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INTRODUCTION

Distribution in the present context refers to the body location of any eruption. This chapter focuses on differential diagnoses for each distribution. Classic and variant distribution patterns for eruptions will be discussed. Sites of predilection for neoplasms will also be mentioned. These differential diagnoses will be divided by reaction pattern and other morphologic attributes, such as pustules or macules of different colors, when useful. Some of the classic distribution patterns that eruptions tend to follow were mentioned in Chapter 1. In this chapter, these and additional locations will be considered, as are listed in the textbox below.

Distribution of Primary Lesions

  • Photoexposed

  • Flexures

  • Intertriginous

  • Seborrheic

  • Extensors

  • Truncal

  • Acral

  • Palms and soles

  • Dorsal hands

  • Digits

  • Webspaces

  • Periorbital

  • Ear

  • Nose

  • Face

  • Generalized

PHOTOEXPOSED ERUPTIONS

In photoexposed eruptions, light plays a role in the pathogenesis of the eruption. As mentioned previously, the sites that are most commonly involved in photoexposed eruptions include one or more sites on the face, neck, the “V” of the chest, the upper back, and the dorsal arms and hands. Other sites, such as dorsal feet and lower legs, may also be involved.

Photoexposed Sites

  • Face

  • Neck

  • “V” of the chest

  • Upper back

  • Dorsal arms and hands

  • Dorsal feet and lower legs

Also note spared areas such as the eyelid creases, the area of the upper neck that is in shadow from the chin, and Wilkonson’s triangle (a triangular area behind the earlobe).

Photoprotected Sites

  • Eyelid creases

  • Upper neck that is in shadow from the chin

  • Triangle behind the earlobe

An etiologic classification of the photoexposed differential diagnosis appears in Table 1.6. Many dermatoses that occur in photoexposed areas are rarely encountered. Some are rare genetically inherited syndromes. Phototoxic and photoallergic eruptions are more common, and medications as a cause, or underlying lupus or some of the porphyrias should always be borne in mind. A comprehensive morphologic differential diagnosis of eruptions with a predilection for photoexposed sites is listed in Table 1.7 by primary lesion. Table 12.1 outlines the photoexposed differential diagnosis by reaction pattern. Psoriasis may be photoacentuated. Some lichenoid drug eruptions (classically from tetracyclines, hydrochlorothiazide, and furosemide) as well as lichen planus actinicus have a photodistribution. Lichen planus actinicus is lichen planus that is localized to sun-exposed areas. There are four subtypes, including a classic plaque type, an annular variant, a dyschromic variant and a pigmented variant. Discoid lupus erythematosus (DLE) and the psoriasiform variant of subacute lupus erythematous (SCLE; see Figure 12.1 and 1.5D) both present with papulosquamous morphology. The annular, polycyclic type of SCLE lacks scale and falls into the dermal reaction pattern category. Tumid lupus is also a dermal reaction pattern disease. Systemic lupus erythematous (SLE) may present with a malar rash or a more widespread photosensitive eruption (both vascular reaction pattern). In severe SLE, there may be epidermal necrosis, and ...

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