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As we have seen in previous chapters, eruptions may manifest lesions with characteristic individual shapes, or characteristic configuration or arrangement of multiple lesions. These attributes of lesions and eruptions can be harnessed as diagnostic tools. As has been previously mentioned, shape is an inherent property of one individual lesion whereas configuration refers to the scheme or arrangement formed by confluent grouped lesions. Sometimes it is difficult to discern whether any given shape is made up of more than one lesion, and so, for convenience, the differential diagnoses of both shape and configuration are discussed in this chapter. For example, papules may be grouped together to form an annular configuration, as is seen in granuloma annulare. Or, a plaque of lichen planus may assume an annular shape if it is depressed in the center and raised at the periphery. Similarly, a line can be made up of a confluence of lesions (configuration), such as the confluent vesicles seen in poison ivy dermatitis, or of one lesion (a shape), such as in dermographism.
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The terms grouping and arrangement may be used synonymously. These terms are applied to lesions that are closely related spatially to each other. Examples include a group of bedbug bites arranged in an annulus or a line.
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An approach to the differential diagnoses of shape, grouping, and configuration was introduced in Chapter 1, including by etiologic classification and by primary lesion. This chapter will delve into these differential diagnoses in detail. This differential diagnostic approach will factor in diagnostic determinants such as reaction patterns and color. Each of the commonly encountered shapes, configurations, and groupings is listed in the textbox below and will be considered in turn. The textbox contains synonyms for each as well as a list of how each entity is formed.
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Commonly Encountered Shapes, Configurations and Groupings
Round—nummular, discoid (shape)
Annular—ring-shaped (shape, configuration, grouping)
Arcuate—part of a ring, arc-shaped (shape, configuration, grouping)
Concentric—rings within rings (configuration)
Polycyclic—formed from confluent annuli (configuration)
Gyrate—forms spirals (configuration)
Figurate—figure-forming (shape, configuration)
Linear—forms a line (shape, configuration, grouping)
Serpiginous—wavy, snake-like (shape, configuration, grouping)
Whorled—forms swirls (shape, configuration)
Reticulate—retiform, net-like, lacy (shape, configuration)
Herpetiform—herpes-like (configuration, grouping)
Agminate—clustered (grouping)
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The physical finding of macules or plaques that are completely round is very rare. Those dermatoses that are completely round are listed in Table 11.1 by reaction pattern and color. Pityriasis rotunda is a very rare dermatosis that may run in families or may be acquired. The acquired form has been associated with malnutrition, tuberculosis, and hepatocellular carcinoma. Plaques of pityriasis rotunda are very thin, are light brown, and have fine scales. The inherited type may resemble ichthyosis. Nummular eczema usually appears with a subacute morphology (scaly erythematous papules and/or plaques, along with round erosions and/or round crusts; see Figure 8.2A,B), but ...