📘 Chapter 104 · Clinical Manifestations of Genetic Metabolic Diseases

Nelson Textbook of Pediatrics 22nd Ed | Key tables: Neurologic & laboratory findings (Table 104.3), neonatal hypoglycemia (104.4), hyperammonemia DDx (104.5), hepatomegaly (104.6), cardiomyopathy (104.7), dysmorphic features (104.8), hydrops (104.9), physical exam findings (104.10), odors (104.11), clinical/lab red flags (104.12–13) and diagnostic algorithm.

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📋 Table 104.3: Neurologic & Laboratory Manifestations in Neonates (IEM)

Presentation	Associated Disorders
Deterioration of consciousness + Metabolic acidosis	Organic acidemias, pyruvate metabolism defects, fatty acid oxidation defects, fructose-1,6-bisphosphatase deficiency, glycogen storage diseases, mitochondrial RC defects, ketone metabolism disorders
Hypoglycemia (see Table 104.4)	Fatty acid oxidation defects, gluconeogenesis disorders, fructose/galactose disorders, glycogen storage diseases, hyperinsulinemic hypoglycemias, organic acidemias, mitochondrial RC defects, citrin deficiency, carbonic anhydrase VA deficiency
Hyperammonemia	Urea cycle defects, organic acidemias, fatty acid oxidation defects, glycine encephalopathy, pyruvate carboxylase deficiency, GLUD1 hyperinsulinism, carbonic anhydrase VA deficiency, Zellweger spectrum
Seizures + Hypotonia	Congenital disorders of glycosylation, antiquitin deficiency (pyridoxine-dependent epilepsy), purine/pyrimidine defects, PNPO deficiency
Neonatal Apnea	Glycine encephalopathy (NKH), multiple carboxylase deficiency, urea cycle disorders, organic acidemias, fatty acid oxidation defects

🔬 Select IEM associated with key neonatal neurologic signs (adapted from Nelson Ch.104).

📋 Table 104.4: Select IEM Associated with Neonatal Hypoglycemia

Category	Specific Disorders
Fatty acid oxidation disorders	Carnitine-acylcarnitine translocase, CPT-Ia, CPT-II, LCHAD, MTP deficiency, MCAD, VLCAD, multiple acyl-CoA dehydrogenase (GA2)
Disorders of ketone metabolism	HMG-CoA lyase deficiency, HMG-CoA synthase deficiency, SCOT deficiency, β-ketothiolase deficiency
Gluconeogenesis defects	Fructose-1,6-bisphosphatase, PEPCK deficiency
Glycogen storage diseases	GSD I (von Gierke), GSD III, GSD VI, GSD IX
Hyperinsulinemic hypoglycemia	HADH, GLUD1 (HI/HA), ABCC8/KCNJ11 (KATP channel), focal/diffuse HI
Organic acidemias / other	Propionic, methylmalonic, isovaleric, multiple carboxylase deficiency, mitochondrial RC defects, citrin deficiency

📋 Table 104.5: Differential Diagnosis of Hyperammonemia

Category	Examples
Urea cycle enzyme defects	NAGS, CPS1, OTC (X-linked most common), ASS (citrullinemia), ASL (argininosuccinic aciduria), arginase 1 deficiency
Organic acidemias	Propionic acidemia, methylmalonic acidemia, isovaleric acidemia
Fatty acid oxidation disorders	Long-chain defects, CPT deficiencies, carnitine transporter defect
Other IEM	Pyruvate carboxylase deficiency, GLUD1 hyperinsulinism, carbonic anhydrase VA deficiency
Acquired / transient	Transient hyperammonemia of newborn, liver failure, valproate toxicity, Reye syndrome

