Most individuals with Canavan disease have the neonatal/infantile form. Although such infants appear normal early in life, by age three to five months, hypotonia, head lag, macrocephaly, and developmental delays become apparent. With age, children with neonatal/infantile-onset Canavan disease often become irritable and experience sleep disturbance, seizures, and feeding difficulties. Swallowing deteriorates, and some children require nasogastric feeding or permanent feeding gastrostomies. Joint stiffness increases, so that these children resemble individuals with cerebral palsy. Children with mild/juvenile Canavan disease may have normal or mildly delayed speech or motor development early in life without regression. In spite of developmental delay most of these children can be educated in typical classroom settings and may benefit from speech therapy or tutoring as needed. Most children with mild forms of Canavan disease have normal head size, although macrocephaly, retinitis pigmentosa, and seizures have been reported in a few individuals.

The spectrum of isolated sulfite oxidase deficiency ranges from classic early-onset (severe) disease to late-onset (mild) disease. Classic ISOD is characterized in the first few hours to days of life by intractable seizures, feeding difficulties, and rapidly progressive encephalopathy manifest as abnormal tone (especially opisthotonus, spastic quadriplegia, and pyramidal signs) followed by progressive microcephaly and profound intellectual disability. Lens subluxation or dislocation, another characteristic finding, may be evident after the newborn period. Children usually die during the first few months of life. Late-onset ISOD manifests between ages six and 18 months and is characterized by ectopia lentis (variably present), developmental delay/regression, movement disorder characterized by dystonia and choreoathetosis, ataxia, and (rarely) acute hemiplegia as a result of metabolic stroke. The clinical course may be progressive or episodic. In the episodic form encephalopathy, dystonia, choreoathetosis, and/or ataxia are intermittent.

Salt and pepper developmental regression syndrome, also known as Amish infantile epilepsy syndrome, is an autosomal recessive neurocutaneous disorder characterized by infantile onset of refractory and recurrent seizures associated with profoundly delayed psychomotor development and/or developmental regression as well as abnormal movements and visual loss (summary by Fragaki et al., 2013). Affected individuals develop hypo- or hyperpigmented skin macules on the trunk, face, and extremities in early childhood (summary by Boccuto et al., 2014). Not all patients have overt seizures (Lee et al., 2016).

Congenital disorders of glycosylation (CDGs) comprise a group of multisystem diseases with mostly severe psychomotor and mental retardation. Type I CDG comprises those disorders in which there are defects that affect biosynthesis of dolichol-linked oligosaccharides in the cytosol or the endoplasmic reticulum (ER), as well as defects involving the transfer of oligosaccharides onto nascent glycoproteins. Type II CDG comprises all defects of further trimming and elongation of N-linked oligosaccharides in the ER and Golgi (Schwarz et al., 2004). CDG1K is a type I CDG characterized by predominant neurologic involvement. Survival ranges from the second day of life to adulthood. The liver is affected in a minority of patients and shows hepatomegaly, edema, ascites, cholestatic jaundice, portal hypertension, and Budd-Chiari syndrome (summary by Marques-da-Silva et al., 2017). For a general discussion of CDGs, see CDG1A (212065).

SCN8A-related epilepsy with encephalopathy is characterized by developmental delay, seizure onset in the first 18 months of life (mean 4 months), and intractable epilepsy characterized by multiple seizure types (generalized tonic-clonic seizures, infantile spasms, and absence and focal seizures). Epilepsy syndromes can include Lennox-Gastaut syndrome, West syndrome, and epileptic encephalopathies (e.g., Dravet syndrome). Hypotonia and movement disorders including dystonia, ataxia, and choreoathetosis are common. Psychomotor development varies from normal prior to seizure onset (with subsequent slowing or regression after seizure onset) to abnormal from birth. Intellectual disability, present in all, ranges from mild to severe (in ~50% of affected individuals). Autistic features are noted in some. Sudden unexpected death in epilepsy (SUDEP) of unknown cause has been reported in approximately 10% of published cases. To date SCN8A-related epilepsy with encephalopathy has been reported in the literature in about 50 individuals.

