Investigation of a Baby Presenting with Metabolic Acidosis
Before launching into a comprehensive investigation procedure it is necessary to establish that there has been firm evidence of metabolic acidosis i.e., that this was not merely a supposition based on low plasma bicarbonate measurements or an observed hyperventilation. Thereafter, the following clinical information is needed:
- what was the baby’s gestational age?
- what is the baby’s postnatal age?
- does the baby have respiratory distress?
- was there any evidence of birth asphyxia or traumatic birth?
- how long has the baby been acidotic?
- is the onset or severity of acidosis related to feeding?
- are the baby’s parents consanguineous?
- is there a family history of acidosis or neonatal death?
The answers to these questions should influence the way in which an investigation is conducted. A baby presenting with sudden acidosis at several months of age with no prior history is much more likely to have an inborn error of metabolism than a sick, premature baby.
One should bear in mind the old clinical adage that ‘common things occur commonly’. That is to say that for most acidotic babies there will be a straightforward aetiology. However one must be careful not to miss a rare diagnosis being masked by an apparently obvious one. For example, septicaemia is a common complication which can easily cause diagnostic confusion for babies with galactosaemia.
All diagnostic possibilities should be considered until a firm diagnosis has been reached. The presenting features of metabolic diseases are not always consistent between patients. For example, the acute organic acidaemias may not necessarily be associated with marked hyperammonaemia, although this is usually the case. It is vital that samples are collected at the time that the baby is symptomatic.
Finally, the complications of acidosis should not be forgotten in the midst of a search for a cause. Hyperkalaemia is a particular danger and unfortunately is often dismissed as factitious when observed in a baby.
The following protocol indicates the important investigations which should lead to a narrower differential diagnosis.