VASCULITIS Inflammation of blood vessels, which is often segmental, may be generalized or localized, and constitutes the basic pathogenetic process in a variety of rheumatic diseases and syndromes. Pathology Vasculitis may follow a variety of etiologic mechanisms, but the histologic abnormalities are limited. In acute lesions, the predominant inflammatory cells are polymorphonuclear leukocytes; in chronic lesions, lymphocytes. The inflammatory process is often segmental, with scattered foci of intense inflammation in an otherwise normal vascular tree. At the affected sites, variable degrees of cellular infiltration and necrosis or scarring within one or more layers of the vessel wall are seen. Inflammation within the media of a muscular artery tends to destroy the internal elastic lamina. Inflammation at any point in the vessel wall tends to resolve by fibrosis and intimal hypertrophy. On occasion, certain distinguishing histologic events may be seen, eg, numerous giant cells, or patchy areas of fibrinoid necrosis where complete sections of the vessel wall have undergone inflammatory destruction and liquefaction. Secondary occlusion of the lumen due to intimal hypertrophy or intraluminal thrombus formation is frequent. In addition, once the vessel wall integrity is breached, RBCs and fibrin may leak into the surrounding connective tissue. Any type and size of vessel may be involved--arteries, arterioles, veins, venules, or capillaries. However, most of the versatile and variable pathophysiology can be ascribed to arterial inflammation, with the potential for partial or total vascular occlusion and subsequent tissue necrosis. Although vasculitis is often segmental or focal, biopsy of even clinically suspected tissue may not always provide definitive histologic diagnosis. However, the intimal and periadventitial fibrous response to a focus of intense vessel wall inflammation frequently extends up and down the vessel from the primary insult, so that the histologic appearance of intimal hypertrophy and fibrosis or perivasculitis would imply the presence of an adjacent area of vasculitis. Classification Classification of the numerous vasculitic disorders according to the size and depth of the predominant vessel involved is most useful (see also individual discussions elsewhere in THE MANUAL). This often reflects the depth of the lesions beginning from the integument and working viscerally. Thus, predominant inflammation of a postcapillary venule with neutrophilic infiltration leads to the typical histologic appearance of leukocytoclastic angiitis, manifesting clinically as palpable purpura and best typified by Henoch-Sch?nlein syndrome or Pseudomonas septicemia. The vascular inflammation of the deep dermal panniculus, mediated mainly by septal perivascular lymphocytes and presenting clinically as tender, deep, indurated red bumps on the arms and legs, is typical of erythema nodosum. Inflammation of medium-sized muscular arteries with the histologic features of a pleomorphic transmural infiltrate, fibrinoid necrosis, destruction of the internal elastic lamina, and postinflammatory aneurysm formation is exemplified by polyarteritis nodosa. When a similar type of process is largely confined to the extracranial carotid tree and is associated with a lymphocytic infiltrate and the formation of giant cells clustered around the luminal aspect of the disrupted elastic lamina, severe headaches may result and giant cell arteritis is recognized. Finally, when there is inflammation of the largest central vessels such as the aorta and its branches, mainly mediated by adventitial or medial lymphocytic infiltration and fibrous scarring with a tendency to postinflammatory stenosis, the loss of major pulses may become clinically evident and Takayasu's arteritis is recognized. Rheumatoid nodules and other lesions of the rheumatic diseases appear to have central foci of vasculitis as their basic pathogenetic mechanism. Much of SLE pathophysiology can be ascribed to vasculitis with or without secondary vascular occlusion, particularly evident in the renal glomerular tufts. Polymyositis or dermatomyositis of childhood frequently includes an element of vasculitis, not only in muscle but also at extramuscular and extracutaneous sites. Even the bland-appearing and extensive intimal proliferation of small arteries typifying progressive systemic sclerosis may be a postinflammatory event. Other syndromes dominated by serious vasculitis include polyarteritis nodosa and Wegener's granulomatosis. Back Fluid & Electrolyte

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