Paediatrics: Metabolic syndromes: metabolic acidosis

Metabolic syndromes: metabolic acidosis

The emergency care of acid–base problems.

Metabolic acidosis may occur as a result of:

•   Abnormal loss of bicarbonate.

•   Abnormal accumulation of hydrogen ions in association with a non-volatile organic anion.

These two states can be differentiated by calculating the anion gap (i.e. the difference between plasma [Na⁺] and the sum of plasma [Cl^–] and [HCO₃^– ]). The normal anion gap is 10–15mmol/L.

Abnormal bicarbonate loss

When metabolic acidosis is due to bicarbonate loss from either the gut or kidney:

•   The anion gap is normal.

•   Hyperchloraemia is usually present.

To distinguish bicarbonate loss from the gut or from the kidney:

•   A history of diarrhoea will distinguish between hyperchloraemia due to GI losses from that due to renal tubular losses.

•   Urine net charge (UNC), calculated as [Na⁺+ K⁺] – [Cl^–], is used to estimate urine ammonium (NH4⁺) when there is no accumulated organic acid. A negative UNC implies the presence of adequate or increased urinary ammonium; therefore the acidosis results from abnormal GI loss of bicarbonate. (Note: Urine ammonium is low in renal tubular disorders.)

IMDs associated with RTA include—galactosaemia; hereditary fructose intolerance; hepatorenal tyrosinaemia; cystinosis; glycogen storage disease I; Fanconi–Bickel syndrome; congenital lactic acidosis; Wilson disease; vita-min D dependency; osteopetrosis with RTA; Lowe syndrome.

Accumulation of organic anion

When metabolic acidosis is due to accumulated organic anion:

•   It is associated with failure to thrive.

•   Tachypnoea may be present.

•   s hypoglycaemia leads to a neurological syndrome.

•   Organic anion may lead to distinct smell of sweat or urine.

•   The anion gap is raised.

The causes include the following:

•   Lactic acidosis: pyruvate accumulation (e.g. pyruvate dehydrogenase deficiency, pyruvate carboxylase deficiency, multiple carboxylase deficiency); NADH accumulation (e.g. defect of mitochondrial electron chain).

•   Ketoacidosis: s to IMD (e.g. maple syrup urine disease, organic acido-pathies, glycogen storage disease, disorders of gluconeogenesis; rare p disorders of ketone utilization, e.g. B-ketothiolase deficiency, succinyl-CoA: 3-ketoacid transferase deficiency.

Organic aciduria: a large spectrum of disorders.