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A 55-year-old homeless man presented to the emergency room with severe shortness of breath, cough and fever. He was admitted to the hospital but rapidly developed respiratory failure and expired. An autopsy was performed. The right lower lobe of the lung showed confluent consolidation with a firm, yellow-red cut surface. Patchy areas of consolidation were also present in the other lobes.
Archive Case and Diagnosis:
This case first appeared as Performance Improvement Program in Surgical Pathology (PIP) 2006, Case 32 and is an Acute lobar pneumonia.
Criteria for Diagnosis and Comments:
The distributed slides show pulmonary tissue with neutrophils filling the alveolar spaces in a near-continuous fashion. The alveolar septa show variable congestion and edema without significant fibrosis. This is an example of acute pneumonia, which, given that it involves an entire lobe, may be classified as lobar pneumonia.
Acute pneumonia may be categorized based on the distribution of disease in the lung. Lobar pneumonia, as the name implies, essentially involves an entire lobe, while bronchopneumonia shows a patchy distribution centered around bronchioles. The designation refers solely to the distribution of disease. Both patterns are usually caused by bacteria and are caused by the same organisms. Similarly, both patterns of disease may coexist in the same patient, and it may be difficult to determine whether any given case is strictly bronchopneumonia vs. lobar pneumonia. The more important task in regard to treating the patient is defining the causative organism and extent of disease.
Classic lobar pneumonia is not commonly seen in modern times given the advent of antibiotic therapy, but may still be encountered in
immunosuppressed or indigent populations. Common causative organisms include Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus
influenzae, Klebsiella pneumonia and Pseudomonas sp. The histology is generally similar regardless of the causative organism. As in this case,
the findings consist of neutrophilic infiltrates within the alveolar spaces with associated congestion and edema of the alveolar septa.
Pneumonia due to Staphylococcus and Klebsiella tends to exhibit more tissue necrosis compared to that caused by Streptococcus, and
Pseudomonas pneumonia may show clusters of organisms surrounding blood vessel walls, but the findings are not specific. Bacterial organisms
may be visible on conventional H&E stains or may be highlighted by stains such as Brown-Brenn or Brown-Hopps, but definitive classification
of the organism should be left to microbiologic culture. During recovery, macrophages may accumulate to remove neutrophilic debris, or the
exudates may transform into organizing fibroblastic tissue. In some cases, the acute pneumonia may evolve into diffuse alveolar damage and
hyaline membranes maybe present in an otherwise typical case of pneumonia.
Acute pneumonia should be distinguished from diffuse alveolar damage (DAD), acute fibrinous and organizing pneumonia (AFOP), eosinophilic pneumonia (EP) and diffuse alveolar hemorrhage with capillaritis (DAH-C).
DAD is a histologic pattern of acute lung injury that is associated with myriad potential underlying etiologies (infection, drug reaction, sepsis, shock, inhalation, etc), or may be idiopathic. DAD is divided into acute (exudative) and organizing (proliferative) phases. The acute phase is characterized by edematous widening of alveolar septa. Hyaline membranes, the classic feature of DAD, are apparent roughly two days after initial injury and reach a peak after roughly 5 days. Blood vessels may contain fibrin thrombi which should not be mistaken for an embolic process. Inflammatory infiltrates are characteristically sparse in DAD. A dominant finding of neutrophils associated with only focal hyaline membrane formation characterizes an otherwise typical acute pneumonia which has evolved into DAD. Conversely, focal neutrophils in a case of otherwise typical DAD may be indicative of an infectious etiology. Special stains for bacteria, fungus/pneumocystis and mycobacteria should be performed in all cases of DAD. Occasionally, cases of DAD may develop a superimposed bacterial pneumonia, and in such cases it may be virtually impossible to sort out whether two processes are present or which one came first. Organizing DAD is generally not confused with acute pneumonia and is characterized by myxoid interstitial fibrosis with prominent type 2 pneumocyte hyperplasia. Significant inflammation is generally not seen, and airspace consolidation is minimal or absent.
AFOP is a recently described histologic pattern of acute lung injury which is currently thought to be a variant of DAD given a similar overall mortality rate. However, unlike DAD, a significant number of cases of AFOP follow a more indolent course with complete recovery. Further study is needed to fully clarify the significance of this histologic pattern. Histologically, AFOP is characterized by organizing intra-alveolar fibrin balls without classic hyaline membrane formation. The process may be patchy or diffuse. The alveolar septa in affected areas may show mild chronic inflammatory cell infiltrates. Significant neutrophilic or eosinophilic infiltrates are not seen. As the process evolves, intra-alveolar organizing fibroblastic tissue may also be seen. These foci often show retention of fibrin cores. It should be noted that organizing intra-alveolar fibrin may occur as a reaction adjacent to, or as a component of other processes such as granulomas, abscesses or neoplasms. Therefore, a diagnosis of AFOP should be made only when organizing intra-alveolar fibrin is the sole finding in a large biopsy specimen. Focal organizing fibrin may also be seen in cases of acute pneumonia, but in such cases the dominant finding is neutrophilic infiltrates, which should not be seen in true cases of AFOP.
Eosinophilic pneumonia (EP) is characterized by varying proportions of intra-alveolar fibrin and macrophages admixed with prominent eosinophils. Eosinophils are generally also present in the interstitium and eosinophilic microabscess formation is often noted. In cases of acute EP, hyaline membranes may be observed but the presence of eosinophils distinguishes these cases from conventional DAD. Diffuse neutrophilic infiltrates, as seen in acute pneumonia, are not a feature of EP.
Diffuse alveolar hemorrhage with capillaritis (DAH-C) is characterized by intra-alveolar blood and hemosiderin-laden macrophage accumulation. The macrophages should contain coarse hemosiderin in contrast to the finely granular pigment in smoker's macrophages. Associated capillaritis is evidenced by accumulation of neutrophils within the alveolar septa with resultant vascular damage. The neutrophilic infiltrates are predominantly located within the interstitium, in contrast to acute pneumonia where the neutrophils are found predominantly within airspaces.