It has puzzled me over the years why I seem to have no reserve when it comes to lung function – I could never run for shortness of breath at school, even more so now I am much older, and my peak flow is always pathetic, achieving under 300 on a good day. Of course it is most likely I am just overweight and unfit, or at a stretch have a degree of asthma. However, a case report published this week caught my attention – a patient with ADPKD who also had cysts in her lungs.
It was a poster presentation entitled “Cystic lung disease in adult polycystic kidney disease” by clinicians from Detroit. The conference was the American Thoracic Society International Conference with the abstract published in Respiratory and Critical Care Medicine:
There have been two previous case reports of cysts appearing in the lungs in patients with ADPKD, both presenting acutely with a pneumothorax. The patient described in the poster was not acutely unwell, but was undergoing assessment for transplant. The only history given was that she had never smoked and had no respiratory symptoms.
Her lung function tests were a little lower than one might expect with reduced vital capacity (working lung volume) and forced expiratory volume (the volume you can breathe out with effort in one second) but her oxygen saturation was normal and her carbon monoxide diffusion capacity was normal: both of these measure the efficiency of her lungs. With a normal chest X-Ray it was s surprise finding on the full body CT scan a cystic lesion in her right lung that led to a detailed chest CT scan. This revealed several cysts in her right lung mostly in the right lower lobe and all with thin walls.
The differential diagnosis of cysts in the lung includes congenital disorders and specific lung infections (pneumocystis or echinococcosis) , both of which would present with clinical symptoms. The appearance on the scans was not consistent with infection or interstitial pneumonia. It was therefore concluded that the cysts in her lung were related to her polycystic kidneys.
One of the pathological processes in ADPKD is a problem with the primary cilium inside the cells. Cilia are filamentous elements found in almost every cell in a human body. There are two main sorts – ones that move (motile) found on the surface of cells such as in the lung cells where they help clear mucus and particles from the lungs, and primary cilia which sit inside the cells and have a role in sensing and signalling. In PKD cysts form because the primary cilia malfunction. Lung cysts in diseases such as cystic fibrosis were thought always to relate to dysfunction of the motile cilia. But it also feasible that lung cysts can form because of abnormalities in the primary cilia in lung cells, which would be the likely mechanism in lung cysts associated with PKD.
While a cyst is usually conceived as a roundish structure it is one of the end results of a process that begins with widening of the initial cylindrical structure. In the ADPKD kidney this is the tubules that develop cysts that get sealed off from the original tubule. In lungs cysts can develop after a process of widening of the previously cylindrical airways (bronchioles) and enlargement of the small sacs at the end of the airways (alveoli).
The widening of bronchioles due to damage and inflammation leads to a clinical picture called “bronchiectasis”. Symptoms include excess mucus with coughing to clear it and repeated infections. Signs can be seen on chest x-Ray’s and CT scans.
In 2008 a review of CT scans in 95 ADPKD patients there were signs of bronchiectasis in 37% of them. This compared with just 13% in a control group. While bronchiectasis is not a described manifestation of ADPKD this study would suggest that lung problems are indeed more common in PKD patients.
More recently another group undertook a similar study but in greater depth and they concluded that ADPKD was associated with both symptoms and signs of bronchiectasis, both worse if the patient was a smoker.
So now I have two possible explanations for my diminished lung function – it could be due to mild bronchiectasis or to cysts related to PKD. The most likely reason remains though, I am just fat and lazy!