The vascular reaction pattern comprises dermatoses that present with red, nonscaly macules, papules, or plaques. The term vascular reaction pattern may be misleading; it does not refer solely to vascular proliferations but rather to any red dermatosis without scale or blisters. Therefore, this reaction pattern can be synonymously referred to as the red reaction pattern. It is an important reaction pattern to be aware of as it houses a broad heterogeneous group of diseases that has, in common, their color.
The Primary Lesions in This Reaction Pattern:
In contrast to the other reaction patterns, dermatoses in this reaction pattern display no scales, vesicles, or bullae, and there are no nodules, either, and so, to the novice clinician, these lesions may appear featureless. Therefore, and especially with red macules, there may be few diagnostic determinants to assist with making a diagnosis. So, it is useful to consider an approach to these lesions.
As mentioned in the previous chapter, the vascular reaction pattern overlaps with the dermal reaction pattern in that red plaques may fit into either of these reaction patterns. Dermatoses that may be classified as either a vascular or a dermal reaction pattern are listed in Table 9.1. Some vascular reaction pattern diseases may develop vesicles and bullae, such as erythema multiforme, Stevens–Johnson syndrome/toxic epidermal necrolysis (SJS/TEN), and fixed drug eruption. Also, bullous pemphigoid may begin with urticarial plaques only—the prebullous phase—and then progress to bulla formation (see Figure 7.22). The vascular reaction pattern may progress to the papulosquamous reaction pattern. An example here is secondary syphilis, which early on may have smooth red macules or thin papules, and later may develop scale. Early erythema annulare centrifugum may similarly lack scale. Early contact dermatitis (eczematous reaction pattern) may present as an edematous erythematous plaque (vascular reaction pattern; Figure 10.1). Langerhans cell histiocytosis spans many reaction patterns (Figure 10.2); there may be scale (papulosquamous reaction pattern), crusting (and so may connote vesicobullous reaction pattern), and purpuric macules (vascular reaction pattern).
Early acute contact dermatitis to poison ivy showing a broad edematous, erythematous plaque on the abdomen.
Langerhans cell histiocytosis displaying many morphologies that span reaction patterns, including purpuric macules.
An important diagnostic consideration in the initial approach to red macules, papules, and plaques is whether they blanch. To perform diascopy, a microscope slide is placed on top of the lesion in question. If the lesion blanches, then extravascular blood is absent. If it does not blanch, the diagnosis is usually extravascular blood, such as in petechiae or purpura.
Perform Diascopy on Red Macules, Papules and Plaques: