Published online: 2021-07-27 THORACIC IMAGING Infectious pneumonia in the immunocompetent host: What the radiologist should know Rohini G Ghasi, Sunil K Bajaj Department of Radiodiagnosis, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi, India Correspondence: Dr. Rohini G Ghasi, Department of Radiodiagnosis, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi ‑ 110 029, India. E‑mail: [email protected] Abstract Lung infections are an important cause of morbidity and mortality, particularly because of the rising antimicrobial resistance. According to the clinical setting, they can be categorized as community‑acquired pneumonia and hospital‑acquired pneumonia. Radiological patterns of lung infections are lobar consolidation, bronchopneumonia, interstitial pattern, and nodular pattern. In addition, typical imaging features of several infections serve as “red flag signs” in reaching a diagnosis or altering the management. It would be prudent for the radiologist to be well informed regarding these aspects of lung infections to be able to make a valuable contribution to the management. Key words: Imaging; immunocompetent; lung infections; pneumonia; radiological signs Introduction incidence of organ transplants, and emergence of newer viruses with atypical imaging features. Radiologists need The most common question put up to an Indian radiologist to be more aware about the dominant imaging patterns in in the setting of pulmonary infection is “does it look different types of these infections and the “red flag” imaging tubercular?” Once tuberculosis is ruled out, most of our signs which make a difference to the management. job is considered done. We need to, however, contribute more to the diagnostic trail. Lung infections caused by Clinical Setting organisms other than Mycobacterium tuberculosis, are, nevertheless, a major cause of morbidity and mortality. The most widely referenced guidelines in pneumonia are the In recent times, there has been an alarming increase in Infectious Disease Society of America (IDSA) and American drug resistance strains. Recent data from the Indian Thoracic Society (ATS) guidelines. A consensus guideline intensive care unit (ICU) setting shows that most of the document was issued by these two societies in 2007.[2] Among pathogens are resistant to common antibiotics.[1] Empirical its various recommendations was the development of locally broad spectrum antimicrobial therapy is one of the causes adapted guidelines. Accordingly, Indian Chest Society and responsible for this. Unusual infections are encountered National College of Chest Physicians (India) developed more frequently now because of increased survival in national pneumonia guidelines.[3] As per the IDSA/ATS acquired immunodeficiency syndrome (AIDS) patients, Access this article online This is an open access article distributed under the terms of the Creative Commons Attribution‑NonCommercial‑ShareAlike 3.0 License, which allows Quick Response Code: others to remix, tweak, and build upon the work non‑commercially, as long as the Website: author is credited and the new creations are licensed under the identical terms. www.ijri.org For reprints contact: [email protected] DOI: 10.4103/0971‑3026.202967 Cite this article as: Ghasi RG, Bajaj SK. Infectious pneumonia in the immunocompetent host: What the radiologist should know. Indian J Radiol Imaging 2017;27:23‑32. © 2017 Indian Journal of Radiology and Imaging | Published by Wolters Kluwer - Medknow 23 Ghasi and Bajaj: Pneumonia in the immunocompetent host guidelines, community‑acquired pneumonia (CAP) is NCCP (I) guidelines.[3] The diagnosis of HAP/VAP is made defined as “an acute infection of the pulmonary parenchyma by the presence of signs and symptoms to acute infection that is associated with at least some symptom of acute with appearance of new or progressive infiltrate on the chest infection, accompanied by the presence of an acute infiltrate radiograph or by positive culture samples. It can be divided on a chest radiograph or auscultatory findings consistent into early or late HAP/VAP depending upon whether the with pneumonia, in a patient not hospitalized or residing in a pneumonia occurs within or after 4 days of admission/ long‑term care facility for more than 14 days before the onset mechanical ventilation, respectively.[2] CT is not routinely of symptoms.[2] The definition itself includes confirmation indicated in the routine diagnosis of HAP/VAP (strong by chest radiograph (moderate recommendation, level recommendation, level III evidence).