Paediatrics: eneral principles of Inherited metabolic disease

Disorders of amino acid metabolism

Due to defects either in the synthesis of (or the breakdown of) amino acids or in the body’s ability to transport amino acids into cells. Most are autosomal recessive. Diagnosis is established by detecting abnormal plasma and urinary amino acid profiles.

Phenylketonuria (PKU)

AR. Occurs in 1/10 000–15 000 live births. In its classical form it is due to a deficiency in phenylalanine hydroxylase. Untreated, brain development is impaired leading to progressive mental retardation and seizures, usually evident by 6–12mths of age. Many children have fair hair and blue eyes. In PKU phenylalanine accumulates and is converted into phenylketones, which are detected in the urine.

• PKU can be managed entirely by a diet low in phenylalanine and high in tyrosine. Adherence to the diet will prevent neurological problems.

• PKU is detected early in a national neonatal biochemical screening programme.

Homocystinuria

Due to deficiency in cystathionine beta-synthase, resulting in increased urinary homocystine and methionine excretion.

Clinical manifestations resemble those of Marfan’s syndrome.

Treatment with high-dose pyridoxine and low-methionine diet, supple-mented with cysteine.

Cardinal features of homocystinuria

•   Eyes: lens subluxation (ectopia lentis); myopia; glaucoma

•   CNS: seizures; neurodevelopmental delay; behaviour problems

•   Skeleton: Marfanoid body habitus; high-arched palate; kyphoscoliosis; arachnodactyly

•   Cardiovascular system: mitral valve prolapse

Thromboembolism risk