Nodular and diffuse dermatitis represents a group of diverse inflammatory conditions, each of which presents with inflammatory cells present in nodules filling portions of the dermis or diffusely filling the entire dermis. As with all inflammatory patterns, there is significant overlap between nodular dermatitis and diffuse dermatitis—many of the same diseases may present with either pattern. Similarly, cutaneous inflammatory conditions which are most often superficial or superficial and deep perivascular infiltrates may at times present with a nodular or diffuse pattern.
In this chapter, we will emphasize utilizing an algorithmic approach in diagnosing nodular and diffuse dermatitis. The concept of recognizing inflammatory conditions by their scanning power or architectural appearance is utilized to some degree in virtually every current dermatopathology text. The pattern approach to histologic diagnosis was firmly established by A. Bernard Ackerman in "Histologic Diagnosis of Inflammatory Skin Diseases, an Algorithmic Method Based on Pattern Analysis," first published in 1978. Much of this discussion is derived from Ackerman's method.
Diagnosis by pattern analysis has now been in use for almost 30 years. It has stood the test of time and its current popularity is due to the fundamental appeal and accessibility of an algorithmic approach and most importantly its utility and accuracy in arriving at a correct diagnosis. While the concept is simple, the application requires understanding of subtleties and much practice. The method involves recognition of various inflammatory patterns at scanning power, in this case nodules and diffuse infiltrates of inflammatory cells. Most patterns and their subdivisions are characterized by prototypes—that is, diseases which are the most common or best known examples of that pattern. Knowing the prototypes is exceedingly helpful in utilizing this approach to pathologic diagnosis. At each "fork" in the algorithm, decisions based on microscopic findings are made by the microscopist until reaching a limited differential diagnosis or specific diagnosis.
In inflammatory infiltrates determined to consist of nodules or diffuse involvement of the dermis, the next key question is "what are the preponderant inflammatory cells that make up the inflammatory infiltrates?" On this basis, nodular dermatitis can be subdivided into inflammatory conditions in which the following cells predominate: lymphocytes; neutrophils, neutrophils eosinophils, and plasma cells; eosinophils and plasma cells with lymphocytes; and histiocytes (granulomatous). Diffuse cutaneous inflammatory infiltrates, similarly, may be composed predominantly of the following inflammatory cells: lymphocytes; neutro-phils; neutrophils and eosinophils; plasma cells; mast cells; abnormal leukocytes; and histiocytes (granulomatous). Nodular and diffuse infiltrates of histiocytes (granulomatous dermatitis) are the subject of Chapter 9. It is of some importance to note that diagnosis of neoplasms also is facilitated by a similar pattern approach. As a consequence, various neoplasms are considered in the differential discussion of inflammatory conditions that follow.
In summary, when first viewing a microscopic slide of a skin biopsy containing nodules or diffuse infiltrates of inflammatory cells, the key question is "which inflammatory cell or cells are predominant?" The following table lists examples of cutaneous conditions that may be characterized by nodular or diffuse cellular infiltrates.
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