Case: Pustular Psoriasis of Pregnancy (Impetigo Herpetiformis)

A 32-year-old woman at 27 weeks 6 days gestation was seen by a dermatologist on July 25, 2018 for a rash on the lower extremities, extensor elbows, and buttocks. The rash began 1 to 1.5 months ago on the legs and was initially associated with erythema and an itchy and burning sensation. The rash developed pustules and had a brown discoloration upon fading. No constitutional symptoms were present. The patient denied family history of autoimmune disease and was only taking vitamin and iron supplements. The patient was seen in 2010 for a rash during her first pregnancy, which was thought to be psoriasiform dermatitis or erythema annulare centrifugum and resolved with triamcinolone acetonide 0.1% cream.

pustular psoriasis in pregnancy

Figure 1. Dermatologic examination reveals erythematous annular plaques of varying sizes with trailing scale on the lower extremities.

Clinical examination demonstrated erythematous annular plaques of varying sizes with trailing scale and small pustules at the border (Figure 1). There were several plaques with raised red borders. The largest plaques were on the lower extremities bilaterally and smaller plaques were on the buttocks and extensor elbows. Dark brown patches were located centrally above the abdomen and abdominal striae were noted.

The patient underwent a punch biopsy of the right posterior calf and the rash improved with clobetasol proprionate 0.05% cream.

Laboratory tests showed an absolute neutrophil count of 6.7 × 103/uL and a C-reactive protein level of 1.0 mg/dL (both within the reference ranges), an increased erythrocyte sedimentation rate of 54 mm/hr, and hypoalbuminemia (albumin = 2.9 g/dL).

Light Microscopic Findings

The biopsy specimen (Figures 2A-D) showed a mild psoriasiform epidermal hyperplasia with an absent granular cell layer. The epidermis was surmounted by a parakeratotic scale that was focally imbued with neutrophils. There were also intraepidermal collections of neutrophils. Within the superficial dermis, there was an interstitial and perivascular neutrophilic infiltration and conspicuous red cell extravasation to the point where this superficial dermal neutrophilic process almost had features reminiscent of an incipient urticarial vasculitic picture. There was attendant vacuous edematous alteration of the dermal papillae. The periodic acid–Schiff stain did not disclose any fungus.

Dermpath figure 2 punch biopsy findings

Figure 2. The punch biopsy specimen showed mild psoriasiform epidermal hyperplasia (A) with intraepidermal collections of neutrophils (B), and parakeratotic scale containing neutrophils (C). An interstitial and perivascular neutrophilic infiltrate was present within the dermis and was associated with significant red cell extravasation (D).

Discussion

The patient’s biopsy demonstrated a sterile pustular psoriasiform dermatitis with a concomitant neutrophil-rich urticarial tissue reaction in the dermis. Along with the clinical presentation of an annular skin rash during the patient’s last trimester of pregnancy, these findings are consistent with a rare dermatosis of pregnancy called pustular psoriasis of pregnancy (PPP), or impetigo herpetiformis (IH). 

Historically, PPP has been included within the same classification scheme as other dermatoses of pregnancy, such as polymorphic eruption of pregnancy, atopic eruption of pregnancy, pemphigoid gestationis, and intrahepatic cholestasis of pregnancy (ie, prurigo gravidarum).1 However, most experts agree that PPP is not pregnancy-specific but rather a variant of generalized pustular psoriasis (GPP) due to the similar clinical presentation and histologic features between the 2 conditions.1-4 The relationship between PPP and GPP is not without controversy, and some researchers argue that PPP is a distinct entity because some patients are not affected by the disease outside of pregnancy.4,5 Currently, PPP is still classified as a dermatosis of pregnancy due to the importance of early treatment.1,4 IH itself is a misnomer, as there is no association with either bacterial impetigo or with the herpes simplex virus.1

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