Metabolic acidosis results from the addition of acids or the removal of alkali. The differential diagnosis in pediatric patients is similar to that in adults, with a few notable exceptions. As in adults, it is useful to distinguish between metabolic acidosis with an elevated anion gap from that in which the anion gap is normal.
In infants and children, the most common cause of metabolic acidosis with a normal anion gap is diarrhea. Diarrheal fluid is rich in bicarbonate and low in chloride, and acidosis occurs when bicarbonate is lost in stool. Laboratory values will reveal a hyperchloremic acidosis. Acidosis resolves with the correction of dehydration.
Renal tubular acidosis (RTA) is the other major cause of non-anion gap acidosis in children. RTA constitutes a group of tubular transport disorders. In type 1, or distal RTA, there is impaired hydrogen secretion in the distal tubule. Inability to secrete hydrogen usually results in a urinary pH greater than 6. In type 2, or proximal RTA, there is inability to resorb bicarbonate at the proximal tubule. Accompanying hypokalemia is common and can be severe. In both types, affected infants commonly present with failure to thrive. They can acutely decompensate during diarrheal illness, when bicarbonate loss in the stool can exacerbate acidosis. Treatment of RTA usually involves the administration of supplemental bicarbonate, which improves acidosis and enables relatively normal growth and development.
The differential diagnosis of metabolic acidosis with an elevated anion gap is substantially the same in children as in adults and implies the presence of an endogenously created or exogenously ingested acid (Table 1.2.8-3). The one notable exception is the possibility of an inborn error of metabolism (IEM). Of the IEMs that cause metabolic acidosis, the most common are methylmalonic, propionic, and isovaleric acidemia. The typical emergency department presentation is a neonate with vomiting, lethargy, and failure to thrive. It is extremely difficult to distinguish these infants from those with more common disorders, especially sepsis. Helpful ancillary studies include evaluation of serum lactate, pyruvate, and ammonia levels. Management requires consultation with a pediatric endocrinologist or geneticist.
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