Metabolic Babies in the ED.. Easy As 1.. 2.. 3

Author: Edward Snelson / Editor: Liz Herrievan / Reviewer: Iain Marshall / Codes: NeoC2, NeoC4, NeoC5, SLO5 / Published: 18/05/2016 / Reviewed: 10/09/2024

Inborn errors of metabolism (IEMs) are disorders of chemical pathways. These diseases lead to dangerous deficiencies or excesses of chemicals in the body. While each individual IEM is rare, together they are much more common, and it is reasonably likely you will have to look after a child with one. Their collective incidence is reported between 1 in 800 to 1 in 2500 births. When it comes to metabolic problems in small children, it is easy to assume that this is a specialist area. It may be scary but it is so simple that I guarantee that nothing in this post is complicated or difficult to remember. In fact, there are really only three things to know about children’s inborn errors of metabolism. That’s right, just three. Let’s start with the hardest one in practice:

Think about the possibility of a metabolic problem

When a baby (In this context, that’s any child that stays within 1 metre of where you left them for five minutes) presents with vague symptoms, the emergency physician should consider five broad categories of causes to the problem. These can all present with similar non-specific symptoms such as reduced feeds, being miserable, vomiting or simply the parents being concerned. Of course, infection is always the most common, but you need to proactively consider them all.

The best way to avoid missing a metabolic problem is to look for any clues of the ‘big five’ listed above. If there is no clear explanation for the child being “off”, don’t just think metabolic. The other causes (don’t forget that injury includes NAI) are just as important and need to be actively considered. But we’re here to talk about metabolic, so what clues point us towards a metabolic cause?

Do extra tests

• THINK AMMONIA
• Blood sugar (+ hyposcreen if low)
• Blood gas
• Electrolytes

A capillary or venous blood sugar level is a routine, (and I do not use that word lightly), test in any baby who is ‘just not quite right’, but for non-specific presentations there are a few more tests that will increase the likelihood of you detecting a metabolic problem early.

The simplest of these is a blood gas: particularly for the lactate, pH and the anion gap. Septic or otherwise ill babies may have a moderately raised lactate, but it can be a clue this is metabolic. Sometimes the lactate is disproportionally high and may even be in double figures, which is a much bigger clue! Similarly, unexpectedly high or low pH results should prompt further consideration of a metabolic problem. Pay particular attention to the electrolytes as these can be quite deranged, which not only may need urgent management but can offer clues towards a metabolic problem you may be facing. I promised not to be too complicated, so I won’t even try and explain the anion gap here, but this DFTB post does a great job.

Ammonia is one of the most important tests you have to do. Hyperammonaemia can occur in a number of metabolic conditions and is extremely dangerous. Ammonia is neurotoxic and the risk of permanent neurological damage and death is directly related to the height and duration of the peak ammonia level. Practically, this goes in different blood bottles in different hospitals so find out which one you need, or more simply take one of each and find out after. The important thing is, this needs to get to the lab as soon as possible. Ideally the sample is cooled on ice as soon as it is taken, but time is more important so don’t wait for the ice!

Save a life

• Stop Feeds
• Give Sugar (IV Dextrose to stop catabolism)
• Rehydrate
• (+- Give ammonia Scavengers)

Early diagnosis and intervention are very important in the long-term outcomes for children with inborn errors of metabolism. How do you do that when you don’t know what the problem is? When a baby presents with vomiting, a lactate of 12, a sodium of 124, a pH of 7.67, dysmorphic features and purple urine, do you know what the diagnosis is? No? Neither do I and I just made the whole thing up. One of the fallacies about metabolic problems in babies is that you have to have learned and understood metabolic medicine. You don’t need to know what the problem is to make a big difference. The diagnosis can wait. There are just three things that are going to help in the first stages.

First, stop the feeds. Most metabolic pathways that go wrong are trying to process complex carbohydrates, fats and proteins. Confiscate these immediately if you want to make sure the condition does not worsen.

