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Abstract
Diffuse lung infiltrates on chest radiography presents a common clinical problem as many diverse pathological processes can cause diffuse lung disease. The presentation of these diseases can vary from acute to chronic and includes a wide array of radiological patterns that are optimally evaluated on High-resolution computed tomography of the chest (HRCT). (1)
”Diffuse” lung infiltrates implies involvement of all lobes of both lungs, but they need not affect all lobes or all lung regions uniformly. (1)
Many different disease processes may present with diffuse lung shadowing on chest radiography. These processes include infection, neoplasm, pulmonary edema, hemorrhage, environmental and occupational lung diseases, aspiration pneumonia, many forms of interstitial lung diseases (ILDs), and others. (1, 2)
Although most disorders with diffuse lung shadowing will be parenchymal processes, some air way diseases such as; bronchiectasis, and cystic fibrosis may present with diffuse lung infiltrates. (1,2) Similarly, some vascular disorder such as veno-occlusive diseases are associated with diffuse lung infiltrates and may be mistaken for ILDs. (3)
Anatomical considerations
The lung has two well-defined interstitial connective tissue compartments arranged in series, as described by Hayek. (4) These are the alveolar wall, interstitium and loose-binding (extra-alveolar) connective tissue (peribronchovascular sheaths, interlobular septa, and visceral pleura). The connective tissue fibrils (collagen, elastin, and reticulin) form a three-dimensional basket like structure around the airways and distal air spaces. (5)
The parenchyma of the lung includes the pulmonary alveolar epithelium, capillary endothelium, and the spaces between these structures, together with the tissues within the septa including the perivascular and perilymphatic tissues. More centrally it includes the peribronchiolar and peribronchial tissues. (6)
Pulmonary infiltrates etiology
Infectious etiologies:
Bacterial, fungal, viral, and mycobacterium pathogens may infect the lungs of both immunocompetent and immunosuppressed patients. (7)
Bacterial infection:
Patients are at risk for infection with traditional nosocomial bacterial and community acquired organisms. (8)
The true incidence of community acquired pneumonia is uncertain because the illness is not reportable, and only 20% to 50% of patients require hospitalization. (9) For any given pathogen, the severity of disease is largely determined by the subject’s age and the presence and type of any co-exiting illness. (10-13)
Common causes of community-acquired pneumonia in patients who require hospitalization, in order of frequency. (12, 13)
• Streptococcus pneumoniae.
• Mycoplasma pneumoniae.
• Chlamydophila pneumoniae.
• Haemophilus influenza.
• Mixed infections.
• Enteric gram negative bacilli.
• Aspiration (anaerobes).
• Legionella.
Immunosuppressed patients are at risk for infection with nosocomial pneumonia caused by pseudomonas aeruginosa, staph aureus, aerobic gram negative bacilli such as klebsiella pneumoniae.
Although legionella is often considered a community-acquired pathogen, legionella may be responsible for hospital out breaks of pneumonia. Anaerobes rarely produce infiltrates in these patients. (14, 15)
Fungal infections:
Fungal pneumonias result in a wide spectrum of illness. Infection with aspergillus is the most common cause of fungal pneumonia in immunocompromised patients. A recent analysis, however, indicates that the incidence of invasive Aspergillus is increasing. (16, 17)
Other medically important fungi that should be considered in the diagnosis of pneumonia include the endemic mycoses (Histoplasma capsulatum, Coccidiodes immitus, and Blastomyces dermatitidis), and emerging fungi such as Trichosporon, fusarium, and Zygomycetes. (18)
Although Candida is often isolated in respiratory secretions, particularly from patients previously receiving broad-spectrum antibiotics, Candida rarely causes pneumonia in patients other than lung transplant recipients. Disseminated Candida, however, may seed the lungs resulting in either local or diffuse infiltrates. (19)
Viral infections:
Several viruses are of concern in the immunosuppressed host: Cytomegalo virus (CMV), Herpes virus, Respiratory Syncytial virus (RSV), influenza virus, Para influenza virus, Adeno virus, and Varicella virus. Over the past 10 to 15 years, viruses have been increasingly recognized as important causes of serious respiratory illness in immunocompromised patients. (20)
These viral infections may result from reactivation of a latent process or reflect a newly acquired infection. As such, seasonal variation in their incidence is seen with RSV, influenza, and Para influenza virus while the development of CMV is related to the patient’s underlying immune status and prior CMV serostatus. RSV and influenza are of particular concern during the fall and winter months. (21)