To correlate the progression of fibrosing lung lesions with an impediment of respiratory function, autopsy lungs from 15 patients who died after ingesting paraquat were submitted to histopathologic and morphometric analysis. The basic lung lesion proved to be a remodeling of alveolar structure caused by deposition of matrix on the septal surface, with proliferating mesenchymal cells. Morphometry, using a geometric model, and stereology showed that the mean thickness D of septa, including the matrix, enlarged after an exponential function of days after ingestion, reaching the alveolar breadth of 200 μm on day 22 and showing the alveoli totally obliterated. The exponential nature of this finding suggested that the paraquat lung is a self-accelerating and therefore a malignant lesion, in contrast to the slowly progressing fibrosis that follows usual interstitial pneumonia. In a whole-lung distribution analysis performed in three cases, D was shown to become increasingly heterogenous over time, with ordinary alveolar tissue remaining in at least part of the lung even in the most advanced case. Thus, heterogenous distribution of the disease was considered to be a sine qua non for a lung disease to run a chronic course.
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