CONTENTS:
3.5 Infectious Lung Conditions
3.5.1 Pulmonary Tuberculosis (TB)
3.5.2 Lung Abscess
3.5.3 Septic Emboli
3.5.4 Fungal Infections
3.5.5 Parasitic Lung Infections
3.6 Vascular Lung Conditions
3.6.1 Pulmonary Arteriovenous Malformations (PAVMs)
3.6.2 Lung Infarcts
3.7 Pneumoconioses (Occupational Lung Disease)
Forward to 3F Benign Lung Tumors . Back to 3D Congenital and Inflammatory Lung Conditions .
3.5 Lung Infection
Infectious granulomas are the cause of approximately 80 % of benign lung nodules. The infectious causes include endemic fungi (e.g., histoplasmosis, coccidioidomycosis), mycobacteria (tuberculous or non-tuberculous). These infectious granulomas often appear as a well-demarcated and fully-calcified solitary pulmonary nodules (SPN).
3.5.1 Pulmonary Tuberculosis (TB)
Infection with mycobacterium tuberculosis causes pulmonary tuberculosis (TB). TB is becoming increasingly common in the developing and western world due to antibiotic resistance and the increase in immune suppressed populations (e.g., due to steroid therapy and HIV/AIDS).
- Primary pulmonary TB infection presents throughout the lung in children. However, TB presents in the upper or lower zones of the lung in adults.
- Post-primary TB infections tend to affect the upper zones of the lungs.
- Miliary TB is evenly distributed throughout both lungs.
Sometimes imaging techniques may find it difficult to detect the initial site of lung infection because it is too small and may be anywhere in the lung. Radiographic evidence of lung infection is seen in 70% of children, and 90% of adults. Cavitation of the infective lung nodule is uncommon in primary TB. Furthermore, cavitation occurs in between 10 % and 30% of cases. In most cases of primary TB infection, the lesion is localized and a ‘caseating granuloma’ forms (‘tuberculoma’) which can then calcify and is known as ‘Ghon focus.’
Figure 3.8 Pulmonary Tuberculosis.
A. Photomicrograph of a section from a lung nodule shows a
granuloma associated with necrosis. (H&E x 60).
B. The same patient’s sputum sample stained with Ziel
Nielsen (ZN) shows acid-fast bacilli (AFBs). (x100)
Types of Tuberculosis
Tuberculomas account for only 5% of cases of post-primary Tuberculosis and appear as a well-defined, rounded mass. Tuberculomas are typically located in the upper lobes of the lungs. They are usually single (80%) and may measure up to 4 cm in size. Small ‘satellite’ granulomatous lesions are seen in most cases. In 20 % to 30 % of cases, lung cavitation may develop. In the majority of cases, the granulomas develop in the posterior segments of the upper lobes (85 %).
Miliary tuberculosis is an uncommon presentation, but carries a poor prognosis. The ‘miliary’ or seeded spread is due to blood-borne dissemination of an uncontrolled infection. Miliary TB is seen in both primary and post-primary tuberculosis. Miliary inflammatory nodules appear which range from 1 mm to 3 mm in diameter, are uniform in size and distribution. If treatment is successful at this stage, little or no residual abnormality remains. The danger is that if untreated, military TB can affect the meninges of the brain (tuberculous meningitis) which has a high mortality.
Diagnosis of tuberculosis begins with an examination of sputum for acid-fast bacilli (AFBs). Histochemical staining and examination under the microscope detects the presence of AFB’s. Culture of the mycobacteria or molecular analysis may be required to select the appropriate anti-microbial treatment.
3.5.2 Lung Abscess
Lung infection with abscess-forming bacteria (eg, Staphylococcus aureus) and Pneumocystis jirovecii (previously Pneumocystis carinii) may present as a Solitary Pulmonary Nodule (SPN), which may cavitate. Rarely, dirofilariasis, a mosquito-borne disease, may present as a SPN. Larvae embolize to the lungs and induce a granulomatous inflammatory response, which results in a non-calcified, sub-pleural that may be mistaken for lung cancer on CT imaging.
Bacteremic patients and patients who aspirate can develop multiple lung abscesses. Lung abscesses are typically round, 0.5 to 3 cm in diameter, and well-defined. They tend to be present at the lung bases, and posterior regions of the upper lobes.
