Menkes disease (MNK) is an X-linked recessive disorder characterized by generalized copper deficiency. The clinical features result from the dysfunction of several copper-dependent enzymes.

N-acetylglutamate synthase deficiency (NAGSD) is an autosomal recessive disorder of the urea cycle. The clinical and biochemical features of the disorder are indistinguishable from carbamoyl phosphate synthase I deficiency (237300), since the CPS1 enzyme (608307) has an absolute requirement for NAGS (Caldovic et al., 2007).

The clinical manifestations of CACNA1C-related disorders include a spectrum of nonsyndromic and syndromic phenotypes, which generally correlate with the impact of the pathogenic variant on calcium current. Phenotypes can include nonsyndromic long QT syndrome (rate-corrected QT [QTc] interval >480 ms); nonsyndromic short QT syndrome (QTc <350 ms), with risk of sudden death; Brugada syndrome (ST segment elevation in right precordial leads [V1-V2]) with short QT interval; classic Timothy syndrome (prolonged QT interval, autism, and congenital heart defect) with or without unilateral or bilateral cutaneous syndactyly variably involving fingers two (index), three (middle), four (ring), and five (little) and bilateral cutaneous syndactyly of toes two and three; and CACNA1C-related neurodevelopmental disorder, in which the features tend to favor one or more of the following: developmental delay / intellectual disability, hypotonia, epilepsy, and/or ataxia.

Sudden infant death with dysgenesis of the testes syndrome (SIDDT) is characterized by sudden cardiac or respiratory arrest, disordered testicular development, and neurologic dysfunction, and is uniformly fatal before 1 year of age (Slater et al., 2020).

2-Methylbutyryl-CoA dehydrogenase deficiency is an autosomal recessive metabolic disorder of impaired isoleucine degradation. It is most often ascertained via newborn screening and is usually clinically asymptomatic, although some patients have been reported to have delayed development and neurologic signs. Therefore, the clinical relevance of the deficiency is unclear (Sass et al., 2008).

Congenital hypothyroidism can also occur as part of syndromes that affect other organs and tissues in the body. These forms of the condition are described as syndromic. Some common forms of syndromic hypothyroidism include Pendred syndrome, Bamforth-Lazarus syndrome, and brain-lung-thyroid syndrome.\n\nSigns and symptoms of congenital hypothyroidism result from the shortage of thyroid hormones. Affected babies may show no features of the condition, although some babies with congenital hypothyroidism are less active and sleep more than normal. They may have difficulty feeding and experience constipation. If untreated, congenital hypothyroidism can lead to intellectual disability and slow growth. In the United States and many other countries, all hospitals test newborns for congenital hypothyroidism. If treatment begins in the first two weeks after birth, infants usually develop normally.\n\nCongenital hypothyroidism occurs when the thyroid gland fails to develop or function properly. In 80 to 85 percent of cases, the thyroid gland is absent, severely reduced in size (hypoplastic), or abnormally located. These cases are classified as thyroid dysgenesis. In the remainder of cases, a normal-sized or enlarged thyroid gland (goiter) is present, but production of thyroid hormones is decreased or absent. Most of these cases occur when one of several steps in the hormone synthesis process is impaired; these cases are classified as thyroid dyshormonogenesis. Less commonly, reduction or absence of thyroid hormone production is caused by impaired stimulation of the production process (which is normally done by a structure at the base of the brain called the pituitary gland), even though the process itself is unimpaired. These cases are classified as central (or pituitary) hypothyroidism.\n\nCongenital hypothyroidism is a partial or complete loss of function of the thyroid gland (hypothyroidism) that affects infants from birth (congenital). The thyroid gland is a butterfly-shaped tissue in the lower neck. It makes iodine-containing hormones that play an important role in regulating growth, brain development, and the rate of chemical reactions in the body (metabolism). People with congenital hypothyroidism have lower-than-normal levels of these important hormones.

SUCLG1-related mitochondrial DNA (mtDNA) depletion syndrome, encephalomyopathic form with methylmalonic aciduria is characterized in the majority of affected newborns by hypotonia, muscle atrophy, feeding difficulties, and lactic acidosis. Affected infants commonly manifest developmental delay / cognitive impairment, growth retardation / failure to thrive, hepatopathy, sensorineural hearing impairment, dystonia, and hypertonia. Notable findings in some affected individuals include hypertrophic cardiomyopathy, epilepsy, myoclonus, microcephaly, sleep disturbance, rhabdomyolysis, contractures, hypothermia, and/or hypoglycemia. Life span is shortened, with median survival of 20 months.

