To the Editor:
There has been speculation among dermatologists that the specificity of the histopathologic diagnosis of psoriasis has been diminishing over time. Although psoriasis is frequently diagnosed clinically, refractory and atypical cases of psoriasis are frequently biopsied and may demonstrate unusual histopathologic features.1 These cases represent a diagnostic challenge, and it may be difficult to determine whether histopathologic features are representative of psoriasis or another dermatologic condition.1 Therefore, nonspecific terms such as “psoriasiform” or “spongiotic psoriasiform” dermatitis may be used to describe these lesions without providing a more definite diagnosis.2,3 Some clinicians have averred that skin biopsies are not useful in making the diagnosis of psoriasis when the diagnosis is in question (M. Lebwohl, oral communication, January 2021).
Despite this general opinion, when clinical correlation is coupled with a systematic histologic method applying well-developed criteria, a definitive diagnosis of psoriasis can be made accurately and repeatedly and distinguished from other inflammatory conditions in which there may be psoriasiform hyperplasia of the epidermis.2 In our dermatopathology practice, we made the unequivocal diagnosis of psoriasis histopathologically 716 times between January 1, 2020 and December 31, 2020, which included the subtypes of psoriasis vulgaris (634), guttate psoriasis (38), inverse psoriasis (29), pustular psoriasis (14), and verrucous psoriasis (1). In no case was the diagnosis “disputed” by the referring clinician, and therapy was instituted in accordance with the diagnosis.
The most consistent histopathologic features of psoriasis include regular acanthosis, suprapapillary thinning, suprabasilar mitoses, parakeratosis, hypogranulosis, dilated blood vessels in the papillary dermis, and intraepidermal and intracorneal neutrophils.2,4 Features more commonly identified in other psoriasiform dermatoses include spongiosis, irregular acanthosis, lymphocytic exocytosis, variable numbers of eosinophils, vertical orientation of collagen bundles, and the absence of intraepidermal neutrophils.2,4
However, the distinction of psoriasis from other psoriasiform dermatoses histologically may be more difficult on volar skin because psoriasis may demonstrate unusual findings such as more spongiosis and look very similar to dyshidrotic dermatitis in some cases. Dermatopathologists can make the diagnosis of psoriasis histopathologically on volar skin when there is limited spongiosis and “tiered” parakeratosis with intracorneal neutrophils.
Additional studies may be helpful for the differentiation of psoriasis from nonpsoriasis psoriasiform dermatitis. For example, Sezer et al3 found that immunohistochemical staining of the proliferation marker Ki-67 was significantly higher in suprabasal and total epidermal cell counts of histopathological specimens of psoriasis than specimens of nonpsoriasis psoriasiform dermatitis and more sensitive than the proliferation marker cyclin D1, which correlates with the histologic detection of suprabasilar mitoses. An important finding of this study was a cut-off value of 75% for the suprabasal/total epidermal cell count ratio, with cases of psoriasis being higher than this value and cases of nonpsoriasis psoriasiform dermatitis being lower than this value.3
In a study by He et al,5 psoriasis was differentiated from atopic dermatitis with 100% accuracy by the expression of nitric oxide synthase 2/inducible nitric oxide synthase by analysis of RNA sequencing from tape-stripped samples of skin. Lesions of psoriasis had higher levels of these genes related to innate immunity as well as TH1 and TH17 polarization. Atopic dermatitis was characterized by TH2 polarization. Further classification of the genetic profiles and immune pathways of other psoriasiform dermatoses by tape stripping may differentiate these entities without the requirement of biopsy.
Although these ancillary methods may someday enhance the diagnostic specificity of psoriasis, they are still largely experimental and not used routinely in practice today. Although psoriasis may be difficult to diagnose when there is associated irritation, there has been partial treatment, lesions are located on volar skin, and in patients with erythroderma, the diagnosis can and should be made with certainty histopathologically. In the hands of good clinicians and skilled dermatopathologists, skin biopsies are still highly valuable in rendering accurate diagnoses, which is more important than ever given the number of excellent treatment options now available.
1. Chau T, Parsi KK, Ogawa T, et al. Psoriasis or not? Review of 51 clinically confirmed cases reveals an expanded histopathologic spectrum of psoriasis. J Cutan Pathol. 2017;44:1018–1026.
2. Jayalakshmy PL, Babitha AM, Sankar S, et al. Histopathological spectrum of psoriasiform dermatitis. J Pathol Nepal. 2016;6:975–980.
3. Sezer E, Böer-Auer A, Cetin E, et al. Diagnostic utility of Ki-67 and Cyclin D1 immunostaining in differentiation of psoriasis vs. other psoriasiform dermatitis. Dermatol Pract Concept. 2015;5:7–13.
4. Mehta S, Singal A, Singh N, et al. A study of clinicohistopathological correlation in patients of psoriasis and psoriasiform dermatitis. Indian J Dermatol Venereol Leprol. 2009;75:100.
5. He H, Bissonnette R, Wu J, et al. Tape strips detect distinct immune and barrier profiles in atopic dermatitis and psoriasis. J Allergy Clin Immunol. 2021;147:199–212.