3.5.3 Septic Emboli
Patients with bacteremia or septic thrombophlebitis may produce septic emboli to the lungs. Septic emboli appear as nodules measuring between 0.5 to 3 cm distributed in the periphery of the lower lobes. Furthermore, cavitation is common, usually producing thin-walled lesions.
3.5.4 Fungal Infections
Fungal infection usually results in multiple pulmonary nodules. The causes include histoplasmosis, blastomycosis, coccidioidomycosis or cryptococcosis.
Invasive aspergillosis is more likely in immunocompromised individuals. Pulmonary nodules caused by fungal infection tend to be 0.5 to 3 cm in diameter and in invasive aspergillosis, nodules are the predominant finding in 90 % of cases. These aspergillus nodules progress to cavitation in about 20 % of cases.
In lung infection with histoplasmosis, nodules may remain unchanged in size for many years and undergo calcification.
Cryptococcus may cause multiple small nodules in patients with HIV infection or occasionally larger nodules or masses in immunocompetent individuals.
3.5.5 Parasitic Lung Infections
Pulmonary larva migrans is due to lung infection with Toxocara canis and Ascaris suum, following ingestion of infected meat or vegetables. Larvae develop from ingested parasite eggs and migrate to the lungs. The CT findings show peripheral nodules surrounded by ‘ground glass‘ opacities.
Paragonimus westermani is a fluke that is found in Japan, Korea, China, the Philippines, and Taiwan. Infection follows ingestion of uncooked fresh crayfish or crab that contain the parasite. Most of the developing flukes penetrate the diaphragm and migrate into the lung, forming multiple cavities with surrounding foci of consolidation.
3.6 Vascular Lung Conditions
Vascular conditions should be borne in mind when considering a localized lung abnormality on imaging. The lung is highly vascularized, containing arteries, veins, capillaries and lymphatics.
3.6.1 Pulmonary Arteriovenous Malformations (PAVMs)
Pulmonary arteriovenous malformation (PAVM) is commonly found in hereditary hemorrhagic telangiectasia (HHT) or Weber-Osler-Rendu syndrome but can also be idiopathic (of unknown cause).
Pulmonary arteriovenous malformations (AVMs) consist of abnormal communications between pulmonary arteries and veins; they can present as either solitary or multiple pulmonary nodules.
A contrast-enhanced CT scan may demonstrate a feeding artery and vein, and pulmonary angiography will also be helpful. Biopsy should be avoided.
3.6.2 Lung Infarcts
Thrombus that may form within peripheral veins, most commonly in the leg veins, may detach and ‘embolize’ (or travel in the blood) to lodge in the lungs. Obstruction of the pulmonary veins causes ischemic death of the lung tissue or ‘infarction.’ Lung infarcts are usually peripheral and wedge-shaped, but if small, they may ‘organize’ and heal by fibrosis to produce nodular scars.
3.7 Pneumoconioses (Occupational Lung Disease)
Coal workers’ pneumoconiosis and silicosis consist of lung fibrosis (scar tissue) in response to inhaled carbon and silica. Both conditions can develop into ‘progressive massive fibrosis’ (PMF) or multiple pulmonary nodules that range in size from 1 to 10 cm.
Beryllium-associated lung disease may also present with multiple pulmonary nodules that on imaging may mimic sarcoidosis. Caplan’s disease is a combination of rheumatoid arthritis and coal-worker’s pneumoconiosis that manifests with multiple pulmonary nodules.
References:
Dye C, Scheele S, Dolin P, Pathania V, Raviglione MG, (for the WHO Global Surveillance and Monitoring Project). (1999) Global Burden of Tuberculosis: Estimated Incidence, Prevalence, and Mortality by Country. JAMA. 1999;282(7):677-686. (Retrieved 14th April 2015): http://jama.jamanetwork.com/article.aspx?articleid=191271
Castranova V, Vallyathan V. (2000) Silicosis and coal workers’ pneumoconiosis. Environ Health Perspect. 2000 Aug; 108(Suppl 4): 675–684. (Retrieved 14th April 2015): http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1637684/
Patient Information:
American Lung Association Pulmonary Vascular Disease. (Retrieved 14th April 2015)
NHS Choices Tuberculosis (TB) – Symptoms. (Retrieved 14th April 2015): http://www.nhs.uk/Conditions/Tuberculosis/Pages/Symptoms.aspx
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