Histidinemia. Histidase or histidine ammonia-lyase deficiency.
Incidence
It is an autosomal recessive disorder. The incidence from newborn screening is 1/10,000, making histidinemia one of the most frequent of the inborn errors of metabolism.
Clinical Characteristics
Symptoms are not present in the neonatal period. They may be present from infancy or childhood to adolescence. Symptoms may be seizures in infancy and childhood, mental retardation and developmental delay from infancy to adolescence, speech difficulty from infancy to adolescence and abnormal behavior in infancy and childhood. Ketones are present from infancy on, diagnosed by ferric chloride test. EEG may be abnormal. Histidase, the defective enzyme is normally expressed only in skin and liver. The block in the conversion of histidine to urocanic acid results in an increased concentration of histidine in blood and urine and the presence of abnormal histidine metabolites in urine. The diagnosis of histidinemia is made on an elevated histidine levels in the blood and increase excretion of histidine and imidazolepyruvic acid in the urine. The urinary metabolite, imidazolepyruvic acid yields positive tests with both ferric chloride and the Phenistix reagent strip. The diagnosis is confirmed by demonstrating the absence or marked reduction of histidase activity and/or the absence of urocanic acid in skin. Treatment with histidine restricted diet will normalize the biochemical phenotype but is not indicated for this probably harmless disorder, although the treatment might be considered in any histidinemic infant who also had clinical abnormalities. The deficient enzyme is histidase or histidine ammonia-lyase. Histidine is elevated in plasma, urine and CSF at all ages including newborn.
Precipitants
no
Provocation Tests
no
Diagnostic Procedures
EB-F. The deficient enzyme is histidase or histidine ammonia-lyase. The diagnosis of histidinemia is made on an elevated histidine levels in the blood and increase excretion of histidine and imidazolepyruvic acid in the urine. The urinary metabolite, imidazolepyruvic acid yields positive tests with both ferric chloride and the Phenistix reagent strip. The diagnosis is confirmed by demonstrating the absence or marked reduction of histidase activity and/or the absence of urocanic acid in skin.