Pulmonary nocardiosis is an infrequent but severe opportunistic infection typified by necrotic or cavitary consolidation in an immunocompromised patient. It is caused by Nocardia spp.
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Epidemiology
The condition is rare in general. Immunocompromised patients, particularly those with impaired cell-mediated immunity related to AIDS and transplantation, are particularly prone.
Incidence rates however may have increased probably due to a combination of improved laboratory detection techniques as well as a greater number of individuals with impaired cellular immunity 2.
Risk factors
Most patients have underlying respiratory disorders ranging from chronic obstructive pulmonary disease (COPD), asthma, and bronchiectasis, to less common disorders such as previous tuberculosis and cystic fibrosis 11.
Pathology
Pulmonary nocardiosis is most commonly caused by Nocardia asteroides, which is an aerobic gram-positive weakly acid-fast bacterium distributed worldwide in soils. Other less common species include Nocardia farcinica and Nocardia otitidiscaviarum 3. It is usually acquired by direct inhalation of contaminated soil.
Location
There may be an increased upper zonal predilection 1.
Radiographic features
Due to a high rate of diffuse hematogenous spread, suspected cases of pulmonary nocardiosis should have a brain MRI to exclude cerebral abscess formation. Other organs such as liver, adrenal, skin, and soft tissues can also be involved.
Plain radiograph / CT
There are several reported radiographic patterns which include:
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lobar or multilobar consolidation
probably the predominant feature 4
focal areas of decreased attenuation may reflect abscess formation
cavitation in 30%
solitary lung masses and/or nodules
mediastinal or hilar lymphadenopathy is not a feature of nocardiosis
Other accompanying features which are non-specific on their own include:
lung cavitating lesions: may occur in up to 1/3 of patients 4
empyema necessitans: can lead to chest wall invasion 4,5
Those with immunodeficiency may show more cavitating lesions and multiple nodules 6.
The temporal progression of radiographic abnormalities tends to be slow.
Different spread patterns have given varying features:
-
endobronchial spread
can occur with cavitation
characterized by the presence of small centrilobular nodules, often associated with bronchial wall thickening and endobronchial debris
-
disseminated pulmonary nocardiosis
consists of numerous nodules of various sizes
Treatment and prognosis
The usual treatment of choice is trimethoprim/sulfamethoxazole (cotrimoxazole). With certain complications, surgical drainage may be required. The prognosis can vary, with those with disseminated nocardiosis having mortality rates up to 40%. Subtherapeutic levels of antibiotics can result in flare-ups while patients are on treatment.
Differential diagnosis
On imaging it can sometimes mimic:
lung cancer: in particular squamous cell carcinoma with cavitation
non-nocardial lung abscess
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