Q. I wonder if you could help me a bit with an unusual case of pneumonia. Our patient is a 56-year-old male with who woke up one morning with dyspnea, fever and chest pain. He came to the emergency department and was diagnosed with pneumonia, put on antibiotics and left to his merry ways. He seemed to be improving over a week until one day the ambulance brought him in with severe dyspnea. Our respiratory team eventually came to the diagnosis of cryptogenic organizing pneumonia/idiopathic bronchiolitis obliterans organizing pneumonia. I’ve tried reading about this but I still don’t quite understand what an “organizing pneumonia” is. Maybe I’m overthinking it and you might be able to shed some light on this.

A. The disease your patient has – bronchiolitis obliterans organizing pneumonia (or BOOP, as it’s more commonly called) – is an uncommon inflammatory disease of the bronchioles and surrounding tissue. It’s sometimes called cryptogenic organizing pneumonia in order to avoid the term bronchiolitis obliterans.

“Bronchiolitis obliterans” is kind of confusing. It’s a term which pathologists use to describe plugging of bronchioles (a fairly nonspecific finding), but which pulmonologists use to imply a chronic, irreversible scarring process resulting in obliteration of small airways (a specific disease often occurring in lung allografts). “Organizing,” by the way, is another fairly non-specific term (meaning that it can be used in several different diseases) used to describe the fibrosis that can develop if a pneumonia persists for a long time.

Back to BOOP. BOOP is a pneumonia, but not an infectious one. By xray, it looks a lot like a severe infectious pneumonia, with extensive, widespread involvement of both lungs. Your patient presents in a typical way for this disease: it looked for all the world like he had pneumonia, but after sending him home on antibiotics, he didn’t get better, and presented again with worsening disease.

The main thing you see under the microscope in BOOP is plugs: little masses of connective tissue rich in fibroblasts that fill the bronchioles and extend into alveolar spaces. Check out the image above: there’s a nice connective tissue plug filling an airway at 7 o’clock. In some plugs, there may be inflammatory cells and capillaries present, giving the lesions a granulation-tissue-like appearance. There is very little, if any, interstitial fibrosis present (a feature commonly seen in other lung diseases, such as usual interstitial pneumonitis).

Although BOOP is not technically an infectious disease, it can be triggered by infectious agents. It can also be triggered by toxic exposures, and it’s more common in patients with autoimmune diseases, like lupus. It can also be idiopathic. The treatment usually involves steroids, and more often that not, patients recover following treatment.