Case 1 History
The patient is a 12-year-old female with recent history of upper respiratory infection, presenting with thin 2- to 3-mm erythematous papules with overlying scale on the trunk and extremities.
Microscopic Findings
Sections show relatively uniform elongation of rete ridges, focal diminution of the granular layer, mounded parakeratosis, and occasional clusters of neutrophils in parakeratosis ( Fig. 3.1 ). There are relatively thin suprapapillary zones of the epidermis with abutting dilated capillaries and extravasated erythrocytes.
Diagnosis
Psoriasis
Clinical Presentation
The clinical morphology of psoriasis is diverse. Subtypes include classic plaque, inverse, erythrodermic, guttate, and pustular. Classic plaque psoriasis presents with sharply circumscribed, symmetric scaled plaques on extensor surfaces. There may be pinpoint bleeding subjacent to scale because vessels reach close to the surface in dermal papillae. Guttate psoriasis, which is illustrated in this case, often follows an upper respiratory infection. The presentation consists of widespread small (2–10 mm) droplike scaly plaques.
Histopathology
The microscopic features of plaque and guttate psoriasis are similar. Key features include acanthosis, parakeratosis with admixed neutrophils, and tortuous dilated vessels in dermal papillae. Additional features include hypogranulosis, subcorneal collections of neutrophils, variable spongiosis, and a dermal infiltrate.
Differential Diagnosis
The differential diagnosis includes spongiotic (eczematous) dermatitis, seborrheic dermatitis, and pityriasis rosea (PR) ( Table 3.1 ). Because psoriasis (but not spongiotic dermatitis) develops through T-helper 17 (Th17)–driven immunologic pathways, markers of Th17 activation can be used as a diagnostic tool. , Immunostaining for interleukin-36 shows strong positivity in the upper epidermis in psoriasis; far less expression is typical of spongiotic dermatitis. ,
| Guttate Psoriasis | Nummular Dermatitis | Seborrheic Dermatitis | Pityriasis Rosea | |
|---|---|---|---|---|
| Parakeratosis | Confluent | Variable | Perifollicular | Variable |
| Acanthosis | Regular | Irregular or slight | Regular | Slight |
| Extravasated erythrocytes | Sometimes | Sometimes | No | Yes |
| Spongiosis | Limited | Yes | Yes (perifollicular) | Yes |
| Eosinophils | Variable | Yes | No | Variable |
Seborrheic Dermatitis
There can be considerable overlap between the findings of psoriasis and seborrheic dermatitis. In some instances, the composite term sebopsoriasis is used as an acknowledgment that the distinction is difficult.
Clinical Presentation
Seborrheic dermatitis presents as scaling erythematous papules and thin plaques in a so-called seborrheic distribution, which includes areas enriched in sebaceous glands, such as the face, nasolabial folds, scalp, eyebrows, and nose. Seborrhea represents a common explanation for dandruff.
Histopathology
Parakeratosis flanking follicular ostia together with psoriasiform epidermal hyperplasia and parafollicular spongiosis represents the key pattern that suggests seborrhea ( Fig. 3.2 ). A superficial lymphohistiocytic infiltrate is also present. Localized parakeratosis in the vicinity of follicular structures stands in contrast to the confluent parakeratosis that typifies psoriasis. Intracorneal neutrophils can be found in both disorders.
Pityriasis Rosea
PR may resemble guttate psoriasis clinically and histopathologically. The cause of PR has not been precisely defined, although some cases have been associated with viral infection.
Clinical Presentation
PR presents as nonpruritic erythematous oval thin plaques with overlying scale near the border of each plaque. The widespread rash along skin lines has been described as occurring in a fir tree–like distribution and is often preceded (heralded) by a solitary plaque on the trunk, known as the herald patch.
Histopathology
Histopathology reveals mounds of parakeratosis overlying foci of spongiosis (the fundamental configuration is that of spongiotic dermatitis) ( Fig. 3.3 ). There may be scattered dyskeratotic keratinocytes and exocytosis of lymphocytes, and sometimes rete have a pointed configuration. The dermis contains a lymphocytic infiltrate, and extravasated erythrocytes can be found. Neutrophils are not common in PR.
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Guttate psoriasis
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Associated with bacterial upper respiratory infection (group A streptococcus)
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Psoriasiform acanthosis, dilated vessels, and parakeratosis with neutrophils
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Seborrheic dermatitis
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Presents in a seborrheic distribution
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Perifollicular parakeratosis
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PR
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Presents in a fir tree-like distribution
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Histopathologic pattern is spongiotic or eczematous
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Case 2 History
The patient is a 32-year-old female who presents with scaly, salmon-pink plaques on the trunk and extremities. There are sharp islands of sparing within the rash. Perifollicular erythema is also noted.
Microscopic Findings
Sections show dilated and plugged follicular ostia with overlying focal parakeratosis in orthohyperkeratosis ( Fig. 3.4 ). Some cases of parakeratosis may show follicular ostia. There is psoriasiform epidermal hyperplasia with a thin granular layer. Orthokeratosis and parakeratosis alternate both vertically and horizontally, which has been termed a checkerboard pattern.
