Adam Lawton and Abi Whitehouse
The radiologist’s report comes back on that baby with respiratory distress and the unusual-looking x-ray. It reads:
You’ve only got about half an hour until handover – Panic!
With no time to open PubMed, you’ve googled CPAM and this article has popped up on your phone. Prepare to dive into the wonderful world of…..
Wait, what? That isn’t what I’m here for… Is it?
OK, let’s break it down. The first thing to remember (or learn for the first time if you never really understood embryology at medical school) is that during development the embryo forms something called a gut tube. This tube stretches out and folds over itself and from this structure derives the gastrointestinal tract – from the oesophagus all the way to the rectum. It includes important organs such as the liver and pancreas, but far more importantly (sorry gastro nerds) it also produces the entire respiratory system from the trachea down to the alveoli. The gut tube is subdivided into three segments called the foregut, midgut, and hindgut. The foregut is the bit which branches to create the respiratory system, and also, as an encore, the oesophagus, stomach, liver, pancreas, gallbladder and bile ducts, and the upper part of the duodenum.
We derive from this newfound embryological knowledge two important concepts:
- The respiratory and gastrointestinal systems are intimately connected, and aerodigestive tract dysfunction often exists when the two systems fight about whether stomach acid should remain in the stomach or should spill over into the airways. This is not how you would design the respiratory and digestive systems if you could start from scratch
- Abnormalities in early development can cause an array of congenital lung abnormalities which are collectively termed bronchopulmonary foregut malformations.
What types of abnormalities are we talking about?
The main purpose of the lungs is to bring air close to blood so that gas exchange can occur. A developing lung therefore wants to contain air spaces and blood vessels that come very close to each other. Without getting too much into the blood supply to the lungs, it’s worth remembering that the lungs have a dual blood circulation. They receive deoxygenated blood from the pulmonary arteries (this is called the pulmonary circulation), and they receive oxygenated blood from the bronchial arteries which branch from the aorta (this is called the bronchial circulation).
The blood from the pulmonary circulation is used for gas exchange. The blood of the bronchial circulation blood is used to bring oxygen and nutrients to the lung tissue.
So what can go wrong?
- Pulmonary sequestration (a.k.a. accessory lung) – this is an extra lump of lung tissue that does not connect to a bronchus, therefore it can receive no air and doesn’t participate in gas exchange. Fortunately, they happen to receive oxygenated blood from the systemic circulation which means they don’t cause a right-to-left shunt. Incidentally, these two features (not having an air supply and receiving blood from the systemic circulation) are the two main characteristics that help decide whether you are looking at a CPAM or a pulmonary sequestration on a CT scan.
- Foregut duplication cysts – there are several types of these cysts, where abnormal budding of the bronchial tree forms a cyst which isn’t connected to the trachea or other bronchi. This means it doesn’t receive an air supply, and therefore these cysts are filled with fluid – unlike the cysts of a CPAM which are filled with air.
- Congenital Pulmonary Airway Malformation – this is what you came here for!
So do I call it a CPAM or CCAM?
Congenital Pulmonary Airway Malformations (CPAMs) used to be called Congenital Cystic Adenomatous Malformations (CCAMs). We are sure that they are congenital malformations, but since they were first described we now know that most are neither cystic nor adenomatous. Therefore, the term CPAM is now preferred, which is a better name because it describes exactly what it is – a congenital abnormality of the airway.
OK enough preamble – what am I looking at?
You are looking at a failure of normal development of the lung tissue. They are hamartomatous lesions (which means they are benign lumps) that are made up of cystic and adenomatous (this just means derived from epithelial tissue that looks glandular) elements.
They are connected to the tracheobronchial tree, hence the cysts are filled with air. Their blood supply is from the pulmonary circulation (which means they receive deoxygenated blood).
They happen really randomly:
- There are no predictive factors such as sex, ethnicity, or maternal age at delivery.
- They can affect any lobe of the lung and there doesn’t seem to be any area that is more likely to be affected than another.
- There is no apparent genetic predisposition (except for in one type, described below).
