The second most important laboratory feature of many inherited
neurometabolic disorders during episodes of illness is metabolic
acidosis that is demonstrated by measuring arterial blood gases and
bicarbonate. An increased anion gap (>16) is observed in many of
these disorders due to the accumulation of fixed acids such lactic
acid, ketoacids and other organic acids
[Figure 3].
Diagnostically, identifying the unmeasured anion is the most important
requirement in patients presenting with metabolic acidosis and
increased anion gap. For this, lactate, 3-hydroxybutyrate, acetoacetate
and organic acids are analyzed. The largest group consisting of organic
acidemias, including entities such as methylmalonic, propionic and
isovaleric acidemias are easily recognized by their typical organic
acid profiles. In addition to specific organic acid intermediates,
plasma lactate is often elevated in organic acidemias as a result of
secondary interference with CoA metabolism. A flowchart for the
evaluation of patients with metabolic acidosis with increased anion gap
[5] is presented in
[Figure 4].
Metabolic
acidosis with a normal anion gap is noted in renal tubular acidosis or
diarrhea. A history of diarrhea is usually sufficient to distinguish
hyperchloremic metabolic acidosis due to excessive gastrointestinal
bicarbonate losses from that arising from renal tubular dysfunction.
Metabolic disorders associated with renal tubular acidosis include
hereditary fructose intolerance, hepatorenal tyrosinemia, cystinosis,
Fanconi-Bickel syndrome, Lowe syndrome, vitamin D dependency and
congenital lactic acidosis due to cytochrome c oxidase deficiency.
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