📋 Table 104.6: Select Metabolic Disorders with Hepatomegaly

Group	Disorders
Amino acid disorders	Tyrosinemia type I, citrin deficiency, urea cycle defects
Carbohydrate disorders	Galactosemia, hereditary fructose intolerance, GSD I, III, IV, VI, IX
Lysosomal storage	Gaucher, Niemann-Pick, mucopolysaccharidoses, I-cell disease
Fatty acid oxidation	Carnitine defects, VLCAD, LCHAD, MADD (GA2)
Mitochondrial/Peroxisomal	Zellweger spectrum, respiratory chain defects

📋 Table 104.7: IEM Associated with Cardiomyopathy

Disorder Category	Examples
Glycogen storage diseases	GSD II (Pompe), GSD III, PRKAG2, GSD IV
Fatty acid oxidation	CACT deficiency, CPT-II, LCHAD/MTP, VLCAD, ACAD9, carnitine transporter defect
Lysosomal storage	Fabry disease, Danon disease (LAMP2), MUCOpolysaccharidoses
Mitochondrial RC defects	mtDNA mutations, SCO2, TMEM70, CoQ10 deficiency

📋 Table 104.8: Dysmorphic Features in IEM

Disorder Group	Examples
Congenital disorders of glycosylation (CDG)	PMM2-CDG, ALG3-CDG, Walker-Warburg
Lysosomal storage	Mucopolysaccharidoses (coarse facies), oligosaccharidoses, sphingolipidoses
Cholesterol biosynthesis	Smith-Lemli-Opitz, desmosterolosis, Conradi-Hünermann
Peroxisomal disorders	Zellweger spectrum (high forehead, large fontanelle, flat face)

📋 Table 104.11: Peculiar Odors in IEM

Odor	Disorder
“Sweaty feet” / acrid	Isovaleric acidemia, glutaric acidemia type II
Maple syrup / burnt sugar	Maple syrup urine disease (MSUD)
Mousey / musty	Phenylketonuria (PKU)
Boiled cabbage / rancid butter	Tyrosinemia type I, hypermethioninemia
Cat urine	Multiple carboxylase deficiency (biotinidase/holocarboxylase synthetase)
Rotten fish	Trimethylaminuria, dimethylglycine dehydrogenase deficiency

🔄 Initial Clinical Approach to a Full-Term Newborn with Suspected IEM (based on Nelson Fig. 104.2)

Step 1: Clinical suspicion — Unexplained sepsis-like picture, lethargy, vomiting, seizures, hypotonia, respiratory distress.

Step 2: First-line STAT labs — Plasma ammonia, glucose, electrolytes, blood gas (pH, HCO3), lactate, urine ketones, and urine reducing substances.

Step 3: Decision tree

🔹 Hyperammonemia + normal pH/alkalosis → Urea cycle defect (OTC, CPS1, ASS, ASL). Treat with IV glucose, ammonia scavengers, arginine, consider dialysis.
🔹 Hyperammonemia + metabolic acidosis + ketosis → Organic acidemia (propionic, methylmalonic, isovaleric). Obtain acylcarnitine/urine organic acids. IV carnitine, biotin, protein restriction.
🔹 Hypoglycemia + hypoketotic → Fatty acid oxidation disorder (MCAD, VLCAD). Acylcarnitine, avoid fasting, IV glucose.
🔹 Hypoglycemia + ketotic + hepatomegaly → Glycogen storage disease or gluconeogenesis defect.
🔹 Metabolic acidosis + normal ammonia + elevated lactate → Pyruvate metabolism disorder, mitochondrial RC defect, or biotinidase deficiency.

Step 4: Advanced testing — Plasma amino acids, urine organic acids, acylcarnitine profile, enzyme assays, and/or genetic testing (exome/panel).

Step 5: Empiric management while awaiting results: IV dextrose (10% at 1.5× maintenance), stop protein for 24h, IV carnitine (50–100 mg/kg), hemodialysis if NH3 > 500 μmol/L.

⚠️ Always rule out sepsis and acquired causes (e.g., valproate toxicity, liver failure).

