| Histopathology No single pathognomonic
lesion is found that would permit certain histopathologic diagnosis of HPS. In fact, the
incipient stages of ARDS can create a picture of pulmonary edema similar to HPS. However,
the total picture is rather distinctive. Pathology in HPS patients is characterized mainly
by pulmonary findings, as well as findings in the spleen, liver, and lymph nodes.
Grossly, the lungs are dense, rubbery and heavy, usually weighing twice as much as the
average lung. They are often found floating in a pool of yellow serous fluid within the
pleural cavity.
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Image 1.: Low-power photomicrograph
showing interstitial pneumonitis and intraalveolar edema. To view large image version,
click directly on the image button. |
The pathologic lesions are primarily vascular with
variable degrees of generalized capillary dilation and edema. Morphologic
changes of the endothelium are uncommon and, when present, consist
of prominent and swollen endothelial cells. Histopathologic lesions
are mainly seen in the lung and spleen. In most cases, the lungs
reveal a mild to moderate interstitial pneumonitis with variable
degrees of congestion, edema, and mononuclear cell infiltration.
The cellular infiltrate is composed of small and enlarged mononuclear
cells with the appearance of immunoblasts. Focal hyaline membranes
are observed, as well as extensive intraalveolar edema and fibrin.
Neutrophils are scanty, and the respiratory epithelium is intact
in typical cases, with no evidence of cellular debris, nuclear fragmentation,
or type II pneumocyte hyperplasia.
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Image 2.: Photomicrographs showing
histopathological features less commonly seen in cases of HPS.
A. Lung showing extensive interstitial and alveolar fibrosis. Note the increased
interstitial cellularity with numerous fibroblasts.
B. Patchy areas of alveolar septal thickening and prominent hyaline membranes.
C. Higher magnification showing typical dense laminated hyaline membranes.
D. Alveolar septum showing prominent type II pneumocyte proliferation.
E. Abundant polymorphonuclear leukocytes fill alveolar spaces with focal destruction of
alveolar septa.
F. Higher power magnification showing the antraalveolar exudate composed mainly of
polymorphonuclear leukocytes, red blood cells, and fibrin.
Original magnifications: A & B, x 50; C, x 100; D, x 158; E, x 50; F, x 158.
To view large versions of these images, click directly on the image
buttons.
Images courtesy Sherif R. Zaki, MD, Ph.D. |
Among patients who die after a longer-than-average course of the disease, and in lung
biopsy specimens from survivors, the histopathologic changes are more characteristic of
exudative and proliferative stages of diffuse alveolar damage. In these cases,
proliferation of reparative type II pneumocytes, severe edematous and fibroblastic
thickening of the alveolar septa with severe airspace disorganization, and distortion of
lung architecture can be seen.
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Images 3 and 4.: Immunoblasts in the
periarteriolar sheath of the spleen. Note prominent nucleoli and high nuclear to
cytoplasmic ratio. Images courtesy Sherif R. Zaki, MD, Ph.D. |
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Other typical histopathologic findings are seen in lymphoid tissues of HPS
patients. These include the presence of immunoblasts within the red pulp and
periarteriolar sheaths of the spleen and paracortex, within sinuses of lymph nodes, and in
the peripheral blood.
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National
Center for Infectious Diseases
