Phenylalanine hydroxylase (PAH) deficiency results in intolerance to the dietary intake of the essential amino acid phenylalanine and produces a spectrum of disorders. The risk of adverse outcome varies based on the degree of PAH deficiency. Without effective therapy, most individuals with severe PAH deficiency, known as classic PKU, develop profound and irreversible intellectual disability. Affected individuals on an unrestricted diet who have phenylalanine levels above normal but below 1,200 µmol/L (20 mg/dL) are at much lower risk for impaired cognitive development in the absence of treatment.

The spectrum of propionic acidemia (PA) ranges from neonatal-onset to late-onset disease. Neonatal-onset PA, the most common form, is characterized by a healthy newborn with poor feeding and decreased arousal in the first few days of life, followed by progressive encephalopathy of unexplained origin. Without prompt diagnosis and management, this is followed by progressive encephalopathy manifesting as lethargy, seizures, or coma that can result in death. It is frequently accompanied by metabolic acidosis with anion gap, lactic acidosis, ketonuria, hypoglycemia, hyperammonemia, and cytopenias. Individuals with late-onset PA may remain asymptomatic and suffer a metabolic crisis under catabolic stress (e.g., illness, surgery, fasting) or may experience a more insidious onset with the development of multiorgan complications including vomiting, protein intolerance, failure to thrive, hypotonia, developmental delays or regression, movement disorders, or cardiomyopathy. Isolated cardiomyopathy can be observed on rare occasion in the absence of clinical metabolic decompensation or neurocognitive deficits. Manifestations of neonatal and late-onset PA over time can include growth impairment, intellectual disability, seizures, basal ganglia lesions, pancreatitis, and cardiomyopathy. Other rarely reported complications include optic atrophy, hearing loss, premature ovarian insufficiency, and chronic renal failure.

3-Hydroxy-3-methylglutaryl-CoA lyase deficiency (HMGCLD) is a rare autosomal recessive disorder with the cardinal manifestations of metabolic acidosis without ketonuria, hypoglycemia, and a characteristic pattern of elevated urinary organic acid metabolites, including 3-hydroxy-3-methylglutaric, 3-methylglutaric, and 3-hydroxyisovaleric acids. Urinary levels of 3-methylcrotonylglycine may be increased. Dicarboxylic aciduria, hepatomegaly, and hyperammonemia may also be observed. Presenting clinical signs include irritability, lethargy, coma, and vomiting (summary by Gibson et al., 1988).

A maple syrup urine disease caused by mutations in BCKDHA.