CHD2-related neurodevelopmental disorders are characterized by early-onset epileptic encephalopathy (i.e., refractory seizures and cognitive slowing or regression associated with frequent ongoing epileptiform activity). Seizure onset is typically between ages six months and four years. Seizure types typically include drop attacks, myoclonus, and rapid onset of multiple seizure types associated with generalized spike-wave on EEG, atonic-myoclonic-absence seizures, and clinical photosensitivity. Intellectual disability and/or autism spectrum disorders are common.

Multiple congenital anomalies-hypotonia-seizures syndrome is an autosomal recessive disorder characterized by neonatal hypotonia, lack of psychomotor development, seizures, dysmorphic features, and variable congenital anomalies involving the cardiac, urinary, and gastrointestinal systems. Most affected individuals die before 3 years of age (summary by Maydan et al., 2011). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of MCAHS, see MCAHS1 (614080). For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

An autosomal dominant condition caused by mutation(s) in the TUBB2A gene, encoding tubulin beta-2A chain. It is characterized by cortical dysplasia and is associated with impaired intellectual development, hypotonia, global developmental delay, cortical dysplasia, and dysmorphic corpus callosum.

Developmental and epileptic encephalopathy-47 (DEE47) is a neurologic disorder characterized by onset of intractable seizures in the first days or weeks of life. EEG shows background slowing and multifocal epileptic spikes, and may show hypsarrhythmia. Most patients have developmental regression after seizure onset and show persistent intellectual disability and neurologic impairment, although the severity is variable. Treatment with phenytoin, a voltage-gated sodium channel blocker, may be beneficial (summary by Guella et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

GNB1 encephalopathy (GNB1-E) is characterized by moderate-to-severe developmental delay / intellectual disability, structural brain abnormalities, and often infantile hypotonia and seizures. Other less common findings include dystonia, reduced vision, behavior issues, growth delay, gastrointestinal (GI) problems, genitourinary (GU) abnormalities in males, and cutaneous mastocytosis.

NEDIM is a neurodevelopmental and neurodegenerative disorder characterized by delayed psychomotor development and infantile or childhood onset of hyperkinetic involuntary movements, including chorea and athetosis. The abnormal movements can be severe, sometimes resulting in inability to sit, walk, speak, or eat. Hyperkinetic movements can be exacerbated by specific triggers, such as stress, illness, or high temperature. Some patients have brain abnormalities, such as cerebral atrophy or thin corpus callosum, and some patients may develop seizures (summary by Ananth et al., 2016 and Danti et al., 2017).

Developmental and epileptic encephalopathy-55 (DEE55) is an autosomal recessive neurologic disorder characterized by onset of refractory seizures in the first weeks or months of life. Affected individuals have an extremely poor outcome, with profoundly impaired intellectual development, absent speech, spastic quadriplegia, and dyskinetic movements. Most have cortical visual impairment and require a feeding tube. Brain imaging shows nonspecific abnormalities, including cerebral atrophy, thin corpus callosum, and abnormal signals in the white matter. Death in childhood may occur. Biochemically, the disorder is associated with impaired synthesis of GPI-anchored proteins (summary by Vetro et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Developmental and epileptic encephalopathy-91 (DEE91) is characterized by delayed psychomotor development apparent in infancy and resulting in severely to profoundly impaired intellectual development with poor or absent speech. Most patients never achieve independent walking. Patients typically have onset of refractory multifocal seizures between the first weeks and years of life, and some may show developmental regression. Additional features, such as hypotonia and cortical visual impairment, are more variable (summary by Myers et al., 2017). For a discussion of genetic heterogeneity of DEE, see 308350.

Developmental and epileptic encephalopathy-59 (DEE59) is characterized by severe global developmental delay apparent in infancy with onset of various types of seizures in the first months of life (range 3 to 11 months). The seizures are usually refractory and are often associated with hypsarrhythmia on EEG, although brain imaging is usually normal. More severely affected individuals may be unable to speak or walk, have poor interaction, and require a feeding tube (summary by the EuroEPINOMICS-RES Consortium et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Hypermethioninemia due to adenosine kinase deficiency is an autosomal recessive inborn error of metabolism characterized by global developmental delay, early-onset seizures, mild dysmorphic features, and characteristic biochemical anomalies, including persistent hypermethioninemia with increased levels of S-adenosylmethionine (AdoMet) and S-adenosylhomocysteine (AdoHcy); homocysteine is typically normal (summary by Bjursell et al., 2011).

