Papulosquamous skin disorders are inflammatory reactions characterized by red or purple papules and plaques with scale. These diseases include psoriasis, pityriasis rubra pilaris, seborrheic dermatitis, pityriasis rosea, pityriasis lichenoides et varioliformis acuta, and parapsoriasis. Lichen planus and lichen nitidus are also considered papulosquamous disorders (see Chapter 12).

Habif TP: Clinical dermatology: a color guide to diagnosis and therapy, ed 4, St Louis, 2004, Mosby.

Psoriasis is a common, genetically determined, inflammatory, and hyperproliferative skin disease. Although there are morphologic variations, the most characteristic lesions consist of chronic, well-demarcated, dull-red plaques (Fig. 7-1A) with silvery scale found commonly on extensor surfaces and the scalp (Fig. 7-1B).

Guttate psoriasis is a variant of psoriasis usually seen in adolescents and young adults. It is characterized by crops of small, droplike, psoriatic papules and plaques (Fig. 7-2A). The word guttate is derived from the Latin gutta, which means “drop.” This type of psoriasis is often found in association with streptococcal pharyngitis, and treatment of the pharyngitis with oral antibiotics may improve or even clear the psoriasis.

Inverse psoriasis refers to psoriasis that involves intertriginous areas (axillae, groin, umbilicus). This distribution is opposite to the usual extensor distribution of psoriasis vulgaris. Psoriatic lesions with both distributions sometimes can be found in the same patients. Clinically, psoriatic lesions found in these “inverse” distributions often do not have scale, but consist of sharply demarcated red plaques that may become macerated and eroded (Fig. 7-2B). Treatment of inverse psoriasis usually involves low-potency (nonfluorinated) topical corticosteroids.

Asymmetrical arthritis, the most common form of psoriatic arthritis, usually involves one or several joints of the fingers or toes. The appearance of this type of arthritis can be similar to subacute gout and include “sausage-like” swelling of a digit due to involvement of the proximal and DIP joints and the flexor sheath (Fig. 7-3). Symmetrical polyarthritis resembles rheumatoid arthritis, but tests for rheumatoid factor are negative, and the condition is clinically less severe than rheumatoid arthritis. Although not common, DIP joint disease of hands and feet is the most classic presentation of arthritis with psoriasis. Destructive arthritis (arthritis mutilans) is a rare, severely deforming arthritis involving predominantly fingers and toes. Gross osteolysis of the small bones of the hands and feet can result in shortening, subluxations, and, in severe cases, telescoping of the digits, resulting in an “opera glass” deformity. Axial arthritis of the spine, which resembles idiopathic ankylosing spondylitis, manifests by itself or with peripheral joint disease.

A careful examination of the nails should be part of the skin exam, especially when evaluating a rash that might be psoriasis. Characteristic nail changes are found in 25% to 50% of psoriatics. These changes include nail pitting, discoloration, onycholysis, subungual hyperkeratosis, and nail deformity. Nail pitting, the most common nail finding in psoriasis, consists of small, discrete, punched-out depressions on the nail surface (Fig. 7-4). Circular areas of nail bed discoloration that resemble oil drops are often seen under the nail plate (hyponychium). The nail can become thin and brittle at the distal edge with separation from the nail bed (onycholysis) or thickened with subungual debris. Ridges, grooves, or even frank deformity of the nail plate can also be seen.