[3] III evidence). The role of the radiologist is, therefore, to confirm the presence of lung infiltrate and its severity. In A comparison of the pathogens responsible for CAP and simple cases, if the clinical improvement upon treatment HAP is presented in Table 1. Most common organism in is satisfactory, then a follow‑up radiograph is not both CAP and early onset HAP is Streptococcus pneumoniae. recommended (Strong recommendation, level II evidence).[3] In more severe cases of CAP requiring inpatient care, Computed tomography (CT) scan is not routinely indicated additional organisms such as Legionella pneumophila and in the diagnosis of CAP (strong recommendation, level II mixed infection in aspiration pneumonitis can be found. evidence).[3] More virulent pathogens such as Staphylococcus aureus and Gram negative bacilli (GNB) can be found in addition Hospital‑acquired pneumonia (HAP) is defined as to the routine organisms in CAP patients requiring ICU pneumonia occurring ≥48 hours after admission not care.[5] However, in India, it is not uncommon to find incubating at the time of admission. Related terms are Methicillin‑Resistant S.aureus (MRSA), Pseudomonas ventilator‑acquired pneumonia (VAP) and healthcare aeruginosa, and Drug Resistant S. pneumoniae (DRSP) associated pneumonia (HCAP). VAP refers to pneumonia in CAP isolates.[6] In late onset (>4 days) HAP, aerobic that arises after more than 48 hours of endotracheal GNB and S. aureus are more common. Anaerobes, intubation. As per the IDSA/ATS consensus statement, Legionella, Pneumocystis, tuberculosis, viruses, and fungi HCAP includes any patient who was hospitalized in an are uncommon causes and occur irrespective of the timing acute care hospital for 2 or more days within 90 days of of acquiring HAP. the infection; resided in a nursing home or long‑term care facility within the past 30 days of the current infection. Imaging Patterns HCAP is included in the spectrum of HAP and VAP, and patients with HCAP need therapy for MDR pathogens.[2,4] In Lobar consolidation the Indian scenario, similar long‑term health care facilities Lobar pattern of consolidation occurs when the alveoli are are uncommon and the term HCAP is avoided in the ICS/ filled with inflammatory exudate as host response to the Table 1: Comparison of pathogens causing CAP and HAP CAP HAP Patient profile Organism Timing of infection Organism % Outpatient Streptococcus pneumoniae Early onset bacterial <4 days S.pneumoniae 5-20<5-15 Mycoplasma pneumonia H.influenzae Haemophilus influenza Chlamydophila pneumonia Respiratory viruses Inpatient S.pneumoniae Late onset bacterial >4 days a) Aerobic GNB 20-60 M.pneumoniae Pseudomonas aeruginosa Enterobecter spp C.pneumoniae Acinetobacter spp H.influenzae Klebsiella pneumoniae L.pneumophila Serratia marcescens Aspiration E. coli Respiratory viruses b) Gram positive cocci 20-40 S.aureus ICU S.pneumoniae Early and late onset pneumonia Anaerobic bacteria 0-35 Staphylococcus aureus L.pneumophila 0-10<1 L.pneumophila P.jiroveci <1<1 Gram negative bacilli (GNB) M.tuberculosis <1<1 H. influenzae Influenza A, B viruses <1 RSV Fungal Aspergillus Candida spp 24 Indian Journal of Radiology and Imaging / Volume 27 / Issue 1 / January - March 2017 Ghasi and Bajaj: Pneumonia in the immunocompetent host infection. The exudate spreads across lung parenchyma hematogenous spread from Staphylococcal pyoderma. through channels of collateral air drift, and hence the Staphylococcal pneumonia can commonly lead to resultant lung opacity is homogenous and contiguous. complications such as lung abscess and empyema. It There is no destruction of lung parenchyma and complete is important to differentiate the two because empyema resolution occurs with treatment. The most common requires drainage whereas medical therapy is curative organism in CAP setting is S. pneumoniae. There is for most lung abscesses.[7,9] Empyema typically forms an predilection for basal segments [Figure 1]. Less commonly, obtuse angle with the lung parenchyma and shows “split L. pneumophila may occur in hospitalized CAP patients. The pleura sign” on CECT due to enhancement of visceral infection begins as unifocal infiltrate but spreads bilaterally. and parietal pleura [Figure 4A]. Lung abscess forms The radiographic picture lags behind clinical status.[7,8] acute angle with lung parenchyma and bronchovascular structures end on the wall of abscess rather than being In HAP, the appearance can be more severe with the splayed [Figure 4B]. In the healing phase, pneumatoceles consolidation being bilateral and extensive. S. pneumoniae can be seen as thin‑walled air filled lung cysts is the most common pathogen in early onset HAP. Klebsiella [Figure 4C], which can rupture to cause pneumothorax. can also cause lobar pneumonia with typical “bulging Pneumatoceles are not specific for S. aureus because they fissure sign” due to copious exudates. Pseudomonas has a can occasionally be seen in Escherichia coli, S.