Pleural, Peritoneal, and Pericardial Effusions
that can be stated is that the cytologic findings are consistent
with pneumonia.
Key cytologic features of effusion caused by pneumonia
• Inflammatory cells of various types depending on the
nature and duration of the pneumonia.
Pulmonary infarct is characterized by a subpleural zone of hem-
orrhagic necrosis of the pulmonary parenchyma, usually in a
cone shape with the base of the cone formed by the pleura. The
necrotic parenchymal tissue induces an inflammatory reaction
on the overlying pleura, which retains a thin zone of viability
owing to its own separate blood supply. It is the area of pleural
inflammation that is responsible for any ensuing exudative effu-
sion. We have also seen examples of peritoneal effusion from
patients with infarct of the small intestine or of the liver.
In pleural effusions from patients with pulmonary infarcts,
the cellularity ranged from slight to profuse, with most speci-
mens being classified as moderate to profusely cellular. Many
of the pleural fluids had no dominant cytologic picture, but
mesothelial cells, histiocytes (including hemosiderophages),
lymphocytes, and neutrophils were present. Some fluids were
eosinophilic, with eosinophil counts ranging from 15 to 66%.
Other fluids were highly cellular, with most cells being neu-
trophils, and some fluids showed a heavy lymphocytosis. In
our cases of small intestinal infarct resulting in effusion, two of
the four specimens contained numerous neutrophils, one con-
tained numerous lymphocytes, and the fourth contained only
a few neutrophils. Again, we have situations in which cytologic
examination of serous effusion does not provide a diagnosis of
a specific pathologic entity, although it may be useful to rule out
the presence of cancer cells.
Key features of infarct
• Non-specific inflammatory picture; and
• Mixture of inflammatory cells.
Pneumothorax is frequently accompanied by effusion, possibly
induced by the presence of air and its impurities in the pleu-
ral cavity as well as any pulmonary parenchymal contents that
also entered the cavity. The cytologic picture of pneumothorax
is nonspecific, but one feature that we have noted in our speci-
mens and that has been noted by others is a high proportion of
eosinophils, possibly due to allergenic impurities in the air.9,38
The presence of air in the pleural cavity may also stimulate mes-
othelial cells to enlarge, manifested by the presence of numerous
hypertrophic mesothelial cells in an accompanying effusion.
Key cytologic features of pneumothorax-related effusion
• High proportion of eosinophilic leukocytes; and
• Hypertrophic mesothelial cells.
It has long been known that the typical pleural effusion caused
by tuberculosis has a high proportion of lymphocytes and very
few or no mesothelial cells, although in tuberculous pleuritis
of short duration the effusion may contain some neutrophils
and mesothelial cells.118-121 The scarcity or absence of meso-
thelial cells in tuberculous pleural effusions is attributable to
deposition of fibrin on the pleural surface, either sealing off
the mesothelial cells or destroying them or both. The presence
of numerous mesothelial cells in a pleural effusion should be
regarded as strong evidence that the effusion is not caused by
tuberculosis. Nontuberculous confluent inflammatory exudates
and neoplastic deposits may either cover the mesothelium or
occupy so much of it that effusions caused by these processes
may also be devoid of mesothelial cells. Most tuberculous pleu-
ral effusions contain 80 to 100% lymphocytes. They may con-
tain small numbers of neutrophils, macrophages, plasma cells,
and red blood cells. Multinucleated giant macrophages and epi-
thelioid cells have been recorded in tuberculous effusions but
must be considered a rarity.122,123
A reported study analyzed T-cell subsets and activation mark-
ers on T cells in pleural effusions in an attempt to distinguish
between tuberculous pleurisy and other lymphocyte-rich effu-
sions.124 However, significant differences were not observed,
leading to the conclusion that phenotypic analysis of lym-
phocytes is of limited diagnostic usefulness in such situations.
At some stage of the tuberculous process, an effusion may
become secondarily infected, possibly by rupture of a tuberculous
cavity of the lungs into the pleural space. In such cases, pleural
empyema may ensue, and the cytologic picture changes to one
of virtually all neutrophils. When empyema is of long duration,
pleural fibrosis may develop, causing it to become encysted.
In this situation, the empyema fluid may lose its cellularity,
resulting in a fluid of low cellularity containing cholesterol
crystals due to the breakdown of cell membranes.
In peritoneal tuberculosis, the predominant cellular profile is
likely to be lymphocytic, although histiocytes, neutrophils, and
sometimes mesothelial cells may also be present. In contrast to
tuberculous pleuritis, the presence of mesothelial cells does not
preclude peritoneal tuberculosis. In tuberculous pericarditis, the
cytologic picture is similar to that of tuberculous pleuritis.
Key cytologic features of tuberculous effusions
• High proportion of lymphocytes and few mesothelial
cells; and
• If secondary infection occurs the effusion becomes
Hepatic cirrhosis
The long-standing transudates occupying the peritoneal cavity
of patients with hepatic cirrhosis are typically sterile, with few or
no acute inflammatory cells. In our analysis of 20 such effusions,
the cellularity was generally low, although six were profusely cel-
lular, with the dominating cell being mesothelial. In six of the
effusions, mesothelial cells were very few or absent. The other
cells consisted almost entirely of lymphocytes and histiocytes in
small numbers. A peritoneal effusion of a patient with hepatic
cirrhosis that contains more than a few neutrophils should raise
the question of whether the patient is developing spontaneous
bacterial peritonitis, a complication of decompensating hepatic
For some reason, peritoneal effusions from patients with
hepatic cirrhosis seem to pose a problem in cytodiagnosis owing
to the presence of hypertrophied mesothelial cells, either soli-
tary or in clusters. Analysis of these cells is no different from
analysis of such cells found in different circumstances. In fact,
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