Developmental and epileptic encephalopathy-76 (DEE76) is an autosomal recessive neurodevelopmental disorder characterized by early-onset, usually refractory, seizures, severely delayed global development, hypotonia, peripheral spasticity, and abnormalities on brain imaging, mainly cerebral atrophy and delayed myelination. Some patients may have additional features, such as scoliosis or microcephaly. The disorder may result in death in childhood (summary by Bell et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Den Hoed-de Boer-Voisin syndrome (DHDBV) is characterized by global developmental delay with moderately to severely impaired intellectual development, poor or absent speech, and delayed motor skills. Although the severity of the disorder varies, many patients are nonverbal and have hypotonia with inability to sit or walk. Early-onset epilepsy is common and may be refractory to treatment, leading to epileptic encephalopathy and further interruption of developmental progress. Most patients have feeding difficulties with poor overall growth and dysmorphic facial features, as well as significant dental anomalies resembling amelogenesis imperfecta. The phenotype is reminiscent of Kohlschutter-Tonz syndrome (KTZS; 226750). More variable features of DHDBV include visual defects, behavioral abnormalities, and nonspecific involvement of other organ systems (summary by den Hoed et al., 2021).

SCN1A seizure disorders encompass a spectrum that ranges from simple febrile seizures and generalized epilepsy with febrile seizures plus (GEFS+) at the mild end to Dravet syndrome and intractable childhood epilepsy with generalized tonic-clonic seizures (ICE-GTC) at the severe end. Phenotypes with intractable seizures including Dravet syndrome are often associated with cognitive decline. Less commonly observed phenotypes include myoclonic astatic epilepsy (MAE), Lennox-Gastaut syndrome, infantile spasms, epilepsy with focal seizures, and vaccine-related encephalopathy and seizures. The phenotype of SCN1A seizure disorders can vary even within the same family.

Developmental delay, impaired speech, and behavioral abnormalities (DDISBA) is characterized by global developmental delay apparent from early childhood. Intellectual disability can range from mild to severe. Additional variable features may include dysmorphic facial features, seizures, hypotonia, motor abnormalities such as Tourette syndrome or dystonia, and hearing loss (summary by Cousin et al., 2021).

Developmental and epileptic encephalopathy-99 (DEE99) is characterized by onset of seizures in early childhood associated with global developmental delay and severely impaired intellectual development. Other features may include hypotonia, quadriparesis, nystagmus, and apnea. Brain imaging may be normal or show nonspecific and variable abnormalities, including cerebral atrophy and polymicrogyria. The severity is variable; some patients die of refractory status epilepticus (summary by Vetro et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Combined oxidative phosphorylation deficiency-27 (COXPD27) is an autosomal recessive multisystem disorder characterized mainly by neurologic features, including delayed development, seizures, abnormal movements, and neurologic regression. Age at onset, ranging from infancy to late childhood, and severity are variable. Other features include hypotonia, myoclonus, brain imaging abnormalities, and evidence of mitochondrial dysfunction in skeletal muscle. Liver dysfunction has also been reported (summary by Samanta et al., 2018). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Developmental delay, impaired speech, and behavioral abnormalities, with or without seizures (DEDISB) is a neurodevelopmental disorder characterized by variably impaired skill acquisition apparent from infancy or early childhood. Affected individuals have predominant language delay with mild fine and gross motor deficits, although they usually are ambulatory by around 3 years of age. Most patients have mild to moderately impaired intellectual development and behavioral abnormalities, including aggression, hyperactivity, and autism spectrum disorder. About half of individuals develop various types of seizures that may be refractory in some. More variable features include dysmorphic facial features, mild ocular anomalies, and nonspecific findings on brain imaging (Thomas et al., 2021).