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The term reaction pattern is used to describe a specific cluster of morphologic findings that groups of diseases have in common. This term is also used by dermatopathologists to describe histopathologically similar groups of disease, but this chapter will focus on clinical reaction patterns that are used to describe physical examination findings rather than histopathologic findings. Each of these reaction patterns will be discussed in more detail in subsequent chapters.

The clinical reaction patterns are an important constituent of the diagnostician’s toolbox. An initial appraisal of the clinical features of a rash can guide a creation of a differential diagnosis based on reaction pattern. Five major clinical reaction patterns can be used to categorize dermatologic disease.

The Five Major Reaction Patterns are

  • Papulosquamous

  • Vesicobullous

  • Eczematous

  • Dermal

  • Vascular/red


The papulosquamous reaction pattern encompasses those dermatoses that present with scaly papules, plaques, or nodules. This is a vast group of diseases that span infectious, inflammatory, neoplastic, and drug-induced disease. Examples include inflammatory diseases such as psoriasis and lichen planus, and neoplastic disease such as actinic keratosis and squamous cell cancer in situ. Papulosquamous diseases can be differentiated from each other by employing the diagnostic boxes of the wheel of diagnosis. An additional diagnostic clue for papulosquamous diseases is the quality and character of the scale present. For example, the scale of psoriasis may be silvery or micaceous (mica-like), and the scale of actinic keratoses is adherent and has a gritty feel. An in-depth description of the papulosquamous reaction pattern is found in Chapter 6.


In the vesicobullous reaction pattern, the primary lesions are vesicles and bullae. Vesicobullous disease may also be accompanied by a host of secondary lesions (including collarettes of scale, erosions, ulcers, and crusts) and associated nonbullous primary lesions (such as urticarial plaques in bullous pemphigoid and erythematous papules and plaques in allergic contact dermatitis). These secondary and adjunctive primary lesions are replete with diagnostic clues and can help to create an approach. Additionally, clinical clues can guide an evaluation of the depth of the cleavage plane of vesicles or bullae and can therefore render important diagnostic information. Finally, elements of the wheel of diagnosis provide additional information, including grouping, configuration, and distribution of lesions. The vesicobullous reaction pattern will be explored in detail in Chapter 7.


The eczematous reaction pattern includes a constellation of findings, such as vesicles, bullae and erythema (of acute eczema), papules, scaling and erosions (of subacute eczema), and lichenification, scaling, and hyper- and hypopigmentation (of chronic eczema). The eczematous reaction pattern overlaps with the vesicobullous (acute eczema) and papulosquamous reaction patterns (chronic eczema). The eczematous reaction pattern is discussed in detail in Chapter 8.


The dermal reaction pattern refers to smooth ...

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