Guideline 12.6 — Inborn Errors of Metabolism — Acute Neonatal Presentation

KEY TAKEAWAYS

Think IEM in any critically ill neonate — especially the "sepsis-like" term infant who deteriorates after an interval of well-being following a normal pregnancy and delivery.
Four first-line tests crack most of the field: ammonia, blood gas (acid-base/anion gap), glucose, and urine ketones.
The high-yield pattern rules:
Hyperammonemia WITHOUT acidosis (often respiratory alkalosis) → urea cycle defect.
High-anion-gap metabolic acidosis + ketosis ± hyperammonemia → organic acidemia.
Hypoglycemia with inappropriately LOW ketones → fatty-acid oxidation defect (or hyperinsulinism).
Don't wait for newborn screening — a sick neonate can die before results return; treat empirically.
Emergency treatment = stop the toxin, reverse catabolism, remove the toxin: stop protein, give high-dextrose (anabolism), and use nitrogen scavengers + arginine ± carnitine, escalating to hemodialysis for severe hyperammonemia.
Marked hyperammonemia is a neurologic emergency — outcome depends on how fast you lower it.

1. Clinical Overview

Clinical Overview

Inborn errors of metabolism are individually rare but collectively important causes of neonatal collapse, and they are eminently treatable when recognized early. The classic scenario is a term infant, product of an uncomplicated pregnancy, who feeds and behaves normally for hours to days and then deteriorates — poor feeding, vomiting, lethargy, hypotonia, seizures, apnea, and coma — often looking exactly like sepsis. Because the accumulating toxins (ammonia, organic acids) injure the brain in proportion to how long they remain elevated, the whole clinical strategy is rapid recognition, rapid metabolic screening, and immediate empiric treatment while specific diagnosis is pursued.

Newborn screening (tandem mass spectrometry) catches many disorders, but results often return too late for the acutely decompensating infant — so the bedside clinician must act on pattern recognition.

2. Definitions

Term	Meaning
IEM	Inherited defect of a metabolic pathway causing toxic accumulation or energy deficiency.
Metabolic "intoxication" disorders	Accumulation of toxic metabolites (urea cycle defects, organic acidemias, aminoacidopathies) — the classic "well then sick" presentation.
Energy-deficiency disorders	Impaired energy production (fatty-acid oxidation defects, mitochondrial/pyruvate disorders) — often hypoglycemia, lactic acidosis, cardiac/muscle involvement.
Critical sample	Blood/urine drawn during the acute crisis to capture diagnostic metabolites (see hypoglycemia chapter).
Nitrogen scavengers	Sodium benzoate/sodium phenylacetate — provide alternative routes for nitrogen excretion in hyperammonemia.

3. Pathophysiology

Pathophysiology

Two broad mechanisms:

Toxic accumulation ("intoxication"): a blocked pathway causes build-up of upstream toxins — ammonia (urea cycle), organic acids (organic acidemias), or amino acids — that poison the brain and other organs. Catabolic stress (birth, fasting, illness, protein load) precipitates crises; hence the symptom-free interval followed by decompensation.
Energy deficiency: impaired ATP generation (fatty-acid oxidation during fasting, mitochondrial/pyruvate disorders) → hypoglycemia, lactic acidosis, and organ (heart, muscle, liver, brain) dysfunction, sometimes from birth.

Genetics: most are autosomal recessive; ornithine transcarbamylase (OTC) deficiency (the commonest urea cycle defect) is X-linked (classically affected boys).

4. Clinical Presentation

Clinical Presentation

The classic tempo: normal at birth → symptom-free interval (hours–days) → deterioration.
Nonspecific/"sepsis-like": poor feeding, vomiting, lethargy → encephalopathy, hypotonia, seizures, apnea, temperature instability, coma.
Clues: central hyperventilation (respiratory alkalosis) in early hyperammonemia (ammonia stimulates the respiratory center); an unusual odor (e.g., maple-syrup in MSUD, sweaty-feet in isovaleric acidemia); a family history of neonatal death or consanguinity.
Energy-deficiency clues: cardiomyopathy, hepatomegaly, dysmorphism, severe hypotonia, hypoglycemia.
Overlap with sepsis: consider IEM alongside infection (and note galactosemia predisposes to E. coli sepsis).

