Page 484 - Textbook of Pathology, 6th Edition

468 type 3-S. pneumoniae causes particularly virulent form of lobar
 TABLE 17.1: Etiologic Classification of Pneumonias.
pneumonia. Pneumococcal pneumonia in majority of cases
A. BACTERIAL PNEUMONIA is community-acquired infection.
I. Lobar pneumonia 2. Staphylococcal pneumonia. Staphylococcus aureus causes
II. Bronchopneumonia (Lobular pneumonia)
pneumonia by haematogenous spread of infection from
B. VIRAL AND MYCOPLASMAL PNEUMONIA another focus or after viral infections.
(PRIMARY ATYPICAL PNEUMONIA)
3. Streptococcal pneumonia. β-haemolytic streptococci may
C. OTHER TYPES OF PNEUMONIAS rarely cause pneumonia such as in children after measles or
I. Pneumocystis carinii pneumonia influenza, in severely debilitated elderly patients and in
II. Legionella pneumonia (Legionnaire’s disease) diabetics.
III. Aspiration (inhalation) pneumonia 4. Pneumonia by gram-negative aerobic bacteria. Less
IV. Hypostatic pneumonia common causes of lobar pneumonia are gram-negative
V. Lipid pneumonia
bacteria like Haemophilus influenzae, Klebsiella pneumoniae
(Friedlander’s bacillus), Pseudomonas, Proteus and Escherichia
6. Leucocyte dysfunctions. Disorders of lymphocytes coli, H. influenzae commonly causes pneumonia in children
including congenital and acquired immunodeficiencies (e.g. below 3 years of age after a preceding viral infection.
AIDS, immunosuppressive therapy) and granulocyte
abnormalities may predispose to pneumonia. MORPHOLOGIC FEATURES. Laennec’s original
description divides lobar pneumonia into 4 sequential
CLASSIFICATION. On the basis of the anatomic part of the
lung parenchyma involved, pneumonias are traditionally pathologic phases: stage of congestion (initial phase), red
hepatisation (early consolidation), grey hepatisation (late
classified into 3 main types: consolidation) and resolution. However, these classic stages
1. Lobar pneumonia seen in untreated cases are found much less often
2. Bronchopneumonia (or Lobular pneumonia) nowadays due to early institution of antibiotic therapy
3. Interstitial pneumonia. and improved medical care.
However, now that much is known about etiology and
In lobar pneumonia, as the name suggests, part of a
pathogenesis of pneumonias, current practice is to follow the lobe, a whole lobe, or two lobes are involved, sometimes
etiologic classification (Table 17.1) which divides pneu- bilaterally. The lower lobes are affected most commonly.
monias into following 3 main groups: The sequence of pathologic changes described below
SECTION III
A. Bacterial pneumonia represents the inflammatory response of lungs in bacterial
B. Viral pneumonia infection.
C. Pneumonias from other etiologies.
In the present discussion, a combined approach of 1. STAGE OF CONGESTION: INITIAL PHASE
etiologic and morphologic classification will be followed. (Fig. 17.6,A). The initial phase represents the early acute
inflammatory response to bacterial infection and lasts for
A. BACTERIAL PNEUMONIA 1 to 2 days.
Grossly, the affected lobe is enlarged, heavy, dark red and
Bacterial infection of the lung parenchyma is the most congested. Cut surface exudes blood-stained frothy fluid.
common cause of pneumonia or consolidation of one or both Histologically, typical features of acute inflammatory
the lungs. Two types of acute bacterial pneumonias are response to the organisms are seen. These are as under
distinguished—lobar pneumonia and broncho-(lobular-) (Fig. 17.7):
Systemic Pathology
pneumonia, each with distinct etiologic agent and i) Dilatation and congestion of the capillaries in the
morphologic changes. Another type distinguished by some alveolar walls.
workers separately is confluent pneumonia which combines ii) Pale eosinophilic oedema fluid in the air spaces.
the features of both lobar and bronchopneumonia and iii) A few red cells and neutrophils in the intra-alveolar
involves larger (confluent) areas in both the lungs irregularly, fluid.
while others consider this as a variant of bronchopneumonia. iv) Numerous bacteria demonstrated in the alveolar fluid
by Gram’s staining.
Lobar Pneumonia
2. RED HEPATISATION: EARLY CONSOLIDATION
Lobar pneumonia is an acute bacterial infection of a part of a (Fig. 17.6,B). This phase lasts for 2 to 4 days. The term
lobe, the entire lobe, or even two lobes of one or both the hepatisation in pneumonia refers to liver-like consistency
lungs. of the affected lobe on cut section.
ETIOLOGY. Based on the etiologic microbial agent causing Grossly, the affected lobe is red, firm and consolidated.
lobar pneumonia, following types of lobar pneumonia are The cut surface of the involved lobe is airless, red-pink,
described: dry, granular and has liver-like consistency. The stage of
red hepatisation is accompanied by serofibrinous pleurisy.
1. Pneumococcal pneumonia. More than 90% of all lobar Histologically, the following features are observed
pneumonias are caused by Streptococcus pneumoniae, (Fig. 17.8):
a lancet-shaped diplococcus. Out of various types,