Pathology of Pityriasis Rubra Pilaris - Top 10 Facts

Pityriasis rubra pilaris (PRP) is a rare erythematous squamous disorder of the skin of unknown etiology.

It was first described by Alaudius Tarral in 1828. In 1856 the Parisian dermatologist Alphonse Devergie named it pityriasis pilaris. In 1889, Besnier renamed the disease to Pityriasis rubra pilaris.

This skin disorder usually starts on the face or scalp and spreads caudally within a few weeks. In juveniles the disease starts on the lower half of the body. Localized lesions have been reported on the elbows and knees.

This lesion is characterized by keratotic follicular papules, well-delineated salmon-coloured erythematous plaques covered with fine powdery scales, interspersed with well demarcated islands of unaffected skin referred to as ‘islands of sparing’.

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There is bilateral orange waxy palmoplantar keratoderma (thickening of the skin of the palms and soles). Image

Although Pityriasis rubra pilaris is usually idiopathic, certain triggering factors have been reported. These factors include trauma, infection (streptococcus, cytomegalovirus and rubella), following vaccination, associated autoimmune diseases, malignancies and drugs. It has also been reported to be associated with defective vitamin A metabolism or vitamin A deficiency.

Pityriasis rubra pilaris occurs equally in men and women and has a bimodal age distribution, with typical onset during the first and fifth to sixth decades.

Griffiths classified Pityriasis rubra pilaris into five subgroups based on age at onset, clinical course, morphologic features, and prognosis.

These types are classic adult type I, atypical adult type II, classic juvenile type III, circumscribed juvenile type IV, and atypical juvenile type V.

More recently, a type VI PRP, or human immunodeficiency virus (HIV)-associated PRP, has been described with different clinical features and a poorer prognosis.

More than 50% of patients are best classified as type I with adult-onset PRP. This form is also characterized by high spontaneous remission rates (80%) within 1-3 years. Clinically, the classical adult (type I) and classical juvenile (type III) forms appear to be the same except for the patient's age.

Microscopic Features:

Key histological features of Pityriasis Rubra Pilaris

1) Follicular Plugging

2) Psoriasiform hyperplasia

3) Alternating orthokeratosis and parakeratosis in both vertical and horizontal direction (checkerboard pattern)

Histology shows psoriasiform hyperplasia, normal or thickened granular layer, hyperkeratosis, alternating orthokeratosis and parakeratosis in vertical and horizontal direction (checkerboard pattern), and follicular plugging. Within the dermis there is usually scanty superficial perivascular infiltrate. Image

Note: Orthokeratotic hyperkeratosis is characterised by hyperkeratosis with non-nucleated cells

Parakeratotic hyperkeratosis is characterised by hyperkeratosis with nucleated cells.

Differential diagnosis-

Unlike psoriasis in pityriasis rubra pilaris neutrophils are not present in the epidermis or stratum corneum and there is no evidence of suprapapillary plate thinning or decrease in the granular layer.

In chronic spongiotic dermatitis there is no parakeratosis in a checkerboard pattern.

Follicular plugging in pityriasis rubra pilaris is an important feature which can help to distinguish this lesion from psoriasis and chronic spongiotic dermatitis.