Differential includes secondary syphilis
The differential diagnosis of guttate psoriasis includes lichen planus, pityriasis rosea, and secondary syphilis.
Lichen planus is characterized by pruritic, planar, polyangular purple papules with a reticular pattern of criss-crossed whitish lines called “Wickham’s striae,” which are areas of epidermal thickening.1
Pityriasis rosea typically presents in a “Christmas-tree” distribution in which the long axis of these oval plaques are oriented along skin lines. Also, the lesions have a distinctive collarette scale, appearing as a fine, wrinkled tissue-like scale surrounding the plaque borders.
Secondary syphilis has numerous signs and symptoms, including rash. In this case, we ruled it out because our patient had no history of sexual contacts and a negative RPR.
Testing confirms suspicions, identifies organism
The diagnosis of guttate psoriasis is established from clinical presentation. While there is no laboratory test specific for diagnosis, as many as 80% of patients will have clinical or laboratory evidence suggestive of a streptococcal infection—specifically tonsillopharyngitis.4 Utilize bacteriologic throat or perianal cultures to isolate the organism. Levels of antibodies to streptolysin O, hyaluronidase, and deoxyribonuclease B may be elevated.4
Urinalysis can be used to rule out associated streptococcal complications, such as poststreptococcal glomerulonephritis. Further serologic evaluation may also include RPR testing to exclude secondary syphilis.
Biopsy is rarely needed to establish the diagnosis, but may be used for confirmation of complicated presentations or to rule out other concerning diagnoses. Histologic findings demonstrate epidermal hyperplasia with small foci of parakeratosis and dermal layer capillary dilation and edema. Infiltrating lymphocytes, macrophages, and polymorpho-nuclear leukocytes may be found at all dermal levels.5