Cohen syndrome is characterized by failure to thrive in infancy and childhood; truncal obesity in the teen years; early-onset hypotonia and developmental delays; microcephaly developing during the first year of life; moderate to profound psychomotor retardation; progressive retinochoroidal dystrophy and high myopia; neutropenia in many with recurrent infections and aphthous ulcers in some; a cheerful disposition; joint hypermobility; and characteristic facial features.

Temtamy syndrome is a mental retardation/multiple congenital anomaly syndrome characterized by variable craniofacial dysmorphism, ocular coloboma, seizures, and brain abnormalities, including abnormalities of the corpus callosum and thalamus (summary by Akizu et al., 2013).

Macrocephaly refers to an abnormally enlarged head inclusive of the scalp, cranial bones, and intracranial contents. Macrocephaly may be due to megalencephaly (true enlargement of the brain parenchyma), and the 2 terms are often used interchangeably in the genetic literature (reviews by Olney, 2007 and Williams et al., 2008). Autosomal recessive macrocephaly/megalencephaly syndrome is characterized by an enlarged cranium apparent at birth or in early childhood. Affected individuals have intellectual disability and may have dysmorphic facial features resulting from the macrocephaly (summary by Alfaiz et al., 2014).

Any complex cortical dysplasia with other brain malformations in which the cause of the disease is a mutation in the TUBG1 gene.

Developmental and epileptic encephalopathy-18 (DEE18) is a severe autosomal recessive neurologic disorder characterized by lack of psychomotor development apparent from birth, dysmorphic facial features, and early onset of refractory seizures. Brain imaging shows a thick corpus callosum and persistent cavum septum pellucidum on brain imaging (summary by Basel-Vanagaite et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

MPPH (megalencephaly-postaxial polydactyly-polymicrogyria-hydrocephalus) syndrome is a developmental brain disorder characterized by megalencephaly (brain overgrowth) with the cortical malformation bilateral perisylvian polymicrogyria (BPP). At birth the occipital frontal circumference (OFC) ranges from normal to 6 standard deviations (SD) above the mean for age, sex, and gestational age; in older individuals the range is from 3 to 10 SD above the mean. A variable degree of ventriculomegaly is seen in almost all children with MPPH syndrome; nearly 50% of individuals have frank hydrocephalus. Neurologic problems associated with BPP include oromotor dysfunction (100%), epilepsy (50%), and mild-to-severe intellectual disability (100%). Postaxial hexadactyly occurs in 50% of individuals with MPPH syndrome.

MPPH (megalencephaly-postaxial polydactyly-polymicrogyria-hydrocephalus) syndrome is a developmental brain disorder characterized by megalencephaly (brain overgrowth) with the cortical malformation bilateral perisylvian polymicrogyria (BPP). At birth the occipital frontal circumference (OFC) ranges from normal to 6 standard deviations (SD) above the mean for age, sex, and gestational age; in older individuals the range is from 3 to 10 SD above the mean. A variable degree of ventriculomegaly is seen in almost all children with MPPH syndrome; nearly 50% of individuals have frank hydrocephalus. Neurologic problems associated with BPP include oromotor dysfunction (100%), epilepsy (50%), and mild-to-severe intellectual disability (100%). Postaxial hexadactyly occurs in 50% of individuals with MPPH syndrome.

X-linked syndromic intellectual developmental disorder-34 (MRXS34) is an X-linked recessive neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability with poor speech, dysmorphic facial features, and mild structural brain abnormalities, including thickening of the corpus callosum (summary by Mircsof et al., 2015).

Macrocephaly, dysmorphic facies, and psychomotor retardation (MDFPMR) is an autosomal recessive neurodevelopmental disorder characterized by large head and somatic overgrowth apparent at birth followed by global developmental delay. Affected individuals have characteristic dysmorphic facial features and persistently large head, but increased birth weight normalizes with age. Additional neurologic features, including seizures, hypotonia, and gait ataxia, may also occur. Patients show severe intellectual impairment (summary by Ortega-Recalde et al., 2015).

Mega-corpus-callosum syndrome with cerebellar hypoplasia and cortical malformations (MCCCHCM) is an autosomal dominant neurodevelopmental disorder characterized by global developmental delay, impaired intellectual development, and characteristic brain abnormalities on brain imaging. Affected individuals have enlargement of the corpus callosum, enlarged ventricles, and cerebellar and brainstem hypoplasia. Other features may include lack of speech development, gait instability, and seizures. Some patients with MAST1 mutations may have impaired intellectual development and/or autism spectrum disorder without significant findings on brain imaging (summary by Tripathy et al., 2018).

Diets-Jongmans syndrome (DIJOS) is an autosomal dominant disorder characterized by mild to moderately impaired intellectual development with a recognizable facial gestalt (summary by Diets et al., 2019).

Developmental and epileptic encephalopathy-98 (DEE98) is characterized by onset of seizures in the first decade (range infancy to late childhood) associated with variable global developmental delay. Other features may include hypotonia, spasticity, and quadriparesis. Brain imaging may be normal or show nonspecific and variable abnormalities, including polymicrogyria. The severity is variable; some patients may 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.

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.

Neurodevelopmental disorder with hypotonia and characteristic brain abnormalities (NEDHBA) is an autosomal recessive disorder characterized by impaired intellectual development with striking radiologic abnormalities of the lateral ventricles (Fasham et al., 2023).