Chapter 13: Cutaneous Reactions to Exogenous Agents

REACTIONS TO TRAUMA AND IRRITATION

Clinical Features

The skin interfaces with and protects from an array of external irritants and other stimuli with pathological effects that range from minimal to severe. Minimal stimuli may result in redness, mild scale, and symptoms such as pruritus and burning. More severe irritation can lead to vesicles, eczematous changes, ulceration, and necrosis. Artifactual dermatitis, ulceration, friction blisters, and calcaneal petechiae are examples of clinical patterns seen in response to trauma and irritation.

Histopathologic Features

The histology associated with trauma and irritation follows the continuum observed clinically. Minimal trauma may result in parakeratosis, crust, mild spongiosis, superficial vasodilatation, and mild papillary dermal inflammation. More severe changes include prominent intracellular edema, spongiosis with vesicle formation and overlying parakeratotic scale-crust, ulceration, and polymorphous dermal inflammation with hemorrhage. The histopathology of ulceration is not specific as to etiology and consists of a polymorphous inflammatory infiltrate, varying degrees of vascular damage, hemorrhage, dermal necrosis, scale-crust formation, and fibrin deposition (Fig. 13-1). Nodular primary localized cutaneous amyloidosis has been reported after blunt trauma to the skin.¹ If foreign material is part of the process, it may be detected within the necrotic mass or in the adjacent dermis. Foreign material may be detected through the use of special stains and by polarized light microscopy. Location may be helpful in arriving at a clinicopathologic correlation; for example, transepidermal elimination of hemorrhage with intracorneal collections in a biopsy specimen from a plantar site would be compatible with the shear injury that produces calcaneal petechiae termed “talon noire.” We also use that term to describe hemorrhages into nail plate in the toes, often biopsied to exclude melanocytic proliferations of the matrix and subungual epithelium.

Differential Diagnosis

Most of the pathologic features associated with external trauma and irritation require clinical correlation because the histologic patterns are not diagnostic in isolation. Certain clinical and histologic features, such as a deep decubitus ulcer on the sacrum of an elderly person, may favor a more precise diagnosis.

REACTIONS TO RADIATION

Ionizing radiation can affect the skin. For the purposes of this text, radiation dermatitis is subclassified as acute, subacute, and chronic radiation dermatitis (Table 13-1). Rarely, a form of fibrosing dermopathy can be seen after diagnostic radiation. Termed “radiation-induced morphea,” it has a variable time of onset ranging from 1 month to 30 years after fluoroscopy, does not appear to be dose-dependent, and unlike radiation fibrosis, does not always remain confined to the radiation port.²