Imaging of Pulmonary Viral Pneumonia 1 - SIRM

STATE OF THE ART: Imaging of Pulmonary Viral PneumoniaFranquetMore than 55 million people die eachyear worldwide, and pneu moniais among the leading causes ( 1 ).Viral infections are an increasingly frequentcause of pulmonary disease worldwide.Unfortunately, the diagnosis ofviral pneumonia still relies heavily onclinician suspicion in the proper setting,which is based on host risk factors,presentation, and exposures. Thediagnostic approach involves a considerationof the likely pathogens on thebasis of the patient’s presenting signsand symptoms and his or her immunologiccondition ( 2 , 3 ). The clinical manifestationsof most viral infections aresimilar. Although most of these infectionsin nonimmunocompromised personsare self-limited, some patients candevelop severe pneumonitis ( 4 ). Clinically,viral pneumonia in adults can bedivided into two groups: so-called atypicalpneumonia in otherwise healthy hostsand viral pneumonia in immunocompromisedhosts ( 5 ). Influenza virus typesA and B account for most viral pneumoniasin immunocompetent adults ( 5 ). Ithas been considered that clinical history,results of physical examination,and imaging features cannot enable theprediction of the etiologic agent. Furthermore,a pathogen is identified in onlyEssentialsn Viral pulmonary infections areclinically important in both immunocompromisedand immunocompetentpatients; despite its limitations,CT is currently the imagingmodality of choice for the evaluationof pulmonary viral infections.n Although the CT features of viralpulmonary infections are usuallynonspecific, precise imaging characterizationis essential; knowledgeof CT findings has a substantialeffect on the treatment ofpatients suspected of having pulmonaryviral infections.n Knowledge of the underlyingpathologic characteristics ofthese diseases aids in the differentiationof viral infections fromother entities that may haveoverlapping imaging features.up to 50%–64% of patients admitted tothe hospital with pneumonia. The recentdevelopment of multidetector helical computedtomographic (CT) scanners capableof rapid scanning and acquisitionof thin sections has revolutionized thethin-section CT technique. Volumetricthin-section CT with thin detectors (0.5–0.625 mm) has become the routine inmany institutions. New serologic testshave also come to the aid of the clinician,and sophisticated molecular methods arebecoming more commonplace in routinediagnosis of acute respiratory diseasein both immunocompetent and high-riskpatients ( 1 , 6 – 8 ). Confirmation of the diagnosisand identification of the specificagent can be accomplished with use oftissue cultures, serologic tests, detectionof viral antigens within respiratory tractsecretions or blood with use of monoclonalantibodies, detection of virusassociatedmolecules with use of in situhybridization or polymerase chain reaction(PCR), and observation of virusinducedchanges cytologically or histologically( 9 , 10 ). The purpose of this reviewis to provide a framework with which tobetter understand respiratory viral infections,the diverse clinical settings in whichthey occur, and their potential importancein these varying clinical contexts. Whileacknowledging the less-than-definitivediagnostic role of CT in their diagnosis,I will also include discussion of thoseconditions in which an understanding ofthe pathologic characteristics can be usefulto the interpretation of thin-sectionCT findings. The epidemiology and pathogenesisof viral infections are discussedin Appendix E1 (online).Histologic Features of Viral InfectionsVarious histopathologic patterns oflung injury have been described in viralpneumonia. Although some of thesepatterns may be relatively unique to aspecific clinical context, others are nonspecificwith respect to either the causeor pathogenesis.Viruses can result in several pathologicforms of lower respiratory tractinfection, including tracheobronchitis,bronchiolitis, and pneumonia. Organizingpneumonia is a nonspecific reparativereaction that may be seen in a varietyof clinical contexts. In other words, organizingpneumonia may result from a varietyof causes or underlying pathologicprocesses, including viral infections.Most respiratory viruses damage cellsdirectly through cytopathic effects mediatedby means of either viral-directedcell lysis or the inhibition of host cellRNA, protein, and DNA synthesis. Otherviruses (eg, cytomegalovirus [CMV], adenovirus,or herpesvirus) may producespecific nuclear changes or characteristiccytoplasmic inclusions ( 11 ) ( Fig 1 ).Cytopathic respiratory viruses (eg,influenza, adenovirus, and herpesvirusgroup) cause a virus-specific lung injurypattern. Influenza virus affects the epitheliumdiffusely and, in severe cases, resultsin necrotizing bronchitis and/or bronchiolitisand diffuse alveolar damage.The histologic features of influenzapneumonia are epithelial necrosis of theairways with submucosal chronic inflammation.Fatal influenza pneumonia representsa necrotizing bronchiolitis withdiffuse alveolar damage, which can behemorrhagic.Adenovirus has its greatest effect inthe terminal bronchioles and may producebronchiolitis or even bronchiectasis( 12 – 14 ). The bronchiolitis may be necrotizingand result in a necrotizing bronchopneumoniasimilar to that seen in severeherpes simplex infection ( 13 ) ( Fig 2 ).The herpes group of viruses (herpessimplex, varicella-zoster, CMV, Epstein-Barr virus [EBV]) may cause focal cytopathiceffects in either the airway oralveoli. The histologic features of herpessimplex lower respiratory tract infectionPublished online10.1148/radiol.11092149Radiology 2011; 260:18– 39Content codes:Abbreviations:AIDS = acquired immunodefi ciency syndromeCMV = cytomegalovirusEBV = Epstein-Barr virusHMPV = human metapneumovirusHTLV-1 = human T-cell lymphotropic virus type 1PCR = polymerase chain reactionRSV = respiratory syncytial virusSARS = severe acute respiratory syndromeAuthor stated no fi nancial relationship to disclose.Radiology: Volume 260: Number 1—July 2011 n radiology.rsna.org 19