Second, give sugar. Giving a dose of IV Dextrose with the aim to stop catabolism. This can and should be given even if the sugar tested is normal. You will then need to give 10% dextrose within the maintenance fluids, keeping the dextrose load high. If you then have high blood sugars you may need to start insulin, although this is something to ask the metabolic team. You have phoned the paediatric metabolic consultant at this point haven’t you? Phone them early, or delegate this to someone, I guarantee they’ll be interested.

Third, rehydrate. Give a 10ml/kg bolus and consider if they may need more. Diluting a toxin is generally a good thing and the baby is almost certainly dry. Then continue normal maintenance fluids as for any other baby, although you may need to be more careful with the electrolytes.

The bonus step to management is starting ammonia scavengers or ‘metabolic medications’ as you may know them. Typically, this is sodium benzoate and sodium phenylbutyrate, but you don’t need to remember them. What you do need to remember is where to find this out and where you actually find them. BIMDG have fantastic, easy to use, resources for all metabolic presentations, known and unknown. Then you have to find these medicines! Many departments have a ‘metabolic drug box’, but others may need to get this from somewhere else, so make sure someone is tasked with finding these.

One dilemma I’ve had before is knowing what the ammonia result means. It’s not a test we use very often so it’s difficult to recognise the normal ranges. Also, some unwell neonates have a high ammonia even if they don’t have a metabolic condition. Guidelines do differ a little on treatment ranges, but a safe and consistent approach would be:

• Ammonia > 100 is abnormal and we should repeat urgently
• Ammonia > 200 more likely metabolic. Repeat urgently, start getting the scavenger infusions ready and call metabolic consultant urgently.

Ok, now that’s all sorted, where are you going to send your patient? Many of these patients will be managed by tertiary metabolic teams, although often this can initially be by remote advice. The exception is a child with hyperammonaemia unresponsive to treatment. These are time critical transfers to a paediatric intensive care where they can get urgent haemofiltration / haemodialysis. Time is brain for these babies.

Hopefully your local transport team have a guideline, but if not here is a good example from NWTS.

Children with known metabolic conditions

Now that you are confident with managing acute undifferentiated metabolic babies, what happens when an older baby or child with a known metabolic condition presents in the same way? Tricky if you’ve never heard of this before. Some of these families have lived with these conditions for many years and will likely know more about them than you. This can be incredibly challenging, especially with the extra pressure that comes from these families knowing this is urgent. I like to break it down into simple steps:

• Do the same emergency management as above. Stop feeds. Give Sugar. Rehydrate.
• The family almost always have a personalised management plan with them. Ask for it. Use it.
• If they don’t have one, or have forgotten it, then you can use the generic management plans on the BMIDG website. It’s the same place you found the emergency plans needed earlier, but you can also find emergency plans for a whole variety of specific metabolic conditions.

Tell me more about metabolic conditions

It wouldn’t be an article on inborn errors of metabolism without an incredibly complicated flowchart. Below is an excellent diagram from The Children’s Mitochondrial Disease Network, which is one of the simplest summaries available, although still daunting to any mortal physician. The best way to look at it is that when a metabolic pathway goes wrong we end up with too much of some things and too little of others.

I promised this wouldn’t be complicated however, so I won’t go into any further detail, however I would recommend a brilliant DFTB module on metabolic disorders which takes this further and goes through some great basics and clinical cases, including an interesting section on the smells associated with different metabolic conditions! So, if you want to know what it means if your patient smells like a swimming pool head over here.

Acknowledgements: Article edited from original by Dr. Edward Snelson. (who credited help from Dr. Judith Gilchrist and Dr. Chris Connolly). I tried to keep as much of the original spirit running through.

Further Reading

1. British Inherited Metabolic Diseases Group (BIMDG)
2. The Children’s Mitochondrial Disease Network
3. Simplified Pediatric Metabolic Emergencies. EM:Rap, 2010.
4. Long B. Inborn Errors of Metabolism. emDocs, 2015.
5. Taciane Alegra. Inherited Metabolic Disorders Module, Don’t Forget the Bubbles, 2020.
6. Guidelines for the Management of Neonatal and Paediatric Hyperammonaemia. North West & North Wales Paediatric Critical Care Network, NHS.