🚩 Clinical Findings That Should Prompt a Metabolic Workup (Table 104.12)

System	Red Flags
Family history	Unexplained sibling death, consanguinity, ethnic predisposition (e.g., Amish MSUD, French Canadian tyrosinemia), recurrent miscarriages
CNS / Neuromuscular	Progressive encephalopathy, developmental regression, intractable seizures, hypotonia → spasticity, dystonia, stroke-like episodes, ataxia, autism regression
Gastrointestinal / Hepatic	Hepatomegaly, Reye-like syndrome, cholestasis, cirrhosis, recurrent vomiting, acute pancreatitis, failure to thrive
Cardiac	Hypertrophic or dilated cardiomyopathy, arrhythmia, unexplained cardiac failure
Renal	Fanconi syndrome, nephrolithiasis, renal tubular acidosis, unexplained renal failure
Ophthalmologic	Cherry-red spot (Tay-Sachs, Niemann-Pick, GM1), cataracts (galactosemia, CDG), corneal clouding (MPS, cystinosis), lens dislocation (homocystinuria)
Dermatologic	Angiokeratomas (Fabry, fucosidosis), alopecia (biotinidase), ichthyosis (multiple sulfatase deficiency, Refsum), abnormal hair (Menkes kinky hair)
Hematologic	Unexplained cytopenias, pancytopenia, hemophagocytosis, megaloblastic anemia (cobalamin disorders)

📊 Laboratory Findings That Should Prompt Metabolic Workup (Table 104.13 & text)

Abnormality	Possible IEM
Hyperammonemia	Urea cycle defect, organic acidemia, fatty acid oxidation defect, transient hyperammonemia
Hypoglycemia (especially hypoketotic)	Fatty acid oxidation, hyperinsulinism, GSD I, gluconeogenesis defect
Metabolic acidosis (high anion gap)	Organic acidemia, lactic acidosis (mitochondrial, pyruvate defects), ketosis
Lactic acidosis	Mitochondrial RC defects, pyruvate dehydrogenase, pyruvate carboxylase, GSD I, biotinidase
Liver dysfunction + hypoglycemia	Galactosemia, tyrosinemia type I, hereditary fructose intolerance, GSD
Pancytopenia + acidosis	Propionic / methylmalonic acidemia, Pearson syndrome

📌 Additional Pearls: Any neonate with recurrent vomiting, lethargy, seizures and negative sepsis workup — obtain ammonia, lactate, urine ketones. OTC deficiency may present with respiratory alkalosis without acidosis. Premature infants: transient hyperammonemia can mimic UCD; treat aggressively but prognosis good.

⚡ Reflex Prompts: Physical Examination Clues to IEM (Table 104.10 style)

📖 Additional Key Physical Findings (Nelson Table 104.10 selected)

Finding	Disorders to Consider
Macrocephaly	Glutaric aciduria type I, Canavan disease, Alexander disease, Tay-Sachs
Microcephaly	Mitochondrial RC defects, serine synthesis defects, cblC deficiency, Smith-Lemli-Opitz
Coarse facial features	Mucopolysaccharidoses, oligosaccharidoses, GM1 gangliosidosis, I-cell disease
Cherry-red spot	Tay-Sachs, Sandhoff, Niemann-Pick type A, GM1 gangliosidosis, sialidosis
Cataracts	Galactosemia, CDG, Wilson disease, Zellweger, Lowe syndrome
Corneal clouding	MPS I, IV, VI, cystinosis, mucolipidoses, tyrosinemia type II
Hepatosplenomegaly	Gaucher, Niemann-Pick, MPS, Wolman, CDG, glycolipidoses
Steely/kinky hair	Menkes disease
Trichorrhexis nodosa (brittle hair)	Argininosuccinic aciduria, biotinidase deficiency
Alopecia + rash	Multiple carboxylase deficiency (biotinidase / holocarboxylase synthetase)