Multiple congenital anomalies-hypotonia-seizures syndrome-2 (MCAHS2) is an X-linked recessive neurodevelopmental disorder characterized by dysmorphic features, neonatal hypotonia, early-onset myoclonic seizures, and variable congenital anomalies involving the central nervous, cardiac, and urinary systems. Some affected individuals die in infancy (summary by Johnston et al., 2012). The phenotype shows clinical variability with regard to severity and extraneurologic features. However, most patients present in infancy with early-onset epileptic encephalopathy associated with developmental arrest and subsequent severe neurologic disability; these features are consistent with a form of developmental and epileptic encephalopathy (DEE) (summary by Belet et al., 2014, Kato et al., 2014). 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 nomenclature and genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Palatal anomalies-widely spaced teeth-facial dysmorphism-developmental delay syndrome is a rare, genetic multiple congenital anomalies/dysmorphic syndrome characterized by global developmental delay, axial hypotonia, palate abnormalities (including cleft palate and/or high and narrow palate), dysmorphic facial features (including prominent forehead, hypertelorism, downslanting palpebral fissures, wide nasal bridge, thin lips and widely spaced teeth), and short stature. Additional manifestations may include digital anomalies (such as brachydactyly, clinodactyly, and hypoplastic toenails), a single palmar crease, lower limb hypertonia, joint hypermobility, as well as ocular and urogenital anomalies.

Neurodevelopmental disorder with microcephaly, cortical malformations, and spasticity (NEDMCMS) is an autosomal recessive disorder characterized by severe to profound global developmental delay, early-onset seizures, microcephaly, and polymicrogyria and/or cerebral atrophy on brain imaging. Most affected individuals are unable to walk or speak and have profoundly impaired intellectual development, as well as axial hypotonia and peripheral spasticity. Rare individuals may be less severely affected (summary by Vandervore et al., 2019).

NARS1-related neurologic disorders encompass NARS1-related neurodevelopmental disorder (NARS1-NDD), a neonatal- or childhood-onset phenotype with central nervous system and peripheral nervous system involvement, and NARS1-related hereditary neuropathy, an adolescent- or early adult-onset hereditary neuropathy. NARS1-NDD manifests with global developmental delay, intellectual disability, microcephaly, ataxia, seizures, and, rarely, neurobehavioral/psychiatric manifestations. Change in muscle tone can manifest either as spasticity or as hypotonia. Peripheral neuropathy with atrophy predominantly of the distal lower limbs can be associated. NARS1-related hereditary neuropathy manifests with mostly motor and sensory impairment involving weakness of predominantly the distal lower limbs and foot deformities, without prominent muscle atrophy. A few individuals have been described with isolated hereditary motor neuropathy associated with foot deformities, ankle contractures, kyphosis, hyperlaxity, and brisk reflexes. To date, 54 individuals from 30 families with NARS1 pathogenic variant(s) have been reported.