Outline of Cutaneous Pathology

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DAVID ELDER, ROSALIE ELENITSAS, GEORGE MURPHY, ADAM RUBIN, XIAOWEI XU, and MISHA ROSENBACH

INTRODUCTION

The diagnosis of disease concerns the ability to classify disorders into categories that predict clinically important attributes such as response to therapy or prognosis. This permits appropriate interventions to be planned for particular patients. Understanding this process involves mastery of the stages of disease, the mechanisms of changes in morphology over time, and the molecular, cellular, gross clinical and epidemiologic reasons for the differences among diseases.

The process of cutaneous diagnosis at a simplistic level might involve the matching of a large number of attributes contained in classical descriptions of skin diseases with the presence or absence of the same attributes in a particular case under consideration. As there are hundreds of diagnostic categories, each having potentially scores of attributes, it is evident that an efficient strategy must be employed to enable diagnoses to be considered, dismissed, or retained for further consideration. Observation of an experienced dermatopathologist reveals a rapidity of accurate diagnosis that precludes the simultaneous consideration of more than a few variables. Indeed, the process of diagnosis by an experienced observer is quite different from that employed by the novice, and is based on the efficient and accurate recognition of combinations or patterns of criteria (1,2). Just as the recognition of an old friend occurs by a process that does not require the serial enumeration of particular facial features, this process of pattern recognition occurs almost instantly, and is based on broad parameters that do not (at least initially) require detailed evaluation.

In clinical medicine, patterns may present as combinations of symptoms and signs, or even of laboratory values. However, in dermatopathology, the most predictive diagnostic patterns are recognized with the scanning magnification of the microscope, or even before microscopy, as the slide is held up to the light, to evaluate the tissue’s profile and distribution of colors. Occasionally, a specific diagnosis can be made during this initial stage of pattern recognition, by a process of “gestalt” or instant recognition, but this should be tempered with a subsequent moment of healthy analytical scrutiny. More often, the scanning magnification pattern suggests a small list of possible diagnoses, a “differential diagnosis.” Then, features that are more readily recognized at higher magnification may be employed to differentiate among the possibilities. Put in the language of science, the scanning magnification pattern suggests a series of hypotheses, which are then tested by additional observations (1). The tests may be observations made at higher magnification, the results of special studies such as immunohistochemistry, external findings such as the clinical appearance of the patient, or the results of laboratory investigations. For example, a broad plaque-like configuration at scanning microscopy of small blue dots near the dermal–epidermal junction could represent a lichenoid dermatitis, or a lichenoid actinic keratosis. At higher magnification, the blue dots are confirmed to be lymphocytes, and one might seek evidence of parakeratosis, atypical keratinocytes, and plasma cells in the lesion, a combination that would rule out lichen planus and establish a diagnosis of actinic keratosis.

Most diagnoses in dermatopathology are established either by the “gestalt” method or by the process of hypothesis generation and testing (differential diagnosis and investigation) just described, but in either case the basis of the methods is the identification of simple patterns recognizable at scanning magnification that suggest a manageably short list of differential diagnostic considerations. This pattern recognition method was first developed and presented in a series of lectures given in Boston by Wallace H. Clark Jr. (3), and has been refined since for inflammatory skin disease by Ackerman (4,5), for inflammatory and neoplastic skin disease by Hood and colleagues (6,7), and more recently by Murphy (8), Barnhill and colleagues (9,10), Maize (11), Weedon (12,13), and McKee and colleagues (14,15). These authors have published texts based more or less extensively on the pattern classification. The present work, however, has been organized upon more traditional lines, in which diseases are discussed on the basis of pathogenesis (mechanisms) or etiology as well as reaction patterns. Such a classification has the advantage of placing disorders such as infections in a common relationship to one another, facilitating the description of their many common attributes. From a histopathologic point of view, however, the novice must learn that some infections, such as syphilis, can resemble disorders as disparate as psoriasis, lichen planus, cutaneous lymphoma, or granulomatous dermatitis.

Because the number of reaction patterns is limited, morphological simulants of disparate disease processes are common in the skin, as elsewhere in the body. For this reason, classification methods based on patterns and those based on pathogenesis are incompatible with each other. To partially circumvent this problem, the present section of this book presents a pattern-based classification of cutaneous pathology based on location in the skin, on reaction patterns, and, where applicable, on cell type, indexed to the more detailed descriptions of the disease entities discussed in other sections of the book. The section has been based (with permission) on the original lecture notes prepared by Wallace H. Clark Jr., MD, in 1965, and on the published works cited above, especially that of Hood and colleagues (6,7).

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