Biochemical disorders Biochemical disorders. Metabolic diseases
Oct 02

Phenylketonuria is the most common amino acid disorder. The screening test is very simply measuring phenylalanine from a blood spot on a newborn screening card from a heel stick. There’s a prerequisite. Children have to have taken in protein in the form of formula for at least 24 hours. If not, you’ll have a false negative. And in these days of early discharge from the hospital where kids barely get a little glucose to suck on, it’s very important to bring them back in the first few days of life to their primary care provider to get an adequate specimen, meeting the prerequisite. The diagnostic test is to do a quantitative plasma amino acid profile so that you can actually quantitate the amount of phenylalanine that’s there. You are all familiar with the clinical features; mental retardation, you can also see autism and seizures. The treatment is the lifetime, preferable, treatment of low phenylalanine in the diet. Just like the diabetics who count calories, these folks have to count grams of phenylalanine, grams of protein as well. The lifetime diet is optimal for yet another reason. High phenylalanine is teratogenic. Half of PKU patients are going to be female, and like most females, they like to grow up and have families and if they go off diet – although it may not harm them very much – this can lead to birth defects in the unborn child, even though the child does not have PKU. If the mother’s PKU, the phenylalanine, the phenylketones, the phenyl lactate across the placenta, are neurotoxic and can also cause some somatic disorders as well such as congenital heart disease. It’s very difficult to control pregnant mothers with PKU because the fetus is growing rapidly, you have to make sure you provide the fetus with enough protein, enough phenylalanine, but not too much. So we usually hospitalize these moms for a week or two to get them on diet. Hopefully they have been on diet before they got pregnant, because as you realize the first six weeks of gestation are the most critical in terms of neurologic development.
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Homocystinuria is very uncommon, compared to PKU, and the screening test in those states that do it is not to measure homocystine, because it’s a little difficult – because it’s an unstable compound – they actually measure methionine. The diagnostic test is to do a complete plasma amino acid profile. I’ll stress again, nobody gets urine amino acids anymore. The only thing you get from urine is organic acids. So don’t waste our time getting urine amino acids. In differential diagnosis of pediatric stroke, you have to make sure that homocystinuria is in your differential.
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How do we treat these kids? For the most part we don’t really treat them with dietary restriction of protein or methionine anymore. But rather we supplement them with either folate or pyridoxine or betaine. Betaine recycles homocystine and methylates it back to methionine, and methionine is non-toxic. And certainly low-dose aspirin for any coagulation.

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