Inborn error of metabolism

Inborn errors of metabolism comprise a large class of genetic diseases involving disorders of metabolism. The majority are due to defects of single genes that code for enzymes that facilitate conversion of various substances (substrates) into others (products). In most of the disorders, problems arise due to accumulation of substances which are toxic or interfere with normal function, or to the effects of reduced ability to synthesize essential compounds. Inborn errors of metabolism are now often referred to as congenital metabolic diseases or inherited metabolic diseases, and these terms are considered synonymous. more...

The term inborn error of metabolism was coined by a British physician, Archibald Garrod (1857-1936), in the early 20th century. He is known for the "one gene, one enzyme" hypothesis, which arose from his studies on the nature and inheritance of alkaptonuria. His seminal text, Inborn Errors of Metabolism was published in 1923.

Major categories of inherited metabolic diseases

Traditionally the inherited metabolic diseases were categorized as disorders of carbohydrate metabolism, amino acid metabolism, organic acid metabolism, or lysosomal storage diseases. In recent decades, hundreds of new inherited disorders of metabolism have been discovered and the categories have proliferated. Following are some of the major classes of congenital metabolic diseases, with prominent examples of each class. Many others do not fall into these categories. ICD-10 codes are provided where available.

• Disorders of carbohydrate metabolism
  • E.g., glycogen storage disease (E74.0)
• Disorders of amino acid metabolism
  • E.g., phenylketonuria (E70.0), maple syrup urine disease (E71.0)
• Disorders of organic acid metabolism
  • E.g., alcaptonuria (E70.2)
• Disorders of fatty acid oxidation and mitochondrial metabolism
  • E.g., medium chain acyl dehydrogenase deficiency
• Disorders of porphyrin metabolism
  • E.g., acute intermittent porphyria (E80.2)
• Disorders of purine or pyrimidine metabolism
  • E.g., Lesch-Nyhan syndrome (E79.1)
• Disorders of steroid metabolism
  • E.g., congenital adrenal hyperplasia (E25.0)
• Disorders of mitochondrial function
  • E.g., Kearns-Sayre syndrome (H49.8)
• Disorders of peroxisomal function
  • E.g., Zellweger syndrome (Q87.8)
• Lysosomal storage disorders
  • E.g., Gaucher's disease (E75.22)

Manifestations and presentations

Because of the enormous number of these diseases and wide range of systems affected, nearly every "presenting complaint" to a doctor may have a congenital metabolic disease as a possible cause, especially in childhood. The following are examples of potential manifestations affecting each of the major organ systems:

• Growth failure, failure to thrive, weight loss
• Ambiguous genitalia, delayed puberty, precocious puberty
• Developmental delay, seizures, dementia, encephalopathy, stroke
• Deafness, blindness, pain agnosia
• Skin rash, abnormal pigmentation, lack of pigmentation, excessive hair growth, lumps and bumps
• Dental abnormalities
• Immunodeficiency, thrombocytopenia, anemia, enlarged spleen, enlarged lymph nodes
• Many forms of cancer
• Recurrent vomiting, diarrhea, abdominal pain
• Excessive urination, renal failure, dehydration, edema
• Hypotension, heart failure, enlarged heart, hypertension, myocardial infarction
• Hepatomegaly, jaundice, liver failure
• Unusual facial features, congenital malformations
• Excessive breathing (hyperventilation), respiratory failure
• Abnormal behavior, depression, psychosis
• Joint pain, muscle weakness, cramps
• Hypothyroidism, adrenal insufficiency, hypogonadism, diabetes mellitus

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