Prader-Willi syndrome (PWS) is characterized by severe hypotonia and feeding difficulties in early infancy, followed in later infancy or early childhood by excessive eating and gradual development of morbid obesity (unless eating is externally controlled). Motor milestones and language development are delayed. All individuals have some degree of cognitive impairment. A distinctive behavioral phenotype (with temper tantrums, stubbornness, manipulative behavior, and obsessive-compulsive characteristics) is common. Hypogonadism is present in both males and females and manifests as genital hypoplasia, incomplete pubertal development, and, in most, infertility. Short stature is common (if not treated with growth hormone); characteristic facial features, strabismus, and scoliosis are often present.

Aromatic L-amino acid decarboxylase deficiency (AADCD) is an autosomal recessive inborn error in neurotransmitter metabolism that leads to combined serotonin and catecholamine deficiency (Abeling et al., 2000). The disorder is clinically characterized by vegetative symptoms, oculogyric crises, dystonia, and severe neurologic dysfunction, usually beginning in infancy or childhood (summary by Brun et al., 2010).

An extremely rare type of arthrogryposis multiplex congenita with the combination of multiple joint contractures with movement limitation and microstomia with a whistling appearance of the mouth that may cause feeding, swallowing and speech difficulties, a distinctive expressionless facies, severe developmental delay, central and autonomous nervous system dysfunction, occasionally Pierre-Robin sequence and lethality generally occurring during the first months of life.

A form of congenital disorders of N-linked glycosylation with characteristics of facial dysmorphism (microcephaly, high forehead, low posterior hairline, strabismus), hypotonia, failure to thrive, intractable seizures, developmental delay, persistent vomiting and gastric bleeding. Additional features that may be observed include fat pads anomalies, inverted nipples, and body temperature oscillation. The disease is caused by mutations in the gene ALG11 (13q14.3).

Hypomyelinating leukodystrophy-12 (HLD12) is an autosomal recessive neurologic disorder characterized by severely delayed or even lack of psychomotor development that becomes apparent in the first months of life. Patients are markedly disabled, with acquired microcephaly, lack of speech, and often lack of spontaneous movement due to hypotonia and spasticity. Brain imaging shows delayed myelination (summary by Edvardson et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080. In a review of the pathogenesis of disorders with prominent dystonia or opisthotonic posturing as a feature, Monfrini et al. (2021) classified HLD12 as belonging to a group of neurologic disorders termed 'HOPS-associated neurologic disorders (HOPSANDs), which are caused by mutations in genes encoding various components of the autophagic/endolysosomal system, including VPS11.

Infantile-onset parkinsonism-dystonia-2 (PKDYS2) is an autosomal recessive complex infantile-onset neurologic disorder characterized by abnormal movements, including parkinsonism, dystonia, and poor fine motor skills, as well as autonomic dysfunction, including abnormal sweating, cold extremities, and poor sleep. Some patients have variable degrees of developmental delay. Features of the disorder are consistent with decreased levels of monoamine neurotransmitters, although levels of these in the spinal fluid are normal (summary by Rilstone et al., 2013). For a discussion of genetic heterogeneity of PKDYS, see 613135.

Congenital central hypoventilation syndrome-2 and autonomic dysfunction (CCHS2) is an autosomal recessive disorder characterized by shallow breathing and apneic spells apparent in the neonatal period. Affected infants require mechanical ventilation due to impaired ventilatory response to hypercapnia, as well as tube feeding due to poor swallowing, aspiration, and gastrointestinal dysmotility. Some patients have other features of autonomic dysfunction, including bladder dysfunction, sinus bradycardia, and temperature dysregulation. Although mild global developmental delay with learning difficulties and seizures were present in the single family reported, it was unclear if these features were related to the hypoventilation phenotype (Spielmann et al., 2017). For a discussion of genetic heterogeneity of CCHS, see CCHS1 (209880).

Neurodevelopmental disorder with dystonia and seizures (NEDDS) is a severe autosomal recessive disorder characterized by hypotonia and dystonic posturing apparent from early infancy. Affected individuals show global developmental delay with inability to walk or speak and have profoundly impaired intellectual development, often with behavioral abnormalities. Additional features may include other extrapyramidal movements, seizures or seizure-like activity, and cerebellar hypoplasia on brain imaging (Sleiman et al., 2022).

Neurodegeneration and seizures due to copper transport defect (NSCT) is an autosomal recessive disorder of copper transport characterized by hypotonia, global developmental delay, seizures, and rapid brain atrophy (summary by Dame et al., 2023).