• Chronic, usually asymptomatic patches or thin plaques with fine scale whose color varies from pink to red-brown; may have associated epidermal atrophy and occasionally poikiloderma (large plaque parapsoriasis).

• Two major forms of parapsoriasis are small plaque (lesions usually <5 cm in diameter) and large plaque (usually >5 cm); digitate dermatosis is a form of the former, whereas retiform parapsoriasis is a variant of the latter (Figs. 7.1 and 7.2).

• While the distribution may be limited or more generalized, there is a tendency for an increase in extent over time; large plaque parapsoriasis can favor the sun-protected ‘girdle’ area; both forms usually occur in adults.

• Controversy exists regarding the percentage of cases of large plaque parapsoriasis that eventually evolve into mycosis fungoides.

• Histologically, parakeratosis and nonspecific spongiotic dermatitis is seen in small and large plaque; large plaque may have a more lichenoid infiltrate.

• Infiltrate of CD4⁺ T lymphocytes, often clonal, in large plaque > small plaque, leading to the term ‘clonal dermatitis’.

• DDx: small plaque – pityriasis rosea (PR), PR-like drug eruption, pityriasis lichenoides chronica, guttate psoriasis, secondary syphilis; large plaque parapsoriasis – patch stage mycosis fungoides (MF), MF-like drug eruption; if a few lesions, consider tinea corporis.

• Rx: topical CS, sunlight, phototherapy (e.g. NBUVB).

• PLEVA, also known as Mucha–Habermann disease, and PLC exist along a clinicopathologic spectrum such that patients can have characteristic lesions of both disorders, either concurrently or in tandem.

• In both forms, there are recurrent crops of papules with individual lesions spontaneously resolving over weeks (PLEVA) to months (PLC); only occasionally is there an obvious trigger (e.g. viral infection, medication); the entire course of the disorder can last for years.

• PLEVA occurs more commonly in younger age groups; it is characterized by widespread erythematous papules that are often crusted but may be vesicular or pustular (Fig. 7.3A–C).

• The onset of PLEVA can be abrupt, and recurrences typically occur for months to years; there is an unusual ulcerative form that is accompanied by fever, lymphadenopathy, arthritis, and mucosal involvement.

• PLC is characterized by pink to red-brown papules with scale and can resolve with post-inflammatory guttate hypopigmentation (Fig. 7.3D,E).

• Histologically, parakeratosis, an interface dermatitis with necrotic keratinocytes, and extravasation of red blood cells are seen; the infiltrate is composed of T cells that are often monoclonal; in PLEVA, the infiltrate may be wedge-shaped and neutrophils may be seen.

• DDx: PLEVA – varicella or other viral exanthem, e.g. Coxsackie, disseminated zoster without a dermatome (more limited duration), lymphomatoid papulosis (often fewer larger lesions), arthropod reactions, small vessel vasculitis; PLC – small plaque parapsoriasis, pityriasis rosea, secondary syphilis, guttate psoriasis, lichen planus.

• Rx: topical CS, prolonged courses of antibiotics (e.g. erythromycin, tetracyclines), phototherapy (e.g. NBUVB); severe cases may require MTX in consultation with a dermatologist.

• Occurs more commonly in adolescents and young adults; in general, individuals are healthy and have no systemic complaints/symptoms.

• The etiology is unknown, but viral infections may serve as a trigger.

• Lesions increase in number and extent over a few weeks but then spontaneously resolve; classically, the initial lesion, known as the ‘herald patch,’ is often the largest (Fig. 7.4).

• The distribution is that of a 1920s bathing suit – proximal extremities and trunk; occasionally, an inverse pattern is seen in which the majority of lesions are in the axillae and/or groin (Fig. 7.5).

• Pink to salmon-colored papules or plaques are round or oval in shape (Fig. 7.6), with their long axes following Langer’s lines of cleavage (Fig. 7.7), creating a ‘Christmas tree’ pattern on the trunk; scale, both fine white centrally and as a collarette at the edge of the lesion, is the most common secondary change, but occasionally crusting, vesicles, purpura, or even pustules may be seen.

• The average duration is 6–8 weeks, with some cases lasting for months.

• Histologically, mounds of parakeratosis are seen, accompanied by spongiosis and a mild perivascular and interstitial lymphocytic infiltrate; extravasation of red blood cells may occur.

• DDx: guttate psoriasis, secondary syphilis (preceding chancre, usually genital; accompanied by malaise, lymphadenopathy, other mucocutaneous signs such as condyloma latum, palmoplantar lesions; positive Venereal Disease Research Laboratory [VDRL] or rapid plasma reagin [RPR] test), pityriasis lichenoides chronica (especially if persistent), nummular dermatitis (if vesicular), pityriasis rosea-like drug eruptions (e.g. ACE inhibitors, metronidazole).

• Rx: topical anti-pruritic lotions or CS (for the minority of patients with associated pruritus), natural sunlight, 14-day course of erythromycin, 10-day course of azithromycin, NBUVB.

• One of the dermatologic disorders that can lead to an erythroderma (Fig. 7.8), often with an onset in the head and neck region; peaks of incidence – first to second decade of life and sixth decade of life.

• It is characterized by a salmon or orange-red color, islands of sparing, follicular papules (including within the relatively spared areas and on the dorsal fingers), and a waxy keratoderma (Fig. 7.9).

• May be exacerbated by exposure to UV light; classic forms spontaneously resolve within 3–5 years (Fig. 7.10).

• Five major forms have been described, with distinctions based on age of onset and distribution, with the adult classic form being the most common (Fig. 7.10); occasionally PRP is familial and can be due to CARD14 mutations.

• Histologically, there is alternating ortho- and parakeratosis, both vertically and horizontally, within the stratum corneum (checkerboard pattern); additional findings are follicular plugging with a shoulder of parakeratosis, acantholysis within the epidermis, and variable inflammation.

• DDx: other causes of erythroderma, in particular psoriasis and Sézary syndrome, and an unusual form of dermatomyositis seen more often in Asians (Wong type); in children, also progressive symmetric erythrokeratoderma; early on, seborrheic dermatitis.

• Rx: oral isotretinion, acetretin, methotrexate, tumor necrosis factor-α inhibitors (mixed results); response should be seen within 6 months.

• Seen primarily in the Far East, Mediterranean basin, and Africa; favors individuals with darker skin phototypes.

• Large, asymptomatic, circular and polycyclic patches with scale that are often hyperpigmented; the lesions are also well demarcated and lack inflammation clinically, occurring on the trunk and extremities.

• Associated with malnutrition and secondarily internal malignancies or systemic infections; occasionally, familial, especially in Caucasians.

• Histologically, resembles ichthyosis vulgaris with hyperkeratosis and a reduced granular layer.

• DDx: leprosy, large plaque parapsoriasis, tinea corporis, tinea versicolor.

• Originally described in the axillae of adults (primarily women), but can involve other intertriginous zones and in infants, the diaper area.

• Red-brown plaques with scale, often thick, that may be related to chronic irritation (Fig. 7.11); there may be secondary maceration and pruritus.

• Diagnostic histologic findings of marked compact parakeratosis with retained keratohyaline granules within the stratum corneum.

• DDx: other causes of intertrigo (e.g. seborrheic dermatitis, inverse psoriasis, candidiasis), irritant or allergic contact dermatitis, erythrasma, Hailey–Hailey disease.

• Rx: discontinuation of topical irritants (e.g. mineral salt-containing crystals used as ‘natural’ deodorants, biodegradable diapers), mild topical CS.