D-glyceric aciduria is a rare autosomal recessive metabolic disorder with a highly variable phenotype. Some patients have an encephalopathic presentation, with severely impaired intellectual development, seizures, microcephaly, and sometimes early death, whereas others have a mild phenotype with only mild speech delay or even normal development (summary by Sass et al., 2010).

Cardiofaciocutaneous (CFC) syndrome is characterized by cardiac abnormalities (pulmonic stenosis and other valve dysplasias, septal defects, hypertrophic cardiomyopathy, rhythm disturbances), distinctive craniofacial appearance, and cutaneous abnormalities (including xerosis, hyperkeratosis, ichthyosis, keratosis pilaris, ulerythema ophryogenes, eczema, pigmented moles, hemangiomas, and palmoplantar hyperkeratosis). The hair is typically sparse, curly, fine or thick, and woolly or brittle; eyelashes and eyebrows may be absent or sparse. Nails may be dystrophic or fast growing. Affected individuals typically have some form of neurologic and/or cognitive delay (ranging from mild to severe). Most individuals have severe feeding issues, which can contribute to poor growth, and many require nasogastric or gastrostomy tube feeding. Many affected individuals have eye findings, including strabismus, nystagmus, refractive errors, and optic nerve hypoplasia. Seizures may be present and can be refractory to therapy.

Individuals with aromatic L-amino acid decarboxylase (AADC) deficiency typically have complex symptoms, including motor, behavioral, cognitive, and autonomic findings. Symptom onset is in early infancy, typically within the first six months of life. The most common initial symptoms are often nonspecific, and include feeding difficulties, hypotonia, and developmental delay. More specific symptoms include oculogyric crises (which occur in the vast majority of affected individuals, typically starting in infancy), movement disorders (especially dystonia), and autonomic dysfunction (excessive sweating, temperature instability, ptosis, nasal congestion, hypoglycemic episodes). Sleep disturbance is present in a majority of affected individuals and can include insomnia, hypersomnia, or both. Mood disturbance, including irritability and anxiety, are also common. Brain MRI is typically either normal or may demonstrate nonspecific abnormalities, such as mild diffuse cerebral atrophy or delayed myelination. Seizures are an uncommon finding, occurring in fewer than 5% of affected individuals.

Phelan-McDermid syndrome-SHANK3 related (PMS-SHANK3 related) is characterized by neonatal hypotonia, absent to severely delayed speech, developmental delay, and minor dysmorphic facial features. Most affected individuals have moderate-to-profound intellectual disability. Other features include relatively large fleshy hands, dysplastic toenails, and decreased perspiration that results in a tendency to overheat. Normal stature and normal head size distinguish PMS-SHANK3 related from other autosomal chromosome disorders. Neurobehavioral characteristics include mouthing or chewing non-food items, decreased perception of pain, and autism spectrum disorder or autistic-like affect and behavior. Some individuals experience regression / loss of skills, epilepsy, ataxic/abnormal gait, and sleep disturbance (difficulty falling asleep and staying asleep, hypersomnia, and parasomnias). Less commonly, affected individuals may have strabismus, vision problems (hyperopia or myopia), cardiac anomalies, renal anomalies, and lymphedema. Those who have PMS-SHANK3 related due to a ring chromosome 22 also have a high risk of developing features of NF2-related schwannomatosis (NF2).

FOXG1 syndrome is characterized by moderate-to-profound developmental delay and intellectual disability, postnatal growth deficiency, congenital or postnatal microcephaly, hyperkinetic/dyskinetic movement disorder, hypotonia, neurobehavioral/psychiatric manifestations (motor stereotypies, impairment of social interaction, abnormal sleep patterns, unexplained episodes of crying, restlessness, and bruxism), feeding difficulties with poor weight gain, strabismus, seizures, spasticity, gastroesophageal reflux, and aspiration. Some individuals have cortical visual impairment, kyphosis, scoliosis, and/or abnormal breathing. Characteristic neuroimaging findings include corpus callosum anomalies (especially a marked, filiform thinning of the rostrum of the corpus callosum), a simplified gyral pattern, and hyperplasia of the fornices.

Neurodevelopmental disorder with movement abnormalities, abnormal gait, and autistic features (NEDMAGA) is characterized by infantile-onset global developmental delay with severe to profound intellectual disability, mildly delayed walking with broad-based and unsteady gait, and absence of meaningful language. Patients have features of autism, with repetitive behaviors and poor communication, but usually are socially reactive and have a happy demeanor. More variable neurologic features include mild seizures, spasticity, and peripheral neuropathy (summary by Palmer et al., 2017).

Neurodevelopmental disorder with impaired language and ataxia and with or without seizures (NEDLAS) is characterized by axial hypotonia and global developmental delay apparent in early infancy. Affected individuals have delayed walking with gait ataxia and poor language development. Behavioral abnormalities also commonly occur. The severity is highly variable: a subset of patients have a more severe phenotype with early-onset seizures resembling epileptic encephalopathy, inability to walk or speak, and hypomyelination on brain imaging (summary by Stolz et al., 2021).

DYRK1A syndrome is characterized by intellectual disability including impaired speech development, autism spectrum disorder including anxious and/or stereotypic behavior problems, and microcephaly. Affected individuals often have a clinically recognizable phenotype including a typical facial gestalt, feeding problems, seizures, hypertonia, gait disturbances, and foot anomalies. The majority of affected individuals function in the moderate-to-severe range of intellectual disability; however, individuals with mild intellectual disability have also been reported. Other medical concerns relate to febrile seizures in infancy; the development of epilepsy with seizures of the atonic, absence, and generalized myoclonic types; short stature; and gastrointestinal problems. Ophthalmologic, urogenital, cardiac, and/or dental anomalies have been reported.

Autosomal recessive intellectual developmental disorder-79 (MRT79) is characterized by global developmental delay apparent from infancy. Affected individuals have mildly delayed walking with an ataxic gait and severely impaired intellectual development with poor or absent speech. Additional features may include postnatal microcephaly and dysmorphic features (Van Bergen et al., 2022).