The misdiagnosis of superficial thrombophlebitis as cutaneous polyarteritis nodosa: features of the internal elastic lamina and the compact concentric muscular layer as diagnostic pitfalls.

The presence of an internal elastic lamina and a compact concentric muscular layer are the cardinal histologic clues for distinguishing a small muscular artery from small muscular vein. However, the subcutaneous muscular veins in the lower legs usually have thick muscular layers with the proliferation of concentric intimal elastic fibers, which resembles the internal elastic lamina of an artery. Moreover, vertical biopsy specimens of the muscular veins can reveal a compact concentric muscular layer with a round luminal appearance, which also resembles the muscular layer in an artery. As these 2 histologic features are commonly accepted as crucial clues for identifying small to medium-sized muscular arteries, it seems that many cases that are histopathologically proven to be deep dermal or subcutaneous arteritis-including cases documented in numerous dermatology, rheumatology, and dermatopathology-related journals as cutaneous polyarteritis nodosa in Behçet's disease and relapsing polychondritis or granulomatous arteritis in nodular vasculitis-are actually consistent with the features of phlebitis or thrombophlebitis. Cutaneous polyarteritis nodosa and subcutaneous thrombophlebitis are usually found in the lower legs and may present with the same cutaneous manifestation of widespread tender or painful nodular erythema. This also accounts for the difficulty in clinically and histopathologically distinguishing between these 2 disorders. Nevertheless, it is important to make a distinction between arteritis and phlebitis because misdiagnosing subcutaneous thrombophlebitis as polyarteritis nodosa may lead to overtreatment with high doses of systemic steroids. Although the veins in the lower legs may have a compact concentric smooth muscle pattern with a round lumen and the intimal elastic fiber proliferation mimicking the characteristic features of arteries, the elastic fibers in the muscular layer are distributed between the bundled smooth muscle in veins, whereas the elastic fibers are scantly distributed in the medial muscular layer in arteries. A diagnostic assessment that is based on the amount of the elastic fibers in the muscular vessel wall more reliably distinguishes a vein from an artery than does the presence or absence of the internal elastic lamina or a smooth muscle pattern.