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Pityriasis rubra pilaris (PRP) is an erythematous papulosquamous dermatosis characterized by small
follicular papules with perifollicular erythema which tend to become confluent but leave zones or islands of
spared normal skin. It typically starts in the head and neck area and then follows a progressive downward
distribution progessing in some cases to an erythrodermic state. This cephalo-caudal progression is
virtually unique to PRP. Palmar plantar keratoderma is almost ubiquitous. Nail changes and alopecia may
occur. Spontaneous remission occurs within six months to two years in fifty percent of cases when no
treatment is given. Recently a role for immune dysregulation has been postulated. In particular, PRP has
occurred in patients ingesting immune dysregulating agents, with underlying endogenous immune dysregulatory
states such as collagen vascular disease and HIV disease, in the setting of unusual psychological stresses,
and in some patients who report inciting triggers such as vaccination and infections with various viruses
and enteric organisms. The eruption is cosmetically quite disfiguring and hence treatment with systemic
vitamin A is usually given in most patients when the diagnosis is made. Regarding the latter, pityriasis
rubra pilaris-like eruptions seem to be a characteristic although uncommon manifestation of patients with
dermatomyositis. These patients show a generalized erythematous papulosquamous eruption with or without
follicular hyperkeratosis and diffuse thickening of the palms and soles. Histological features of
dermatomyositis associated PRP include keratotic plugging of the follicular infundibulum and features of
erector pili myositis.
Histopathology
Virtually all cases show moderate to marked epidermal hyperplasia. In a minority of cases, the epidermal
hyperplasia may alter with zones of epithelial attenuation. The stratum corneum is strikingly altered,
assuming an ichythyotic pattern of hyperkeratosis.
Orthohyperkeratosis is dominant, but scattered parakeratotic foci within the greatly thickened stratum
corneum are a ubiquitous finding. The granular cell layer is markedly abnormal, being increased and often
containing large, irregularly-shaped keratohyalin granules resembling those encountered in epidermolytic
hyperkeratosis. Peripheral margination of these large, abnormal granules may result in perinuclear vacuolar
alteration. Follicular hyperkeratosis is typical. Acantholytic dyskeratosis is observed in the majority of
biopsies of PRP in our experience; such foci may be very minute, averaging no more than one hundred microns
in diameter, but in some cases where acantholytic dyskeratosis is conspicuous, the clinical presentation may
suggest a primary vesiculobullous disorder. The lymphocytic infiltrate in the dermis is superficially
confined, and ranges from sparse to intense and can be accompanied by eosinophils and plasma cells. There
is variable exocytosis of inflammatory cells in the epidermis. PRP differs from psoriasis by the absence of
the true psoriatic diathesis. In particular, the dermal papillae capillaries are not ectatic and do not lie
in intimate apposition to thinned superpapillary plates. Neutrophil-imbued parakeratosis is an uncommon
finding.
The historic criteria used for diagnosis for PRP include alternating ortho- and parakeratosis in vertical
and horizontal planes associated with keratotic follicular plugging, parakeratosis at the shoulders of the
follicular ostia to form a collarette, psoriasiform hyperplasia and a sparse dermal inflammatory infiltrate
comprising predominantly lymphocytes.
In a recent study, the following morphologic parameters were predictors of PRP over psoriasis:
- an increased and abnormal granular cell layer. The abnormality refers to the large and irregularly
shaped, peripherally disposed keratohyalin granules.
- acantholytic dyskeratosis.
- follicular plugging
- absence of neutrophil imbued parakeratosis, granular cell layer diminution, and dilated vessels in
intimate apposition to thinned suprapapillary plates.
The inflammatory cell infiltrate within the dermis may be quite heavy and include plasma cells and
eosinophils. The epidermis shows evidence of a hyperproliferative state characterized by dysmaturation and
prominent suprabasilar mitotic activity.
Differential Diagnosis
Another differential diagnostic consideration is seborrheic dermatitis; pathological changes in both PRP and
seborrheic dermatitis are often centered around the hair follicle. However, in seborrheic dermatitis the
ichthyosiform scale, the abnormal granular cell layer and acantholytic diskeratosis are not observed. In
seborrheic dermatitis, the dominant pathology is one of a spongiotic dermatitis and neutrophil embued
parakeratosis in and around the follicular ostia,unlike the usual absence of neutrophils within the
parakeratotic scale in PRP. Nonetheless, we have identified neutrophils in the parakeratotic scale crust in
a minority of cases of PRP, hence, that finding in isolation should not be used as an absolute criterion to
exclude a diagnosis of PRP. In addition to antigenic triggers and immune dysregulation being implicated in
the pathogenesis of PRP, abnormalities in keratins and retinoid binding defects may play a role.
particularly in the familial forms. Regarding the distinction of PRP from the ichthyosiform dermatoses, in
the context of a congenital ichthyosis such as lamellar ichthyosis or ichthyosis vulgaris or an acquired
ichthyosis, the striking epidermal hyperplasia that accompanies most cases of PRP is not present, and as
well, such cases are virtually devoid of any inflammatory cell infiltrate. In ichthyosis vulgaris, be it
congenital or acquired, the granular cell layer is diminished to absent, unlike the prominent granular cell
layer encountered in PRP. The clinical presentation should allow easy distinction of those cases of PRP
showing epidermolytic hyperkeratosis from congenital epidermolytic hyperkeratosis.
In addition to psoriasis, other differential diagnostic considerations would include the nutritional
dermatoses including vitamin A deficiency. However granular cell absence is the role and as well the scale
is almost exclusively parakeratotic. Finally zones of epidermal atrophy are seen in lesions of nutrional
dermatosis.