Lethal neonatal rigidity and multifocal seizure syndrome (RMFSL) is a severe autosomal recessive epileptic encephalopathy characterized by onset of rigidity and intractable seizures at or soon after birth. Affected infants achieve no developmental milestones and die within the first months or years of life (summary by Saitsu et al., 2014).

Encephalopathy-hypertrophic cardiomyopathy-renal tubular disease syndrome is a rare mitochondrial disease due to a defect in coenzyme Q10 biosynthesis that manifests with a broad spectrum of signs and symptoms which may include: neonatal lactic acidosis, global developmental delay, tonus disorder, seizures, reduced spontaneous movements, ventricular hypertrophy, bradycardia, renal tubular dysfunction with massive lactic acid excretion in urine, severe biochemical defect of respiratory chain complexes II/III when assayed together and deficiency of coenzyme Q10 in skeletal muscle. Cerebral and cerebellar atrophy can be seen on magnetic resonance imaging and multiple choroid plexus cysts and symmetrical hyperechoic signal alterations in basal ganglia have been observed on ultrasound.

Mitochondrial complex IV deficiency nuclear type 13 (MC4DN13) is an autosomal recessive metabolic disorder characterized by the onset of hypertrophic cardiomyopathy soon after birth. Affected individuals have hypotonia, weakness, and failure to thrive, resulting in death in infancy. Laboratory studies show increased serum lactate and decreased levels and activity of mitochondrial respiratory complex IV (summary by Baertling et al., 2015). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

The two forms of deoxyguanosine kinase (DGUOK) deficiency are a neonatal multisystem disorder and an isolated hepatic disorder that presents later in infancy or childhood. The majority of affected individuals have the multisystem illness with hepatic disease (jaundice, cholestasis, hepatomegaly, and elevated transaminases) and neurologic manifestations (hypotonia, nystagmus, and developmental delay) evident within weeks of birth. Those with isolated liver disease may also have renal involvement, and some later develop mild hypotonia. Progressive hepatic disease is the most common cause of death in both forms.

Combined oxidative phosphorylation deficiency-37 is an autosomal recessive multisystem disorder apparent at birth or in the first months of life. Affected individuals have hypotonia, failure to thrive, and neurodegeneration with loss of developmental milestones, as well as liver dysfunction. Some patients may have hypertrophic cardiomyopathy, loss of vision and hearing, and/or seizures. Mitochondrial respiratory dysfunction is apparent in liver and skeletal muscle tissue. Most patients die in childhood (summary by Zeharia et al., 2016). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Hypotonia, hypoventilation, impaired intellectual development, dysautonomia, epilepsy, and eye abnormalities (HIDEA) is an autosomal recessive neurodevelopmental syndrome characterized by global developmental delay, poor or absent speech, hypotonia, variable ocular movement and visual abnormalities, and respiratory difficulties, including hypoventilation, and sleep apnea. Patients may have significant breathing problems during respiratory infections that may lead to early death (summary by Rahikkala et al., 2019).

Developmental and epileptic encephalopathy-78 (DEE78) is a severe neurologic disorder characterized by onset of refractory seizures in the first days or months of life followed by severely impaired intellectual development. Additional features may include cortical visual impairment, hypotonia, and abnormal movements, such as spasticity (summary by Butler et al., 2018). One family with an attenuated disease course has been reported (Maljevic et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

The concentration of vitamin B2 in the blood circulation is below the lower limit of normal.

Jeffries-Lakhani neurodevelopmental syndrome (JELANS) is an autosomal recessive disorder characterized by hypotonia, early-onset seizures, and global developmental delay apparent from infancy. Affected individuals have motor delay, speech delay, and impaired intellectual development, and about half of patients are nonambulatory and/or nonverbal. Some patients have cardiac arrhythmia, but congenital cardiac septal defects are only rarely observed. Additional features may include feeding difficulties, recurrent infections, ocular defects, and nonspecific dysmorphic features. Premature death due to cardiac arrhythmia or epilepsy may occur (Jeffries et al., 2024).