5. Diagnostic Approach

Diagnostic Approach

First-line ("metabolic panic") labs — send immediately:

Ammonia (free-flowing sample, on ice, run urgently)
Blood gas (acid-base; calculate anion gap)
Glucose
Lactate
Urine ketones (dipstick)
Plus electrolytes, CBC, LFTs, and save a critical sample.

The algorithm (very high-yield):

Pattern	Points to
↑ Ammonia, NO acidosis (often respiratory alkalosis)	Urea cycle defect (± transient hyperammonemia of the newborn)
High-anion-gap metabolic acidosis + ketosis ± ↑ ammonia	Organic acidemia (e.g., methylmalonic, propionic, isovaleric)
Hypoglycemia + LOW/absent ketones	Fatty-acid oxidation defect (or hyperinsulinism)
Hypoglycemia + reducing substances in urine, ± cataracts, ± E. coli sepsis, liver dysfunction	Galactosemia
Lactic acidosis (persistent)	Mitochondrial / pyruvate metabolism disorder

Confirmatory (targeted): plasma amino acids, urine organic acids, plasma acylcarnitine profile, specific enzyme/genetic testing — guided by the pattern. Newborn screening supports diagnosis but shouldn't delay treatment.

Ammonia thresholds: neonatal ammonia >~100–150 is elevated; >1000 is markedly elevated and a neurologic emergency.

6. Management (the metabolic emergency)

Management (the metabolic emergency)

Principle: stop the toxin, reverse catabolism, remove the toxin — and call the metabolic service now.

Stop protein/feeds — remove the offending substrate (temporarily).
Reverse catabolism / drive anabolism — high-dextrose IV (generous glucose infusion rate, e.g., D10 or higher via central access) ± intralipid to halt endogenous protein breakdown; add insulin if hyperglycemic to keep anabolism going. (Exception: in a suspected fatty-acid oxidation defect, glucose is still first-line, but avoid lipid.)
Treat hyperammonemia:

Nitrogen scavengers (sodium benzoate / sodium phenylacetate) provide alternative nitrogen excretion.
Arginine (for urea cycle defects) supports the cycle.
L-carnitine (organic acidemias / FAO) aids toxin conjugation/excretion.
Hemodialysis is the most effective removal for severe/refractory hyperammonemia — escalate early for very high or rising ammonia.

Correct acidosis (sodium bicarbonate for severe metabolic acidosis), electrolytes, and hypoglycemia.
Cofactor trials where the picture fits: hydroxocobalamin (B12) for methylmalonic acidemia, biotin for biotinidase/multiple carboxylase deficiency, thiamine for thiamine-responsive MSUD variants, pyridoxine (B6) for pyridoxine-dependent seizures.
Disease-specific diet: e.g., galactose-free (lactose-free) formula for galactosemia; avoid fasting in FAO.
Supportive care (airway/ventilation for encephalopathy, seizure control) and urgent transfer to a center with a metabolic team and dialysis capability.

7. Monitoring

Monitoring

Serial ammonia (until falling and controlled), blood gas/anion gap, glucose, lactate, electrolytes.
Neurologic status (encephalopathy grade), seizures (EEG as needed).
Response to therapy (ammonia trend guides escalation to dialysis).
Growth/nutrition once stabilized and on specialist-directed diet.

8. Complications

Complications

Neurologic injury proportional to the duration/degree of hyperammonemia (developmental delay, cerebral palsy, epilepsy); coma and death if untreated.
Metabolic crises recur with catabolic stress (illness, fasting) throughout life.
Organ-specific damage (liver, heart, kidney) in energy-deficiency disorders.

9. Safety Warnings

Safety Warnings

⚠️ Consider IEM in every critically ill/"septic" neonate — especially "well then suddenly sick" after a normal pregnancy.
⚠️ Send ammonia early and handle it correctly (free-flowing, on ice, run stat) — a delayed/mishandled sample misses the diagnosis.
⚠️ Don't wait for newborn screening to treat.
⚠️ Marked/rising hyperammonemia = emergency — start scavengers/arginine and escalate to dialysis early.
⚠️ Reverse catabolism aggressively — stopping protein alone isn't enough; give generous glucose.
⚠️ Consider galactosemia in a jaundiced/hypoglycemic infant with E. coli sepsis — start lactose-free feeds.

