Coronavirus: what it does to your lungs

What actually happens to people infected with the coronavirus? The vast majority develop two main symptoms: a dry cough and a fever. Most also report fatigue and muscle aches and pains, and data released in just the last few days suggests that a lot of patients also develop gastrointestinal symptoms, including diarrhoea. The biggest impact though does appear to be on the respiratory system. So what is the virus doing to produce these effects? Phil Sansom asked lung pathologist Sanjay Mukhopadhyay to explain…

Sanjay - In the most asymptomatic - so people who don't have any symptoms - the best hypothesis would be that these viral particles just sort of stay there and evoke minimal reaction from the tissue. As you get to more and more symptoms, it would be plausible that the tissue reaction actually involves inflammation; that means the immune cells come in to destroy the virus, and that the interplay between the virus and the immune cells causes further destruction. And then the most severe people it seems are developing a very severe form of lung damage, that we call as pathologists diffuse alveolar damage...

Phil - Diffuse alveolar damage?

Sanjay - That's correct. And diffuse alveolar damage is essentially what happens if you get any kind of very severe injury to the lung, regardless of the stimulus. Conceptually, diffuse alveolar damage as seen by a pathologist is sort of the equivalent of seeing a building strewn all over the ground in a damaged way. You see the end result of what happened; you'd don't necessarily see what caused it.

Phil - What does it look like then?

Sanjay - What happens is: the alveoli - which is the air sacs, right, the very last portion of your lung where the gas exchange goes on - they're like little balloons. Every time you inhale, those balloons get bigger, and when you exhale, those balloons get smaller. Oxygen is supposed to traverse the wall of those balloons and then go inside the blood vessels that are in the substance of the balloon. The thickness of the balloon therefore has to be extremely thin so that gas exchange can occur. When the damage occurs you get a lining of sort of debris that lines the inner side of those alveoli or air sacs. It looks pink under the microscope. And as time goes on, the walls of the alveoli tend to get really, really thick. And you can imagine if the thin alveoli are necessary for breathing, that thick alveoli would be very bad for breathing.

Phil - Do you have to go into someone's lungs and take a look at the tissue? Is there an easier way to tell that they've got this kind of damage?

Sanjay - Almost nobody, or very few people, actually come to the stage where doctors actually need to send a piece of tissue off to a pathologist to look at under a microscope. And so most of the patients who are sick and come to the hospital get a chest X-ray or a CT scan, and on those, normal lungs look black because they're filled with air. Now when you get an infection, you start to get what are known as opacities in the lungs. And that means that the lung, instead of being black, starts to look white.

Phil - So white long on the scan equals probably this kind of damage?

Sanjay - Yes, that's correct. And the more the white, the worse it is.

Phil - So you have these two things then that you talked about: the war between the virus and the immune system, and then also this diffuse alveolar damage. How do those things connect to the symptoms that the coronavirus causes?

Sanjay - I'm not sure there's a complete one-to-one correspondence between the symptoms and what's going on, but it is plausible at least that as the virus goes down from the back of the throat down into the windpipe, that damage - or the back and forth damage - actually causes irritation to those cells, and that to some extent accounts for the cough that's going on. It is also possible that the damage in the air sacs and all that debris that that's producing is also producing some degree of cough. So both the action in the airways as well as the action in the air sacs all could be leading to the cough that's going on in these patients.

Phil - And how does what you're seeing in these people's lungs compare to something like the flu?

Sanjay - Overall the immune response and the kind of damage that's occurring in the lungs is likely to be very similar to the flu, and to other similar viruses that we've seen in the past. Now I will add a caveat that we are in very early days, but from the very early reports it seems like it is almost identical to what we have known with other viruses.

Phil - But they have different symptoms?

Sanjay - In terms of just the standard symptoms of cough, shortness of breath, and fever, those are not very significantly different between SARS coronavirus and other severe viruses.

Phil - The people who have this diffuse alveolar damage - I assume the worst-off are those people that we see needing ventilators because they have trouble getting enough oxygen - is that damage to their lungs permanent?

Sanjay - Depends. In general, people with diffuse alveolar damage develop a syndrome that's called acute respiratory distress syndrome. In about half of these patients the outcome is fatal. So it's a very severe and irreversible form of disease in about half or 50% of patients. In the other half of patients, there's sort of a mixed outcome. Some of those patients can recover completely, but there are others who go on to develop scarring in the lungs or what we call fibrosis. That can cause problems in terms of breathing and cough in the long term.

Phil - And for the people who don't have this diffuse alveolar damage, who only have a very mild case of the infection, will they come out with any sort of issues with their lungs?

Sanjay - They shouldn't! We have been focusing on the very, very severe end of this spectrum, right? But we must remember that the majority of patients actually don't get to that point. The vast majority of patients either have no symptoms, or they have symptoms that are relatively mild - so cough and shortness of breath - but nowhere near developing the acute respiratory distress syndrome. It seems like many of those patients recover completely and then can go back home.