Lactic Acidosis and Developmental Delay in an Infant
A 7-month-old boy is admitted with lethargy and rapid breathing after a mild febrile illness. He has had poor feeding and hypotonia since birth, and his mother reports that he has never rolled over. He is the first child of first cousins. On examination he is tachycardic, with Kussmaul breathing, no organomegaly. Arterial blood gas shows pH 7.28, bicarbonate 12 mmol/L, anion gap 22 mmol/L; lactate 7.2 mmol/L and pyruvate 0.72 mmol/L; ammonia is normal; glucose 3.2 mmol/L. Urine organic acids show no methylmalonic acid or propionic acid. Brain MRI shows symmetric lesions in the basal ganglia. The metabolic team is considering a ketogenic diet while awaiting confirmatory testing. The parents ask whether the diet could harm him if the diagnosis is different.
1. Calculate the lactate-to-pyruvate ratio and use it to distinguish pyruvate dehydrogenase deficiency from a respiratory chain defect. Which diagnosis does this ratio favor?
Read the worked answer
Lactate-to-pyruvate ratio = 7.2 ÷ 0.72 = 10. A normal ratio (usually less than 20) with high lactate and high pyruvate points to pyruvate dehydrogenase deficiency rather than a respiratory chain defect, where NADH/NAD+ rises and the ratio is typically greater than 20-25. The normal ammonia and absence of methylmalonic or propionic acid make organic acidemias less likely. The basal ganglia lesions fit energy failure. However, the ratio is a guide, not a diagnosis; fasting, sample handling, and sepsis can alter it. Confirm with enzyme assay or genetic testing.
2. Explain the biochemical rationale for a ketogenic diet in this disorder and one situation in which it could be harmful.
Read the worked answer
PDH converts pyruvate to acetyl-CoA. In PDH deficiency, pyruvate cannot enter the TCA cycle and is shunted to lactate. A ketogenic diet provides acetyl-CoA and ketones, bypassing PDH and reducing lactate production. It can be harmful if the true diagnosis is a fatty acid oxidation defect, because fasting and high fat intake can trigger hypoglycemia and metabolic crisis; it may also be risky in some respiratory chain disorders. Therefore the diet should be started only with metabolic supervision and after excluding other diagnoses.
3. What confirmatory test would you order, and what counseling points follow from the parents being first cousins?
Read the worked answer
Order PDH enzyme activity in cultured skin fibroblasts and molecular testing of PDH genes, especially PDHA1, which is X-linked, and other nuclear genes. Because the parents are first cousins, autosomal recessive disease is more likely, but X-linked PDHA1 can also occur. If autosomal recessive, recurrence risk is 25% for each pregnancy; if X-linked, risk differs by sex. Counsel about the son's risk, prenatal diagnosis, and the uncertainty before results. Consanguinity increases the chance that both parents carry the same recessive variant.