Vasculitis is a nonspecific term that encompasses a large and heterogeneous group of disorders that are characterized by inflammation of blood vessels. No uniform classification system for vasculitis exists, although subtypes are often categorized by the size of the vessels involved, the type of circulating immune complexes, and other histopathologic and clinical features. In many cases, damage to the blood vessels results in leakage of red blood cells and the development of palpable purpura on the skin. Other common cutaneous signs of vasculitis include skin ulcerations, Raynaud's phenomenon, and livedo reticularis. It is important to distinguish a primary systemic vasculitis from one associated with medications, infection, malignancy, or a connective tissue disorder, as the best course of treatment may be different.

Several types of vasculitis are more likely than others to be mistaken for cellulitis. Among these are polyarteritis nodosa and variants of leukocytoclastic (hypersensitivity) vasculitis, such as erythema elevatum diutinum, and urticarial vasculitis. Kawasaki disease and immunoglobulin A vasculitis (formerly Henoch-Schönlein purpura) may present with some lesions that mimic cellulitis, but this disease occurs predominantly in children. On rarer occasions, other vasculitides, such as granulomatosis with polyangiitis, Behçet's syndrome, cryoglobulinemia, and livedoid vasculopathy, may present similarly to cellulitis. Other primary vasculitic syndromes include eosinophilic granulomatosis with polyangiitis, microscopic polyangiitis, Takayasu arteritis, and giant cell (temporal) arteritis. Vasculitis may occur in association with such diseases as rheumatoid arthritis, systemic lupus erythematosus, and dermatomyositis.

Polyarteritis nodosa (PAN) refers to a necrotizing vasculitis of small- and medium-size arterioles. It is the least distinctive of the vasculitides. The exact etiology is unknown, but it likely involves immune complex deposition, autoantibodies, inflammatory mediators, and adhesion molecules. Any organ may be affected, but PAN most commonly involves the skin, peripheral nerves, kidneys, joints, and GI tract. Symptoms may include malaise, fever (in 50%), weakness, myalgias, arthralgias, abdominal pain, cutaneous ulcers, livedo reticularis, testicular pain, and weight loss. Polyarteritis nodosa has been associated with infection with hepatitis B, hepatitis C, HIV, CMV, parvovirus B19, HTLV, and streptococci. There appears to be an association with inflammatory bowel disease. Polyarteritis nodosa usually affects individuals in mid-to-late adulthood. It is slightly more common in men, and there is no apparent racial predilection.

"Leukocytoclastic vasculitis" (LCV) is a pathologic term that refers to inflammation of small blood vessels, usually with an inflammatory infiltrate and necrosis of the vessel wall. This form of vasculitis may be localized to the skin, where it is most commonly seen as palpable purpura, or it may also involve the other organs such as the kidneys, the joints, or the GI tract. The exact pathogenesis remains elusive, but LCV also appears to involve circulating immune complexes, autoantibodies, and inflammatory mediators.

Erythema elevatum diutinum refers to a rare subtype of LCV. It is associated with upper respiratory infections (especially streptococcal), HIV, hematologic disease, and rheumatologic disorders. It presents with arthralgias and erythematous to violaceous papules, plaques, and nodules with a predilection for extensor surfaces.

Urticarial vasculitis is also considered to be a variant of leukocytoclastic vasculitis. This presentation may be idiopathic, or it can occur in association with serum sickness, connective tissue disorders, infections, and after the administration of potassium iodide or nonsteroidal anti-inflammatory agents. Urticarial vasculitis affects mainly women. The skin lesions last 3-5 days. Episodic arthralgias are a major clinical manifestation and affect the wrists, fingers, knees, ankles, and toes. General features include fever, malaise, myalgias, lymphadenopathy, and hepatosplenomegaly.

Unlike cellulitis, the lesions of vasculitis often are multifocal and/or bilateral. Ulceration, palpable purpura, and livedo reticularis may be present. Be sure to ask the patient about the presence of specific symptoms such as myalgias, arthralgias, hematuria, or neuropathy.