The phenotypic spectrum of sepiapterin reductase deficiency (SRD), which ranges from significant motor and cognitive deficits to only minimal findings, has not been completely elucidated. Clinical features in the majority of affected individuals include motor and speech delay, axial hypotonia, dystonia, weakness, and oculogyric crises; symptoms show diurnal fluctuation and sleep benefit. Other common features include parkinsonian signs (tremor, bradykinesia, masked facies, rigidity), limb hypertonia, hyperreflexia, intellectual disability, psychiatric and/or behavioral abnormalities, autonomic dysfunction, and sleep disturbances (hypersomnolence, difficulty initiating or maintaining sleep, and drowsiness). Most affected individuals have nonspecific features in infancy including developmental delays and axial hypotonia; other features develop over time.

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).

Baraitser-Winter cerebrofrontofacial (BWCFF) syndrome is a multiple congenital anomaly syndrome characterized by typical craniofacial features and intellectual disability. Many (but not all) affected individuals have pachygyria that is predominantly frontal, wasting of the shoulder girdle muscles, and sensory impairment due to iris or retinal coloboma and/or sensorineural deafness. Intellectual disability, which is common but variable, is related to the severity of the brain malformations. Seizures, congenital heart defects, renal malformations, and gastrointestinal dysfunction are also common.

GRIN1-related neurodevelopmental disorder (GRIN1-NDD) is characterized by mild-to-profound developmental delay / intellectual disability (DD/ID) in all affected individuals. Other common manifestations are epilepsy, muscular hypotonia, movement disorders, spasticity, feeding difficulties, and behavior problems. A subset of individuals show a malformation of cortical development consisting of extensive and diffuse bilateral polymicrogyria. To date, 72 individuals with GRIN1-NDD have been reported.

Mild non-BH4-deficient hyperphenylalaninemia (HPANBH4) is an autosomal recessive disorder characterized by increased serum phenylalanine (HPA) usually detected by newborn screening and associated with highly variable neurologic defects, including movement abnormalities, such as dystonia, and variably impaired intellectual development. Laboratory analysis shows dopamine and serotonin deficiencies in the cerebrospinal fluid, and normal tetrahydrobiopterin (BH4) metabolism. Evidence suggests that treatment with BH4 and neurotransmitter precursors can lead to clinical improvement or even prevent the neurologic defects if started in infancy (summary by Anikster et al., 2017). The phenotype is highly variable: some patients may present with later onset of juvenile or young adult nonprogressive dopa-responsive parkinsonism reminiscent of early-onset Parkinson disease (168600). These patients benefit from treatment with L-dopa (summary by Straniero et al., 2017). In a review of HPA, Blau et al. (2018) noted that molecular screening for DNAJC12 mutations should be mandatory in patients in whom deficiencies of PAH (612349) and BH4 metabolism have been excluded.

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.

SLC6A3-related dopamine transporter deficiency syndrome (DTDS) is a complex movement disorder with a continuum that ranges from classic early-onset DTDS (in the first 6 months) to atypical later-onset DTDS (in childhood, adolescence, or adulthood). Classic DTDS. Infants typically manifest nonspecific findings (irritability, feeding difficulties, axial hypotonia, and/or delayed motor development) followed by a hyperkinetic movement disorder (with features of chorea, dystonia, ballismus, orolingual dyskinesia). Over time, affected individuals develop parkinsonism-dystonia characterized by bradykinesia (progressing to akinesia), dystonic posturing, distal tremor, rigidity, and reduced facial expression. Limitation of voluntary movements leads to severe motor delay. Episodic status dystonicus, exacerbations of dystonia, and secondary orthopedic, gastrointestinal, and respiratory complications are common. Many affected individuals appear to show relative preservation of intellect with good cognitive development. Atypical DTDS. Normal psychomotor development in infancy and early childhood is followed by later-onset manifestations of parkinsonism-dystonia with tremor, progressive bradykinesia, variable tone, and dystonic posturing. The long-term outcome of this form is currently unknown.