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Inflammatory disease of small airways can present multiple histopathological patterns based on varying etiologies and clinical factors. Some of these entities are listed in the following table.

Diffuse alveolar damage (DAD) is the histopathological substrate for most patients diagnosed clinically with the acute respiratory distress syndrome. The two terms are not synonymous, however, as some patients presenting with acute unexplained respiratory failure will be found to have a more specific disease. The benchmark for DAD is the hyaline membrane. Some authors have used the term acute interstitial pneumonitis for cases where the etiology of the DAD is unknown, and restricted the term DAD to cases where the etiology is known. The disease, when usually sampled, is subacute (weeks), principally alveolar rather than interstitial (alveoli filled by myxoid fibrosis), and not a pneumonitis (principally fibrosis rather than inflammation).

The two most common diseases encountered in an open biopsy for chronic lung disease are usual interstitial pneumonitis (UIP) and bronchiolitis with patchy organizing pneumonia (BPOP). The initial determination is to separate these two (), and the distinction is important histologically and clinically. UIP is a disease with temporal and spatial heterogeneity and continuous progression. The patient is likely to get worse even with therapy. BPOP is a disease often representing a single event, so the disease tends to be all of about the same age, usually 4–6 wk in duration when biopsied. By definition, the disease is centered on bronchioles and therefore focal and classically micronodular.

The purpose of this chapter is not to present an exhaustive list of lung infections or agents causing these infectious processes. Such an atlas has recently been published by the American Registry of Pathology and the Armed Forces Institute of Pathology in Washington, DC. Instead, examples of a select group of pulmonary infections seen in a consultative format are illustrated here. More importantly, various cases are presented to help the pathologist identify histological patterns that should trigger consideration of a diagnosis of infection of the lung even though stains for specific organisms may have been negative or, in a consultative format, unavailable. Furthermore, cases with nonspecific histological findings that could have followed infectious processes are instructive when indications for culture techniques and special stains are defined.

The diagnosis of Wegener’s granulomatosis (WG) began with a clinicopathological triad of necrotizing granulomatous inflammation of the upper and lower respiratory tracts, angiitis, and necrotizing glomerulonephritis based on tissue examined at autopsy. However, only one-half of the patients in the initial series of Godman and Churg had the complete triad. Limited and ulcerative or tumefactive forms can occur in many organs. Initial biopsy diagnosis of established disease was followed by the early histological diagnosis of limited disease. This was followed by diagnosis on the basis of anti-neutrophilic cytoplasmic antibody (ANCA) without tissue confirmation. Today, therapy prevents most cases from progressing to multi-organ involvement. The most common cause of death is renal failure resulting from glomerulonephritis, and this can occur with days or even hours, so the pathological evaluation of a lung biopsy for WG is a medical emergency. Pulmonary and cardiovascular complications of the disease are much less common causes of morbidity or mortality. The evolution of the ANCA test has provided substantiating evidence for cases with equivocal histology and for separation of WG from microscopic polyangiitis and from other forms of disseminated vasculitis.

Hypertensive changes are usually ancillary to other pulmonary pathology, although biopsies may be done in patients with subacute cor pulmonale or congenital heart disease specifically to identify the type and degree of hypertension. Hypertensive changes occur in pulmonary blood vessels of all sizes but are usually quantified in small muscular arteries. The grading of pulmonary hypertension in musuclar arteries used here is a modification of the system used by Heath and Edwards and by Wagenvoort and Wagenvoort. The description of Heath and Edwards includes six grades. Because grades 4–6 are closely related in their pathogenesis and clinical significance and because grade 6 precedes grades 5 and 4, all three are grouped together here into a single grade. Plexogenic arterial hypertension (grade 4) usually occurs in only severe and untreated congenital heart disease and in severe and rapidly progressive idiopathic pulmonary hypertension. A subdivision of classic grading as applied to preoperative lung biopsies from patients scheduled for corrective surgery for congenital heart disease is discussed separately. Each higher grade of pulmonary hypertension incorporates the changes of the lower grades as well.

In this chapter, a series of cases is presented that have provoked significant thought on the part of the consulting and consulted pathologists. Most of these cases don’t fall neatly into the context of other chapters in this volume. A number of these cases illustrate patterns of pulmonary disease that are not diagnostic and challenge the pathologist to offer an intelligent differential diagnosis. A brief discussion of some of these relevant diagnoses follows.

The pathology of the pleura revolves largely around the diagnosis of diffuse malignant mesothelioma (DMM). The most important clinical distinction is drawing the line between various forms of reactive mesothelial hyperplasia and DMM. The diagnosis of DMM and distinguishing DMM from other pleural neoplasms will be discussed in Chapter 10.

The vast majority of cases in this chapter illustrate pertinent histological features of malignant tumors of the lung. There are, however, peculiar pulmonary neoplasms which may be confused with common bronchogenic carcinomas. Below is a list of these neoplasms, some of which are illustrated.

The term mesothelioma has been applied to two very different neoplasms. The first is the diffuse malignant mesothelioma (DMM), which covers the serosal surface, has both epithelioid and mesenchymal characteristics histologically, is virtually always fatal, and is caused by asbestos. The second is the fibrous mesothelioma, which forms a discrete mass, usually can be cured by resection, and is not related to asbestos. It is derived from the submesothelial fibroblast and thus has a histogenesis different from DMM. Its radio- graphic appearance as a baseball in the chest differs dramatically from DMM, but a needle biopsy of the tumor may show hyperchromatic spindle cells difficult to differentiate from a fibrosarcomatous or desmoplastic subtype of DMM.