1. Clinically, prurigo pigmentosa presents itself mostly as intensely pruritic erythematous papules, papulovesicles, and, at times, vesicles, that involute in several days, leaving behind pigment in a netlike pattern. The lesions are distributed symmetrically and with predilection for the back (especially in the center of it in a "band" that often is wedge-shaped in cranio-caudal distribution), chest, neck, and lumbosacral region, tend to become confluent, and often assume bizarre shapes. Exacerbations and recurrences are the rule. The female:male ratio is 2:1, and the mean age of patients at the time of onset is 25 years. No family history can be obtained. There seems to be proclivity of prurigo pigmentosa for Asians; caucasians are affected, but less commonly.

2. The differential diagnosis of prurigo pigmentosa clinically includes dermatitis herpetiformis, linear IgA dermatosis, and acute lupus erythematosus. The reticular pattern of pigmentation at the end stage of the disease must be distinguished from that of confluent and reticulate papillomatosis of Gougerot and Carteaud and from macular amyloidosis. Each of the latter diseases may be identified for what it is by virtue of distribution of lesions, morphologic attributes of individual lesions, histopathologic findings, and response to therapy.

3. Histopathologically, prurigo pigmentosa begins with a sparse upper part of the perivascular infiltrate of neutrophils that, shortly thereafter, is found scattered in the interstitium of the superficial reticular dermis, in an edematous papillary dermis, and in the lowermost part of the slightly spongiotic epidermis. Those neutrophils then move swiftly through the epidermis, where they are distributed diffusely, but mostly in the upper part of it, as solitary units and as abscesses, or within spongiotic vesicles. A fully developed lesion is characterized by necrotic keratocytes disposed as solitary units, in clusters, or en masse in the vicinity of neutrophils. Eosinophils and lymphocytes, the latter of which may assume a patchy lichenoid pattern, come to predominate over neutrophils, both in the dermis and the epidermis. Spongiosis and ballooning may lead to intraepidermal vesiculation and extensive vacuolar alteration to subepidermal clefts and blisters. Scale-crusts house neutrophils and a few lymphocytes. Later, when lymphocytes predominate, the epidermis becomes slightly hyperplastic and parakeratotic. Eventually, melanophages appear in the papillary dermis and the epidermis becomes hyperpigmented. The essential changes sometimes are complicated by signs of scratching and rubbing. A biopsy of an early urticarial papule produces a specimen that is likely to exhibit findings that are diagnostic readily; biopsy of an older papule or papulovesicle also may yield a specimen from which sections of tissue house findings that can be diagnosed with confidence histopathologically as being those of prurigo pigmentosa. A lesion that is resolving may not be diagnosable by conventional micoscropy. Staining by immunofluorescence should be performed when diagnosis of prurigo pigmentosa is suspected; it is negative in prurigo pigmentosa, thereby permitting differentiation of it from dermatitis herpetiformis, linear IgA dermatosis, and acute lupus erythematosus.

4. Differential diagnosis histopathologically of prurigo pigmentosa at an early stage of the process includes urtica and the initial expression of leukocytoclastic vasculitis. Later, dermatitis herpetiformis, linear IgA-dermatosis, acute lupus erythematosus, eruptive psoriasis, and pustular dermatophytosis are among the considerations diagnostically. When lymphocytes come to predominate, Mucha Habermann disease has to be taken into account. Each of the forementioned diseases can be diagnosed with specificity by virtue of characteristic morphologic findings.

5. Diamino-diphenyl-sulfone (dapsone, DDS), sulfimethoxazol, minocyclin, doxycyclin, and macrolides are effective treatments for prurigo pigmentosa. Those drugs inhibit migration and/or function of neutrophilis and, thereby, can interrupt prurigo pigmentosa at an incipient stage of the process, when the infiltrate consists almost exclusively of neutrophils. No improvement results from either antihistamines or corticosteroids administered systemically.

6. Prurigo pigmentosa is distinctive clinically, histopathologically, and biologically. No cause of it has yet been established and pathogenesis of it awaits elucidation.

7. The name prurigo pigmentosa is misleading because the disease is not a prurigo, that is, a response to rubbing and scratching, and because the essential lesion is a red papule, not a pigmented macule. The designation tells nothing about the histopathologic findings crucial to characterization of it at an early stage, namely, neutrophils in the upper part of the dermis and in the epidermis. It makes no sense, however, to change the name of the disease unless all the other misnomers in dermatology are changed, too.