An introduction to nutritional treatment in inborn errors of metabolism--different disorders, different approaches.

Treatment of metabolic disease aims to restore homeostasis, where possible. This can be achieved in a number of ways. For disorders of intermediary metabolism, treatment involves a thorough understanding of the disorder and the pathogenesis of the deleterious effects The various approaches indicated may involve substrate restriction, replacement of deficient products, removal of toxic metabolites or stimulation of residual enzymes. Newer therapies include enzyme replacement and gene therapy. Often, the cornerstone of treatment is dietary. Substrate restriction includes not only a diet low in the substrate indicated by the disorder, but also strict calorie support in times of illness to avoid catabolism. Useful levels of substrate restriction may require the use of supplements of "medical foods", for example amino acid mixtures. Provision of the deficient products is important in disorders affecting energy metabolism. To understand the problems involved in nutritional treatment it is helpful to consider examples of different types of disorders. In Maple syrup urine disease (MSUD), treatment with a very strict low-protein diet, supplemented by a branched-chain-free amino acid mixture is successful, but each intercurrent illness is hazardous, regimens for sick days vital, and strict lifelong treatment is needed. Treatment for phenylketonuria is similar in restricting a substrate but there is no tendency for systemic illness if the phenylalanine levels are too high. Disorders of the urea cycle are difficult dietary challenges because while a very low-protein diet is required, no specific amino acid needs to be avoided and there is a fine line between adequate protein intake and chronic catabolism. Fatty acid oxidation disorders affect energy production and can be detected by newborn screening using tandem mass spectrometry. For long-chain fatty acid disorders, long chain fats must largely be avoided and medium-chain fats must be substituted while strictly avoiding catabolism. Glycogen storage disorders require strict attention to providing carbohydrate, at all times including throughout the night. Many patients with inborn errors do not need any specific dietary therapy, (eg those with storage or neurodegenerative disorders), although all children benefit from an optimal diet, and sick children need this especially.

Information overload--new technologies, can we store the data?

Computerization within newborn screening programs is a developing issue. To date two basic approaches to data storage have been used: (1) a storage system for babies diagnosed with a disorder, (2) a comprehensive system with long-term details for all patient samples, tests performed, test results and interpretations. It usually provides efficient real-time reports for various clinical and quality control requirements and easy access to an inborn errors registry. Within the last decade there have been two new technologies adapted to routine use in newborn screening laboratories: (1) tandem mass spectrometry for selected amino acids and acyl carnitine, and (2) DNA mutational analysis of PCR products. Both technologies present data storage challenges. Both are capable of providing large files of information for a sample. Consideration must be given to how these data are stored, whether all results including a graphical representation or DNA sequence data are kept or whether only final results for specific analytes are stored. Many new analytical technologies can only be incorporated into routine programs as a result of advances in hardware and software allowing better access to, and storage of, data.

Evaluating outcomes of newborn screening programs.

Newborn screening is a medical intervention. For every program, there should be evidence of its effect and effectiveness. The four questions to be addressed, very broadly, are: What is the effectiveness for case-finding (sensitivity, specificity, and positive predictive value)? What are the benefits of early detection versus clinical detection? What harm results from the program? Are the costs reasonably balanced in relation to benefits? Ideally, there would be randomized controlled trials (RCTs) of screening for each disorder. In practice, power calculations reveal that for very rare disorders this is not feasible. The numbers of screened and unscreened babies required would be huge, and trials would last for decades. There have only been RCTs of newborn screening for cystic fibrosis (birth prevalence 25-40 per 100,000 in Caucasians). No such trials were ever attempted for hypothyroidism, with a similar birth prevalence, and it may not now be ethical to mount one. Instead, lower orders of evidence must be used. Double-blind randomized controlled trials should be planned for not-so-rare disorders where possible. Where it is not feasible, careful planning and collection of data, plus the use of both historical controls and contemporaneous controls from other regions may have to suffice. To introduce programs with no plans for full evaluation is not best practice. Evaluation of outcomes of all kinds, not simply of case-finding, must be mandatory. Data for case-finding should be collected actively, with systematic searching for missed cases. Data about benefits need to be collected in well-planned long-term studies, although short-term benefits are also valuable. Good studies of harm, mainly from false positive results, are urgently needed. The problem of costs and benefits is difficult, and a "reasonable balance" rather than positive cost/benefit ratio seems desirable.

Newborn screening--is it really that simple?

The incorporation of tandem mass spectrometry (MSMS) into an existing newborn screening program is an evolving process. Limited worldwide experience has ensured that all stages of reliability testing need to be followed. These include a literature review to establish methodology and analytes/disorders for testing and a pilot screening project including assaying archival samples from subjects with proven target disorders. Algorithms used for analyte concentrations and the relationships of various analytes to one another for resample criteria need to be continually reassessed to maximise screening specificity, sensitivity and positive predictive value. Since 1st of April 1998, the NSW Newborn Screening Program has screened 320, 848 babies using electrospray MSMS for selected amino acids and acyl camitines. Screening for amino acids has led to requests for 415 repeat samples with 94 babies referred for further testing. Of these 73 had a disorder of amino acid metabolism, including 43 with persistent hyperphenylalaninemia (36 of whom had PKU, 2 had a pterin pathway defect, 5 HPAA). Screening for acyl carnitines has led to requests for 245 repeat samples with 63 babies referred for further investigation. Of these 44 had a diagnosed disorder, including 15 with medium chain acyl CoA dehydrogenase deficiency. Five babies with confirmed disorders detectable with MS/MS had negative test results. The cost of screening using MSMS was only $A0.50 more than the method for screening for PKU and homocystinuria alone (ie the bacterial inhibition assays) and has allowed detection of an additional 74 babies at least 48 of whom have a diagnosis for which early treatment seems clearly beneficial. MSMS has shown a sensitivity of 95.9% and specificity of 99.8% in our laboratory with a positive predictive value of 18%.