Inborn Errors of Metabolism Flashcards
What should come to mind when a previously healthy infant develops poor feeding, hypoglycemia, lethargy, vomiting, tachypnea, apnea, irritability, seizures, coma, apnea?
Inborn errors of metabolsim
What is often the crucial clue for answers regarding inborn errors of metabolism?
Antecedent events, even if they seem inconsequential (like feeding)
What should you focus on with questions regarding inborn errors of metabolism?
Specific differences since many are quite similar
Use a logical approach
If you are presented with a lethargic or comatose infant, what is appropriate to order?
CBC, electrolytes, serum ammonia level, and urine organic acids
If you are given values for electrolytes, what should you do?
Measure the anion gap…
Serum Sodium - [Chloride + Bicarbonate]
What is normal anion gap?
8-12 mEq/L
What should an elevated anion gap in an infant who is lethargic or comatose make you think of?
Elevated ammonia
What 2 symptoms do all of the metabolic disorders present with?
- Vomiting
- Lethargy
*So distinguishing the fine differences between them can be a challenge… key phrases and wording distinguish one from the other
What does it mean if they emphasize that the infant is afebrile?
ID tree is the wrong tree…look for other clues in the question that this isn’t an infection (normal WBC count and platelet count)
Metabolic acidosis, Elevated serum ammonia (NH4)…disorders?
- Propionic acidemia
- Methylamalonic acidemia
- Fatty acid oxidation defects
ABG normal, Elevated serum ammonia (NH4)…disorders?
- Urea cycle defect
2. Transient hyperammonemia
Metabolic acidosis, Normal serum ammonia (NH4)…disorders?
- MSUD
2. Some organic acidemias
ABG normal, Normal serum ammonia (NH4)…disorders?
- Aminoacidopathy
- Galactosemia
- Non-ketotic hyperglycinemia
What is a normal serum ammonia value in a newborn?
Below 110 mcmol/L
In a kid with galactosemia, why might they have a metabolic acidosis?
From sepsis… pure galactosemia has a normal ABG
What are 3 examples of organic acidemias?
- Methylmalonic acidemia
- Propionic acidemia
- Isovaleric acidemia
Which organic acidemia has an odor of sweaty feet?
Isovaleric acidemia
What common problem can present with a metabolic acidosis?
Sepsis
How do organic acidemias typically present?
With a “drunk-like” intoxicated picture…but the disease is life threatening
What are important lab findings in organic acidemias?
- Elevated serum ammonia levels (in many, but not all)
- Acidosis
- High anion gap
- Ketonuria
When do organic acidemias often present?
First 2 days after the introduction of protein in the diet
What is the most important initial step after diagnosing an organic acidemia?
HYDRATION…to maintain good urine output
What is indicated after you hydrate your patient you have diagnosed with organic acidemia?
Appropriate diet
What is the most important laboratory study for organic acidemia?
Urine organic acid levels
When they present you with an infant with a septic appearance with sepsis ruled out, what is likely cause?
Metabolic (generally)
What should be done next if they present you with an infant with a septic appearance with sepsis ruled out?
Obtain a serum ammonia level
Additional correct choices: Lactic acid level, serum pyruvate, total and free carnitine, acetylcarnitine
Why can granulocytopenia and thrombocytopenia occur with organic acidemias?
Because metabolic acidosis can suppress bone marrow
If you have a history consistent with organic acidemia and a low platelet and WBC count, is this normal?
Could be…metabolic acidosis (from organic acidemias) can suppress bone marrow leading to granulocytopenia and thrombocytopenia
How does a child with organic acidemias present?
Decreased appetite, falling down frequently, delayed developmental milestones, with no overt physical abnormalities or dysmorphology
*Think of someone who is drunk…think of organic acids as very powerful alcoholic drinks
Since brain tumors and organic acidemias can both present with balance problems and vomiting, what can help distinguish?
If it is a brain tumor they will hint that the symptoms have been progressively worse… might also mention morning headaches and visual disturbances
How are fatty acid oxidation defects inherited?
Autosomal recessive pattern
Name 4 fatty acid oxidation defects?
- Medium chain acyl-CoA dehydrogenase deficiency
- Long chain acyl-CoA dehydrogenase deficiency
- Very long chain acyl-CoA dehydrogenase deficiency
- Glutaric aciduria
How do infants with a defect in fatty acid metabolism present?
Hypoglycemia and hepatomegaly
*Fatty acid oxidation defects may or may not present with hepatomegaly…it may be part of the presentation on boards however
What preceding signs should you watch for with fatty acid oxidation defect?
Preceding benign illness during which oral intake was decreased (this could occur in a toddler)
When do many of the signs of fatty acid oxidation defects present?
With fasting (why you have to watch for illness with associated oral intake)
True or false: In between episodes of fasting, a child with a defect in fatty acid metabolism is fine?
True
What is seen in urine and serum with defects in fatty acid metabolism?
Absence of reducing substances and ketones in urine
Normal serum amino acids
How is definitive diagnosis of a defect in fatty acid metabolism made?
Via plasma acylcarnitine profile
How do urea cycle defects present?
With hyperammonemia in the absence of acidosis and ketosis. Also look for a symptom-free period, followed by hypotonia and coma.
What acid-base issues can be present with urea cycle defects?
Respiratory alkalosis as well as lactic acidosis
What causes symptoms consistent with encephalopathy (vomiting, lethargy, coma) in urea cycle defects?
Hyperammonemia
What is treatment for urea cycle defects?
Reduce serum ammonia (NH4) by reducing protein intake and increasing glucose intake by IV… dialysis might be needed on a PRN, usually acute, basis
What does a neonate with hyperammonemia, acidosis, and ketosis have?
Organic aciduria
What does a neonate with no acidosis and ketosis, but hyperammonemia have?
Urea cycle defects
Infant with hypoglycemia, seizures, hepatomgealy, failure to thrive…most helpful measure to determine etiology?
Most important measurements to determine the etiology of hypoglycemia in infants is urine measurement of ketones and reducing substances
*Don’t fall for cortisol and growth hormone (if macrosomic) or insulin level measurement
When do infants with galatosemia present?
After their first meal containing lactose (breast milk or formula)…infants with galactosemia will appear normal at birth
After consuming lactose, how do babies with galactosemia present?
Non-specific findings…poor feeding and failure to thrive
Which states test for galactosemia?
All of them…but symptoms may develop before the newborn screen results are recieved
What should be done for a newborn with symptoms of galactosemia or a positive screen for galactosemia?
Changed immediately to a soy-based formula and repeat NBS
Name more specific findings of galactosemia
- Abdominal distension with hepatomegaly
- Hypoglycemia
- Non-glucose reducing substances in the urine
- Lethargy and hypotonia
What should you think with a history of prolonged jaundice and/or infection with gram negative organisms including E. Coli?
Galactosemia
What is galactosemia due to?
Deficiency of galactose-1-phosphate uridyltransferase (GALT)
How is definitive diagnosis of galactosemia made?
Measuring GALT in RBCs
What is treatment of galactosemia?
Soy formula…galactose-free diet
Failure to treat galactosemia can result in what?
Cataracts, intellectual disability, and/or liver disease
True or False: Cataracts due to galactosemia are reversible with diet change?
True
Since several glycogen storage diseases presents with hepatosplenomegaly, what buzz word should make you think galactosemia?
Positive reducing substances in urine
What are 3 other diseases that should be in your differential diagnosis for galactosemia?
- Lactose intolerance
- Maple Syrup Urine Disease
- Urea Cycle Defects
How is lactose intolerance different from galactosemia?
Presents later in childhood…much more benign