Heredity is the passing of genes from one generation to the next. Children inherit their parents' genes. Hereditary metabolic disorders develop when children inherit defective genes that control metabolism. There are different types of inherited disorders. In most hereditary metabolic disorders, both parents of the affected child carry a copy of the abnormal gene. Because usually two copies of the abnormal gene are necessary for the disorder to occur, usually neither parent has the disorder. Some hereditary metabolic disorders are X-linked, which means only one copy of the abnormal gene can cause the disorder in boys.
There are hundreds of metabolic disorders and most of them are extremely rare.
Most of the foods and drinks people ingest are complex materials that the body must break down into simpler substances. This process may involve several steps. The simpler substances are then used as building blocks, which are assembled into the materials the body needs to sustain life. The process of creating these materials may also require several steps. The major building blocks are
Metabolism is this complicated process of breaking down and converting the substances ingested.
Metabolism is carried out by chemical substances called enzymes, which are made by cells in the body. If a genetic abnormality affects the function of an enzyme or causes it to be deficient or missing altogether, various metabolic disorders can occur. These disorders usually result from one or both of the following:
Metabolic disorders are classified by the particular building block that is affected.
Before birth, some hereditary disorders of metabolism (such as phenylketonuria and lipidoses) can be diagnosed in the fetus by using the prenatal screening tests amniocentesis or chorionic villus sampling. Usually, a hereditary metabolic disorder is diagnosed by using a blood test or examination of a tissue sample to determine whether a specific enzyme is deficient or missing. Genetic testing is also sometimes used.
After birth, many of these disorders are detected by routine newborn screening tests. For a complete list of routine newborn screening tests by state, see the National Newborn Screening and Global Resource Center web site. However, newborns are not screened for many of the less common hereditary metabolic disorders, and doctors do tests for those disorders only when they suspect a problem.
Doctors may suspect a hereditary disorder of metabolism during a physical examination. Symptoms may provide clues as well. For example, children who have sweet-smelling urine may have maple syrup urine disease, or children who smell like sweaty feet may have isovaleric acidemia. Eye problems, an enlarged liver or spleen, heart problems (such as cardiomyopathy), or muscle weakness may indicate other hereditary metabolic disorders.