Chapter 134

Over 400 human diseases that are due to inborn errors of metabolism are now recognized, and this number is constantly increasing. However, the incidence of inborn errors may well be underestimated, because diagnostic errors are frequent. Despite the relative abundance of new case reports, there is considerable evidence that many of these disorders remain undetected or misdiagnosed. Several factors conspire to make the clinical diagnosis of inborn errors of metabolism (IEM) difficult.

IEM are individually rare but collectively numerous. The recent application of tandem mass spectrometry (tandem MS) to newborn screening and prenatal diagnosis has enabled presymptomatic diagnosis for some IEM. However, for most IEM, neonatal screening tests are either too slow, too expensive, or too unreliable; consequently, a simple method of clinical screening is mandatory before initiating sophisticated biochemical investigations. The clinical diagnosis of IEM relies upon a limited number of principles:

• Consider IEM in parallel with other more common conditions; for example, sepsis or anoxic-ischemic encephalopathy in neonates, and intoxication, encephalitis, and brain tumors in older patients
• Be aware of symptoms that persist and remain unexplained after the initial treatment and the usual investigations have been performed
• Collect blood and urine samples at the right time in relation to an acute illness
• Suspect that any neonatal death may be due to an IEM, particularly deaths that are attributed to sepsis
• Carefully review all autopsy findings
• Do not confuse a symptom (such as peripheral neuropathy, retinitis pigmentosa, cardiomyopathy, etc) or a syndrome (such as Reye syndrome, Leigh syndrome, sudden infant death, etc) with etiology
• Remember that an IEM can present at any age, from fetal life to old age
• Know that although most genetic metabolic errors are hereditary and transmitted as recessive disorders, the majority of individual cases appear sporadically
• Initially consider inborn errors that are amenable to treatment (mainly those that cause intoxication). Do not miss a treatable disorder.
• In acute emergency situations, undertake first those few investigations that are able to diagnose treatable IEM: First take care of the patient (emergency treatment) and then the family (genetic counseling).

In this section, inborn errors amenable to treatment are printed in bold. Additional information and diagnostic checklists are available online.

### Classification

The vast majority of IEM involve abnormalities in enzymes and transport proteins. However, all the metabolic disorders can be divided into the following two large clinical categories.

#### Category 1

Includes disorders that either involve only one functional system (such as the endocrine system, immune system, coagulation factors, or lipoproteins) or affect only one organ or anatomic system (such as the intestine, renal tubules, erythrocytes, or connective tissue). Presenting symptoms are uniform (e.g., a bleeding tendency in coagulation factor defects or hemolytic anemia in defects of glycolysis), and the correct diagnosis is usually easy to guess even when the basic biochemical lesion gives rise to systemic consequences.

#### Category 2

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