References
- Bonomo RA, Korman N, Nagashima-Whalen L, Briggs J, Graham R, Salata RA. Pityriasis rubra pilaris: an
unusual cutaneous complication of AIDS. Am J Med Sci 1997 Aug;314(2):118-21
- Clayton BD, Jorizzo JL, Hitchcock MG, Fleischer AB Jr, Williford PM, Feldman SR, White WL. Adult
pityriasis rubra pilaris: a 10-year case series.J Am Acad Dermatol 1997 Jun;36(6 Pt 1):959-64
- Gillum PS, Golitz LE. Psoriasiform dermatitis. In : Barnhill R, Crowson AN, Busam K, Granter S ed's.
Textbook of Dermatopathology. New York : McGraw-Hill Co, 1998.55-68.
- Magro CM, Crowson AN. The clinical and histological features of pityriasis rubra pilaris : A
comparative analysis with psoriasis. J Cutan Pathol 1997;24:416-424.
- Toussaint S, Kamino H. Noninfectious erythematous, papular, and squamous diseases. In: Elder D,
Elenitsas R, Jaworsky C, Johnson B, Ed's. Lever's Histopathology of the skin, 8th Ed'n, Philadelphia:
Lippincott-Raven, 1997:151-184.
- Requena L, Grilli R, Soriano L, Escalonilla P, Farina C, Martin L. Dermatomyositis with a pityriasis
rubra pilaris-like eruption: a little-known distinctive cutaneous manifestation of dermatomyositis.Br J
Dermatol 1997 May;136(5):768-71
- Weedon D. The psoriasiform reaction pattern. Skin Pathology. Edinburgh : Churchill Livingstone.
1997:65-82.
OTHER PSORIASIFORM DERMATOSES
LICHEN SIMPLEX CHRONICUS
Introduction/Pathogenesis/Clinical
Lichen simplex chronicus eventuates when a dermatitis persists over a sustained period of time, aquiring a
thickened scale due to repeated rubbing. When such a process is localized to one or a few sites and is
exaggerated, lesions are referred to as "prurigo nodularis" or "picker's nodules".
Histomorphology
Llichen simplex chronicus manifests pronounced psoriasiform hyperplasia of an epidermis overlaid by a thick
orthohyperkeratotic scale. Inflammation is variable but is usually mild and is predominantly composed of
lymphocytes which manifest little exocytosis; the epidermis characteristically does not manifest spongiosis
or vesiculation. In prurigo nodularis, the epithelial hyperplasia becomes striking or pseudoepitheliomatous
in character, mimicking well differentiated squamous cell carcinoma. Some authors believe that the origin
of the elongated epithelial tongues in prurigo nodularis is from adnexal structures of the skin, thus, the
islands of well-differentiated squamous epithelium manifest terminal differentiation, such as keratohyalin
granules. This is in contrast to well-differentiated squamous cell carcinoma, where the epithelial nests in
the dermis do not manifest terminal differentiation. In prurigo nodularis much of the elevation of the
lesion is due to fibroplasia involving both the papillary and reticular dermis. In contrast, in squamous
cell carcinoma, the fibroplasia is restricted to the immediate territory of the invasive tumor tongues, and
is a delicate stromal fibroplasia which is fibroblast-rich, unlike the cell-poor fibroplasia of prurigo
nodularis.
Differential Diagnosis
The differential diagnosis of lichen simplex chronicus is that of psoriasiform dermatitis: psoriasis,
seborrheic dermatitis, pityriasis rubra pilaris, etc., which can be distinguished from lichen simplex
chronicus by the presence of significant inflammatory infiltrates and the other criteria described above.
As lesions of lichen simplex chronicus and prurigo nodularis relate to pruritus, nerve density may be
increased and there may be an abundance of mast cells in the territory of cutaneous nerves, a finding which
may play a role in the pathophysiology of the fibroplasia. When an obvious source of pruritis cannot be
demonstrated, some authors recommend investigations for underlying cause of pruritus, such as hepatobiliary
disease and lymphoreticular neoplasia.
References
- Gillum PS, Golitz LE. Psoriasiform dermatitis. In : Barnhill R, Crowson AN, Busam K, Granter S ed's.
Textbook of Dermatopathology. New York : McGraw-Hill Co, 1998.55-68.
- Toussaint S, Kamino H. Noninfectious erythematous, papular, and squamous diseases. In: Elder D,
Elenitsas R, Jaworsky C, Johnson B, Ed's. Lever's Histopathology of the skin, 8th Ed'n, Philadelphia:
Lippincott-Raven, 1997:151-184.
- Weedon D. The psoriasiform reaction pattern. Skin Pathology. Edinburgh : Churchill Livingstone.
1997:65-82.
INTERFACE DERMATITIS
Cell-poor vaculopathic interface dermatitis
Cell-poor vaculopathic interface dermatitis is defined by basilar keratinocyte and subepithelial
vacuolopathy unaccompanied by a significant inflammatory cell infiltrate. Usually, there is some lymphocyte
tagging along the dermal-epidermal junction pointing to the immunopathogenetic basis, namely, cellular
cytotoxicity. This may be either in the context of a type 4 immune reaction or of antibody dependent
cellular cytotoxicity.
The differential diagnoses of a cell-poor interface dermatitis include:
- erythema multiforme
- graft-versus-host disease
- a morbiliform viral exanthum
- a morbiliform drug reaction
- collagen vascular disease, specifically systemic lupus erythematosus, dermatomyositis, and mixed
connective tissue disease
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