Approach to interstitial lung disease (diffuse parenchymal lung disease): Clinical sciences

Interstitial lung disease, or ILD, is a group of lung disorders that cause inflammation and scarring of the lung parenchyma.

Based on the underlying cause, ILDs can be subdivided into four main categories: exposure-related ILDs, caused by inhaling harmful substances like asbestos;

iatrogenic ILDs, which occur as a result of side effects from certain medications or radiation therapy;

ILDs related to systemic diseases, seen in conditions like sarcoidosis; and finally, idiopathic ILDs, where the causes are unknown, like idiopathic pulmonary fibrosis.

Okay let's begin our assessment of a patient presenting with a chief concern suggesting ILD. First obtain a focused history and physical examination, as well as pulmonary function tests, or PFTs, and chest X-ray.

Patients typically report shortness of breath on exertion, and chronic, dry cough.

The physical exam might reveal wheezing, and diffuse inspiratory crackles. There can also possibly be finger clubbing, cyanosis, and low oxygen saturation.

You might also see signs of extrapulmonary systemic disease like polyarthritis or thickened skin, but that will depend on the cause of ILD.

Here’s a clinical pearl! In ILD, patients often present with a normal oxygen saturation when they are at rest, which is about 95-100%, but it drops during physical activity as their body's oxygen demand rises. So, if a patient initially shows normal oxygen saturation, test for exercise-induced hypoxemia by checking pulse oximetry while they ambulate.

Next, PFTs typically reveal reduced diffusing capacity of the lungs for carbon monoxide and might show a restrictive pattern on spirometry.

Finally, a chest X-ray might reveal diffuse bilateral reticular opacities, which appear net-like in texture,often in the lower and lateral lung zones. With these findings, consider ILD and order a high-resolution chest CT to confirm your diagnosis.

On a chest CT, the main findings typically include reticulations which are thin septal opacities, or ground glass opacities. The term Ground glass comes from its similar hazy appearance to glass which has been given a matte finish.

In this next radiograph we see a good example of honeycombing, which appears as clusters of enlarged air spaces surrounded by thickened and fibrotic walls. If you see this, think ILD.

Now that we have our diagnosis, let’s talk about possible causes, starting with exposure-related ILDs. If your patient presents with a history of prolonged exposure to inhaled inorganic material, like asbestos and silica, then immediately suspect pneumoconiosis.

There are various types, depending on the kind of inhaled materials, and even though history taking is key and imaging features often overlap, there are some classic imaging findings that may help the diagnosis.

For example, silicosis can have classic perihilar lymph node calcifications or ‘eggshell calcifications’.

Next lets look at Asbestosis, which will typically have calcified pleural plaques. In both cases, you are dealing with a type of pneumoconiosis.

Now, let's consider if your patient has a history of exposure to inhaled organic material, like bird droppings, fungi, or mold.

Additionally, the CT scan shows centrilobular ground glass nodules and air trapping which are abnormal lucent areas. They’re termed air trapping as air is trapped in these lobules, and appears black, or lucent, on x-ray and CT due to its low density.

With these findings you should consider hypersensitivity pneumonitis.

Your next step is to obtain a bronchoalveolar lavage, or BAL.

If BAL shows lymphocytosis, diagnose hypersensitivity pneumonitis.

Lastly, if your patient has a smoking history, consider smoking-related ILDs.

Now, they might be between 20 and 40 years old and have a history of spontaneous pneumothorax.

A chest CT often reveals irregular thick-walled cysts and nodules in the middle and upper lung zones. With these findings, think about pulmonary Langerhans cell histiocytosis.

However, your patient might be between 30 and 60 years old.

In this case you might see CT findings of ground glass opacities, possibly with centrilobular nodules.

In this case, there are two possibilities: respiratory bronchiolitis interstitial lung disease or desquamative interstitial pneumonia. To tell them apart, you need a BAL, or a lung biopsy.

BAL might show brown-pigmented macrophages, known as smokers' macrophages, with no lymphocytes. On the biopsy, you might see macrophages surrounding the bronchioles, called bronchiolocentric macrophages.

In this case, you are dealing with respiratory bronchiolitis-interstitial lung disease.