A different kind of baby blues…
Dr Agatha Okah; Dr Priyesh Parekh; Dr Hema Kannappan
We’ve all heard of haemoglobin and know what it does… but does the word methaemoglobin ring a bell? In this article we are going to refresh your memory and remind you (or perhaps teach your for the first time!) all about methaemoglobinaemia.
Case Study
Imagine you are the paeds reg on a busy weekend shift in a district general hospital. You receive a fast bleep to the emergency department for a 6 week old baby. This baby came with a long history of watery, non-bloody diarrhoea, vomiting and poor weight gain.
He is a term baby, taking 120-150ml/kg/day of formula and EBM, and had an otherwise normal antenatal period. During triage he suddenly became floppy and dusky in colour.
When you examine the baby, he is maintaining his own airway, with no increased work of breathing, but his SPO2 is 64%. He is pale, with grayish discolouration of the skin, perioral cyanosis, HR of 167 and a CRT of 4 seconds. He is lethargic, has a normal BM, and has nothing else significant on examination.
Let’s take a second and think about your first steps – the title of this article may be giving you a bit of bias as to the end diagnosis but we should make sure we cover all bases! Your main differentials are going to include cyanotic heart disease or sepsis, but we all know small babies can present with weird and wonderful things!
The nurses have already given oxygen via face mask and cardiac monitoring is on. You get a cannula in (first attempt, kudos to your skills under pressure) and complete a septic screen, and also take some bloods for a gas, ammonia, clotting, etc. You give a fluid bolus of 10ml/kg and start IV antibiotics.
Your blood gas comes back:
With oxygen, the baby’s SPO2 only improved to 85%. CXR and ECG were normal, and your consultant has come in to support. The gas was confusing as the metabolic acidosis didn’t really seem to fit with the other parameters, so your consultant decides to call the local transport/PICU team for advice on the need for further respiratory support or cardiac review, as there is no out of hours echocardiography in your hospital. You ask your consultant what the raised MetHb means, but they aren’t sure.
A quick google (of appropriate sources!) shows:
What is methaemoglobin?
Methaemoglobin is formed when the iron in haemoglobin is oxidised to a different state (Fe3+ rather than Fe2+ if you’re interested in the chemistry!). This results in irreversible binding of oxygen and a functional anaemia, with hypoxia not responsive to oxygen therapy.
| MetHb | associated symptoms |
| <10% | Symptoms unlikely |
| MiLD (10-30%) | Cyanosis, fatigue, dizziness, headache |
| Moderate (30-50%) | Tachypnoea, tachycardia, confusion, chest pain, dyspnoea |
| Severe (50-70%) | Respiratory depression, Coma, seizures, cardiac arrhythmias |
| >70% | Potentially fatal |
The PICU consultant receives the SBAR and agrees that cardio review is needed, along with further respiratory support. Before they hang up the call, your consultant asks them about the high metHb. After a quick discussion they realise this might be the cause of all the trouble!
The PICU consultant asks for an infusion of methylene blue.
The baby was treated with IV methylene blue over a period of 15 minutes. Following this a rapid improvement was seen, with the MetHb level falling to 3.3% within 3 hours, and SpO2 improving to 100%. He remained clinically stable and was discharged few days later.
It was found that his methaemoglobinaemia was precipitated by severe dehydration following protracted diarrhoea. He was managed as CMPA and followed up under the gastroenterology team.
…So what is methaemaglobinaemia?
Methaemoglobinaemia is rare, characterised by increased percentage of MetHb in circulation. This results in a leftward shift of the oxygen dissociation curve leading to impaired oxygen delivery.
Methaemoglobinaemia can be either congenital or acquired:
Congenital
- Cytochrome B5 reductase deficiency
- Pyruvate Kinase deficiency
- G6PD
Acquired
- Drugs
- GTN
- Dapsone
- Sulphonamides
- Environmental
- Nitrates
Neonates and infants are actually at a higher risk of developing methaemoglobinaemia as they have lower activity of cytochrome B5 reductase. There have been several reports of an association between raised MetHb and cow’s milk protein allergy (CMPA)!
Key Learning
- Any patient who presents with cyanosis or low SpO2 out of keeping with their clinical state, and which does not improve with supplemental oxygen, think methaemoglobinaemia!
- When reviewing blood gases always remember to check methaemoglobin if the patient is cyanosed.
- Methaemoglobinaemia can only be diagnosed via blood gas!
- Oxygen therapy will only improve oxygen saturations marginally until treatment with methylene blue
Authors
Dr Agatha Okah ST4, University Hospital Coventry and Warwickshire; Dr Priyesh Parekh GP, Queensbridge Group Practice; Dr Hema Kannappan, Paediatric consultant with gastroenterology SPIN, University Hospital Coventry and Warwickshire