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Definition and aetiologies
The small vessel vasculitides of the lungs1-3 (table1) are inflammatory destructive processes that affect the arterioles, venules, and alveolar capillaries located within the interstitial compartment. An intense infiltration of activated neutrophils results in fibrinoid necrosis and dissolution of arteriolar and venular walls, thus compromising the vascular lumen. Accompanying the arteriolitis and venulitis is a distinct interstitial (alveolar wall) component referred to as necrotising pulmonary capillaritis.4-9 Necrotising pulmonary capillaritis can occur in isolation, in the absence of histological evidence of arteriolitis or venulitis. It is recognised by a marked interstitial neutrophilic infiltration and many of these cells are undergoing leucocytoclasis or fragmentation (fig 1). Because these neutrophils are constantly undergoing cell death (apoptosis), pyknotic cells and nuclear fragments (dust) accumulate within the lung parenchyma. The interstitial space becomes broadened by oedema, fibrin, and the neutrophilic infiltrate and eventually undergoes fibrinoid necrosis (fig 2). During this process the integrity of interstitial capillaries is damaged, permitting red blood cells to traverse the now incompetent alveolar capillary basement membranes and freely enter the interstitial compartment and flood alveolar spaces. Clinically this is referred to as diffuse alveolar haemorrhage, and this accompanies most episodes of small vessel vasculitis in the lungs regardless of aetiology. Fibrin and neutrophils also traverse the damaged alveolar capillary basement membranes and enter the alveolar spaces with the red blood cells. Other histological features in the small vessel vasculitides of the lung include arteriolar and capillary thrombosis, organising haemorrhage, epithelial type 2 cell hyperplasia, and eventually the accumulation of free parenchymal haemosiderin and haemosiderin containing macrophages in alveolar spaces (fig 3). With time, and after repeated episodes of diffuse alveolar haemorrhage due to pulmonary capillaritis, both interstitial pulmonary fibrosis and a progressive obstructive lung disease with the physiological and computed tomographic appearance of emphysema have been described.9 …
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