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INTRODUCTION

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Inborn errors of metabolism (IEMs), although rare, are becoming more recognized in pediatrics.

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Common examples of inborn errors of metabolism are shown in Table 74-1.

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TABLE 74-1

Common Examples of Inborn Errors of Metabolism

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Newborn screening tests have aided clinicians in diagnosing most of these disorders shortly after birth. However, some infants and children will present later in life, and it is crucial for pediatric ICU providers to recognize these disorders quickly so treatment can be initiated.

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Children with undiagnosed metabolic disorders oftentimes present with:

  • Varying states of shock

  • Cardiac insufficiency and/or cardiomyopathies

  • Feeding intolerance, failure to thrive, vomiting, and lethargy

  • Profound dehydration with or without metabolic acidosis

  • Altered mental status with or without hyperammonemia

  • Intractable seizures

  • Developmental delay

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These predominately autosomal-recessive disorders often result in significant enzyme deficiencies impairing mitochondrial function. Mitochondria therefore:

  • Cannot break down proper by-products of certain metabolic pathways, which impairs their function on the cellular level

  • Accumulate dysfunctional upstream metabolites, which becomes toxic to the cell and patient

  • Shunt metabolites into other pathways, which get overloaded or are unable to process these metabolites

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WORKING UP A METABOLIC DISORDER

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Physicians who suspect an underlying metabolic disorder or a patient in metabolic crisis should immediately evaluate the following on physical exam:

  • Vital signs: Assess for fever or hypothermia, abnormal changes in blood pressure and pulse pressure, alterations in heart rate and rhythm, and variations in respiratory effort (irregular breathing or large Kussmaul breaths)

  • General appearance: Observation of coarsened facial features, body odor/smell

  • Ophthalmologic exam: Consider a distinct eye exam to evaluate for cataracts, changes in retinal pigmentation

  • Cardiovascular exam: Evaluate for signs of cardiogenic shock; assess for brisk or delayed capillary refill, bounding pulses, displaced point of maximal impulse (PMI), murmurs

  • Abdominal exam: Direct palpation for hepatosplenomegaly

  • Neurologic exam: Note baseline Glasgow Coma Score (GCS) for encephalopathy, hypotonia or hypertonia, brisk tendon reflexes, pupillary irregularities, or cranial nerve deficits

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Initial laboratory studies should include the following:

  • Arterial blood gas to evaluate for changes in blood pH and the presence of metabolic acidosis and lactate production

  • Rapid blood glucose to evaluate for hypoglycemia

  • Rapidly run iced ammonia levels on free-flowing blood samples

  • Basic metabolic panels to assess bicarbonate levels and electrolyte derangements

  • Pyruvate levels

  • Blood and urine ketones/β-hydroxybutyrate

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