than one type of seizure. The frequency of seizures in different individuals with this condition ranges from hundreds per day to fewer than one per month. In many individuals, the seizures are described as refractory because they do not respond to therapy with anti-epileptic medications.

Other signs and symptoms of SCN8A-related epilepsy with encephalopathy include intellectual disability that may be mild to severe. Some affected infants have normal early development but begin to lose previously acquired skills (developmental regression) and have a gradual loss in thinking ability (cognitive decline) when epilepsy develops. Problems with movement are common, and about half of affected infants cannot perform intentional movements. Behavior disorders may also occur.

In rare cases, individuals with this condition die unexpectedly for no known reason (sudden unexpected death in epilepsy or SUDEP).  https://medlineplus.gov/genetics/condition/scn8a-related-epilepsy-with-encephalopathy