What is this widespread, mildly itchy rash that ‘came out of nowhere’?

Speedy skins: In the next in our series of quick quizzes in dermatology, can you diagnose this young adult’s widespread and mildly itchy rash that came on suddenly?

The Case

A 19-year-old university student presents with a 2-week history of a widespread rash that ‘came out of nowhere’. It started on the trunk and has spread to the arms and thighs. The lesions are mildly itchy. He recalls having a sore throat about 3 weeks ago that resolved without antibiotics.

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What’s the diagnosis?

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Fast facts: guttate psoriasis

Definition / diagnostic criteria

Guttate psoriasis is an acute eruptive form of psoriasis characterised by the sudden onset of multiple small (≤1 cm), drop-like (‘guttate’) erythematous papules with fine scale.

Diagnosis is clinical and supported by:

Acute onset of numerous small scaly papules.
Predominant involvement of the trunk and proximal limbs.
Typical history of preceding infection, especially streptococcal.

Epidemiology

Guttate psoriasis most commonly affects children, adolescents, and young adults. It is a recognised presentation of psoriasis and may be the first manifestation of the disease.

It is frequently triggered by streptococcal infection, typically following pharyngitis. There is no strong sex predilection.

Diagnosis

Clinical features

Patients present with a sudden eruption of multiple small, “drop-like” erythematous papules with fine scale. The rash predominantly affects the trunk and proximal limbs, with relative sparing of the face, palms, and soles.

Lesions are usually mildly pruritic. The condition may occur as an isolated episode or evolve into chronic plaque psoriasis.

Investigations

Investigations are not routinely required as diagnosis is clinical.

Some authorities suggest that, rarely, a throat swab or ASO titre might be helpful to confirm streptococcal infection, but in practice this is hardly ever needed. Similarly, skin biopsy may very occasionally help diagnosis in uncertain cases.

Differential diagnosis

Pityriasis rosea – herald patch and Christmas-tree distribution.
Viral exanthem – more systemic features.
Seborrhoeic dermatitis – greasy scale in seborrhoeic areas.
Secondary syphilis (rare) – consider if atypical features or risk factors.

Treatment

Management is usually conservative as guttate psoriasis is often self-limiting.

Topical therapy includes:

Emollients for symptomatic relief.
Moderate potency topical corticosteroids.
Vitamin D analogues (e.g. calcipotriol).

If disease is widespread or persistent, referral for narrowband UVB phototherapy may be appropriate.

Antibiotics are only indicated if there is evidence of an active streptococcal infection; they do not routinely alter the course of the rash.

Prognosis

Guttate psoriasis often resolves within weeks to months. However, up to one-third of patients may go on to develop chronic plaque psoriasis. Recurrence may occur, particularly following further streptococcal infections.

Further reading

NICE CKS. Psoriasis
Primary Care Dermatology Society. Psoriasis guidance
British Association of Dermatologists. Psoriasis patient information leaflet

AI was used to assist in development of this article, with thorough review by our medical advisors before publication

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