Pityriasis rubra pilaris
Several types have been described, but their causes are unknown. A defect in vitamin A metabolism was once suggested but has been disproved. The familial type has an autosomal dominant inheritance.
The familial type develops gradually in childhood and persists throughout life. The more common acquired type begins in adult life with redness and scaling of the face and scalp. Later, red or pink areas grow quickly and merge, so that patients with pityriasis rubra pilaris are often erythrodermic. Small islands of skin may be ‘spared’ from this general erythema, but even here the follicles may be red and plugged with keratin (Fig. 6.8). Similarly, the generalized plaques, although otherwise rather like psoriasis, may also show follicular plugging.
The palms and soles become thick, smooth and yellow. They often fissure rather than bend. The acquired form of pityriasis rubra pilaris generally lasts for 6– 18 months, but may recur. Even when the plaques have gone, the skin may retain a rough scaly texture with persistent small scattered follicular plugs.
There are usually no complications. However, wide-spread erythroderma causes the patients to tolerate cold poorly.
Psoriasis is the disorder closest in appearance to pity-riasis rubra pilaris, but lacks its slightly orange tinge. The thickening of the palms and soles, the follicular erythema in islands of uninvolved skin, and follicular plugging within the plaques, especially over the knuck-les, are other features that help to separate them.
A biopsy may help to distinguish psoriasis from pityriasis rubra pilaris; but, even so, the two disorders share many histological features.
The disorder responds slowly to systemic retinoids such as acitretin (in adults, 25–50 mg/day for 6– 8 months;). Oral methotrexate in low doses, once a week may also help. Topical steroids and kerato-lytics (e.g. 2% salicylic acid in soft white paraffin) reduce inflammation and scaling, but usually do not suppress the disorder completely. Systemic steroids are not indicated.