10. Common Mistakes

Common Mistakes

Anchoring on sepsis and never sending an ammonia.
Mishandling the ammonia sample (delay/tourniquet) → falsely reassuring or uninterpretable.
Waiting for newborn-screening results before treating a decompensating infant.
Stopping protein without giving glucose (catabolism continues).
Delaying dialysis in severe hyperammonemia.
Missing cofactor-responsive disorders (B12, biotin, thiamine, B6).
Overlooking galactosemia (with E. coli sepsis) and continuing lactose feeds.

11. Clinical Pearls

Clinical Pearls

💡 "Well then sick" term infant = think IEM.
💡 Ammonia + gas + glucose + ketones solve most of the puzzle at the bedside.
💡 Hyperammonemia without acidosis = urea cycle; with high-gap acidosis + ketosis = organic acidemia.
💡 Hypoglycemia with low ketones = fatty-acid oxidation defect.
💡 Give sugar to stop the body eating itself — anabolism is treatment.
💡 When ammonia is very high, dialysis beats drugs alone — escalate early.

12. Summary Table

Summary Table

Domain	Bottom line
When to suspect	Critically ill/"septic" term neonate; well then sick after normal pregnancy
First-line labs	Ammonia, blood gas/anion gap, glucose, lactate, urine ketones (+ critical sample)
↑NH₃, no acidosis	Urea cycle defect
High-gap acidosis + ketosis ±↑NH₃	Organic acidemia
Hypoglycemia + low ketones	Fatty-acid oxidation defect (or hyperinsulinism)
Emergency Rx	Stop protein → high-dextrose (± lipid/insulin) → scavengers + arginine ± carnitine → hemodialysis for severe ↑NH₃
Cofactors	B12 (MMA), biotin, thiamine (MSUD), B6 (seizures)
Specific	Galactose-free formula (galactosemia); avoid fasting (FAO)
Outcome driver	Speed of lowering ammonia / removing toxin

13. Step-by-Step Bedside Algorithm

CRITICALLY ILL NEONATE ("sepsis-like"; well then sick after normal pregnancy)
        │
   (treat for sepsis in parallel — cultures + antibiotics ± acyclovir)
        │
        ▼
SEND FIRST-LINE METABOLIC LABS NOW:
   AMMONIA (free-flow, on ice, STAT) + BLOOD GAS/anion gap + GLUCOSE + LACTATE + urine KETONES
   (+ save critical sample; electrolytes, LFTs, CBC)
        │
        ▼
PATTERN?
   ├─ ↑Ammonia, NO acidosis ───────────► urea cycle defect
   ├─ High-gap acidosis + ketosis ±↑NH₃ ► organic acidemia
   ├─ Hypoglycemia + LOW ketones ──────► fatty-acid oxidation defect (or hyperinsulinism)
   ├─ Hypoglycemia + reducing substances/E. coli sepsis ► galactosemia → lactose-free formula
   └─ Persistent lactic acidosis ──────► mitochondrial/pyruvate disorder
        │
        ▼
START EMERGENCY TREATMENT (don't wait for newborn screening):
   1. STOP protein/feeds
   2. HIGH-DEXTROSE IV (reverse catabolism) ± lipid ± insulin
   3. For hyperammonemia: nitrogen SCAVENGERS + ARGININE ± CARNITINE
   4. Correct acidosis/electrolytes/glucose
   5. Cofactor trials (B12/biotin/thiamine/B6) per pattern
        │
        ▼
Severe or RISING ammonia / not responding? ──► HEMODIALYSIS (escalate early)
        │
        ▼
Confirmatory tests (plasma amino acids, urine organic acids, acylcarnitines, genetics)
   + URGENT metabolic team / transfer → long-term diet & emergency plan

14. References to Verify

Confirm each against the primary source and your metabolic service before clinical or published use.

1.Emergency management: Saudubray J-M, et al. and other standard references on emergency treatment of neonatal metabolic decompensation (protein removal, anabolism, toxin removal). (Verify doses.)
2.Hyperammonemia: consensus guidelines on urea cycle disorder management (nitrogen scavengers, arginine, dialysis thresholds). (Verify dosing/thresholds.)
3.Diagnostic algorithm: standard "ammonia + acidosis + ketones + glucose" approach references. (Verify interpretation.)
4.Newborn screening: current national/state tandem-mass-spectrometry panels (RUSP). (Verify local panel — and that treatment shouldn't await results.)
5.Cofactor-responsive disorders: references for B12 (MMA), biotin, thiamine (MSUD), pyridoxine. (Verify.)

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Chapter 12.6 — Inborn Errors of Metabolism — Acute Neonatal Presentation