Secondary lesions are those lesions that are left behind when primary lesions resolve, or when primary lesions are altered through scratching or rubbing, for example. Secondary lesions may hold morphologic clues that can be leveraged to make a diagnosis, even in the absence of primary lesions. This chapter summarizes secondary lesions and the diagnostic information that they can offer. In this chapter, reference will also be made to the hypo- or hyperpigmented postinflammatory macules that are left behind after lesions resolve and the diagnostic clues that they provide.
Scales represent a heterogeneous group. Some may be an inherent part of the primary lesion (and not a secondary lesion, by its strict definition), such as when they lie atop papules and plaques. In this situation, scales can render a wealth of diagnostic information. Examples include the silvery scales of psoriasis, or the gritty scale of an actinic keratosis. Chapter 6 provides more details about primary scales, including a comprehensive morphological list as seen in Table 6.3. On the other hand, scales may be true secondary lesions, when they are seen after resolution of primary lesions. An example is the superficial desquamative scale that may be seen after an exanthematous drug eruption. Desquamative scale is also seen when infections resolve, such as in a treated cellulitis or erysipelas. The term peeling can also be applied to the desquamation that it seen when an inflammatory disorder resolves. Superficial scales can be peeled off the skin surface. Sheets of scale can be peeled off in staphylococcal scalded skin syndrome. This is a toxin-mediated process, where the exotoxins produced by phage group II staphylococci cleave desmoglein 1 in between keratinocytes. Sheets of superficial epidermis can be peeled away. This appearance is also known as exfoliative scale. Exfoliative scales are seen in pemphigus foliaceus following rapid rupture and repair of superficial bullae. Sheets of desquamative scale can also sometimes be peeled off when severe sunburn resolves.
Another example of a secondary scale is the collarette. The term collarette refers to a circumferential margin of fine scale that is loosely adherent peripherally and centrally detached. Its presence implies a preexisting inflammatory papule or plaque, or vesicle or bulla.
A collarette of scale implies a preceding inflammatory papule or plaque, or a preceding vesicle or bulla.
Collarettes are seen when the superficial bullae of bullous impetigo resolve, or in any of the superficial autoimmune blistering diseases (see Chapter 7), such as pemphigus vulgaris.
Examples of Secondary Scales
Desquamation/peeling: Fine superficial scales that peel off the skin surface
Exfoliative scale: Sheets of desquamative scale
Collarette: A circumferential margin of fine scale that is ...