A lung nodule is a small, focal abnormality often discovered on a chest imaging test like a computed tomography (CT) scan or X-ray. It appears as a small, round or oval-shaped white shadow in the lung tissue. These findings are common, and the vast majority—approximately 95%—are benign. The nodule itself rarely causes shortness of breath, but its existence may signal a separate underlying process that affects breathing.
Defining Lung Nodules and Their Typical Presentation
A lung nodule is defined as a growth in the lung that measures less than three centimeters in diameter. Any lesion larger than this is classified as a lung mass, which carries a higher probability of malignancy and is evaluated differently. Lung nodules are frequently discovered incidentally, meaning they are spotted on a scan performed for an unrelated reason.
Most nodules are small and do not interfere with lung function, making them asymptomatic, causing no cough, pain, or breathing problems. Common causes for benign nodules include scar tissue from old infections, such as tuberculosis or fungal diseases like histoplasmosis. Inflammation from non-infectious conditions, like sarcoidosis or rheumatoid arthritis, can also create granulomas, which appear as nodules on imaging.
How Size and Location Determine Symptom Development
While most nodules are silent, a nodule may directly cause shortness of breath. The specific location of the growth is important in determining its potential to cause breathing difficulty. Nodules situated peripherally seldom cause symptoms because they are far from the main airways.
Conversely, nodules that develop centrally, near the trachea or the large main bronchi, are far more likely to cause problems. As a nodule in this location grows, it can physically press on or narrow the airway, creating a central airway obstruction. This narrowing restricts the flow of air, leading to symptoms like wheezing, persistent coughing, and breathlessness.
A large nodule, often approaching the three-centimeter limit or being classified as a mass, can also induce shortness of breath by occupying a significant portion of the lung volume. This space occupation reduces the total area available for gas exchange, impairing oxygen transfer into the bloodstream. Additionally, a large nodule may cause post-obstructive atelectasis—the collapse of the lung tissue downstream from the blockage—further compromising breathing function.
Underlying Conditions That Cause Both Nodules and Shortness of Breath
In many cases, the nodule itself is not the source of breathing difficulty. Instead, the underlying systemic disease process that led to the nodule’s formation is the true cause of shortness of breath. This distinction is important for accurate diagnosis and effective management.
Acute infections, such as bacterial pneumonia or chronic infections like tuberculosis, cause inflammation and can leave behind granulomas that appear as nodules. The inflammation from the active infection is the primary driver of breathlessness, while the nodule is a secondary finding. Treating the infection often relieves the shortness of breath, even if the residual nodule remains visible on subsequent scans.
Systemic inflammatory and autoimmune disorders affect the entire body, including the lungs. For example, sarcoidosis is characterized by the formation of multiple small nodules (granulomas) throughout the lungs. However, the associated interstitial lung disease causes widespread scarring and restricted lung capacity, leading to shortness of breath. Similarly, patients with rheumatoid arthritis can develop rheumatoid nodules, but the breathlessness is usually due to interstitial lung disease.
For individuals with a history of smoking, conditions like Chronic Obstructive Pulmonary Disease (COPD) are the direct cause of shortness of breath. These individuals are also at a higher risk of developing lung nodules. In these cases, the underlying destructive lung disease is responsible for the breathing impairment, not the presence of a single nodule.
When to Seek Medical Evaluation and Diagnostic Steps
New or worsening shortness of breath in a person with a known lung nodule should prompt a medical evaluation, especially if the symptom is sudden, accompanied by chest pain, or if the individual is coughing up blood. The diagnostic process focuses on evaluating both the symptom and the nodule to determine the cause. Initial evaluation of breathing function often includes measuring blood oxygen levels and may involve pulmonary function tests (PFTs) to assess how well the lungs move air.
To evaluate the nodule itself, physicians follow guidelines that consider the nodule’s size, shape, and the patient’s risk factors, such as a history of smoking. For small nodules with a low risk of malignancy, the standard approach is active surveillance, which involves serial CT scans over time to monitor for any change in size or appearance. Since cancerous nodules tend to grow relatively quickly, surveillance helps differentiate them from stable, benign lesions.
If a nodule has concerning features, such as a larger size or an irregular, spiculated border, additional testing may be ordered. A positron emission tomography (PET) scan can provide metabolic information, as cancerous tissue typically consumes more sugar than benign tissue. The definitive diagnostic step is a biopsy, which involves obtaining a small tissue sample from the nodule, often through a bronchoscopy, to be examined under a microscope and confirm the cause.