On imaging they have a variable appearance. Some types consist of a small number of big cysts, and others have a large number of little cysts.
How are they categorised?
CPAMs are categorised according to the location in the respiratory system from which they originate. The various types of CPAM have different radiological appearances, morbidity and mortality statistics, and associated congenital anomalies – a full discussion of these is beyond the scope of this article. If you’d like to delve deeper, take a look at the review articles listed at the end.
What’s the big deal?
These sections of abnormal lung tissue might cause the following problems:
With the recent explosion in the use of ultrasound imaging for various indications, we are picking up more CPAMs. Some of these patients would have remained completely asymptomatic and never known they had a focal lung abnormality had it not been identified on imaging. Probably only a quarter of affected people have symptoms at birth.
Aside from gas exchange, the other primary function of the lung is immune defence against inhaled pathogens. Any focal area of abnormal lung tissue will have suboptimal defence against pathogens, and can therefore be a focus for infection. Beware the child who presents with recurrent pneumonia – is there an underlying lung abnormality? In these cases, surgical resection may be helpful.
Respiratory failure due to mass effect
Lesions can compress what would otherwise be normal lung tissue, preventing normal development. If enough lung tissue is compromised then there is insufficient tissue available for adequate gas exchange, and the result is respiratory failure.
If a cyst is connected to the tracheobronchial tree, and it bursts, then a connection can be made between the airways and the pleural space. Air will then leak into the pleural space – a pneumothorax has occurred.
Pleuropulmonary blastoma (PPB) is a rare lung cancer. Children with CPAM have a higher incidence of PPB than the general population. PPB is particularly common in type 4 CPAM, with some researchers now suggesting that type 4 CPAM is actually always PPB.
As you might imagine, lesions which occur high in the tracheobronchial tree cause the entire lung to be severely affected and consequently these patients do not survive long after birth due to profoundly impaired gas exchange. Death can also occur in utero as a consequence of fetal hydrops.
What do we do about them?
Once a congenital abnormality is discovered, careful examination should be made to see if any other congenital abnormalities are present. In some cases, CPAM progresses to hydrops in utero, due to progressive compression of vascular structures, which is generally fatal without treatment (early delivery, steroids, and fetal surgery have all been described). In other cases, the CPAM will ‘involute’ – or spontaneously resolve on subsequent prenatal ultrasound scans. However, these lesions still require postnatal follow up as the US scans become less sensitive as the pregnancy progresses and these lesions are sometimes still present on postnatal imaging.
Asymptomatic cases receive serial imaging to ensure that they do not progress to symptomatic cases. This includes chest x-ray shortly after birth and then CT imaging in the first few months of life.
Symptomatic cases (respiratory distress, hypoxia) or cases thought to be a high risk of developing pleuropulmonary blastoma undergo surgical resection of the lesion.
So what do I say in handover?
This depends on how hard you want to flex on your colleagues. You can either admit to having read this article or pretend that you remember all about CPAM from your MRCPCH revision. Either way, conclude with the following phrase:
I think we need to discuss this patient with our local paediatric respiratory service, as they will require follow-up and possibly surgery.
There is much more to be said about the different types of CPAM, but they’re beyond the scope of this casual introduction to the wonderful world of niche respiratory pathology. If you’d like to learn more, or if you are desperate for further sources because the boss has asked you to prepare a short presentation for tomorrow, check out the review articles below. All of them are open-access and should therefore be freely accessible at work or home without a log-in.
- Mehta PA, Sharma G. Congenital Pulmonary Airway Malformation. [Updated 2020 Aug 10]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan https://www.ncbi.nlm.nih.gov/books/NBK551664/
- Disu EA, Kehinde OA, Anga AL, et al. Congenital pulmonary airway malformation: A case report of a rare cause of neonatal respiratory distress and review of the literature. Niger J Clin Pract. 2019;22(11):1621-1625.
- David M, Lamas-Pinheiro R, Henriques-Coelho T. Prenatal and Postnatal Management of Congenital Pulmonary Airway Malformation. Neonatology. 2016;110(2):101-115.