Diagnosis and investigation of pityriasis rosea

The cause of this common, self-limiting skin condition remains unclear, explains Dr Waseem Chaudhry.

Pityriasis rosea occurs worldwide and affects all races. Reported incidence is 0.3-3 per cent. In temperate climates, it is more common during spring and autumn.

It is equally common in men and women, although some reports suggest that women are more susceptible.

The cause remains unclear, but its natural history is similar to viral exanthema, which has led to the thinking that pityriasis rosea has a viral cause. Various infective agents have been suspected but not confirmed, including fungi, spirochetes, streptococci and legionella.

Pityriasis rosea is now thought to have a viral origin because herpes virus-like particles have been isolated from lesions.

Various drugs, such as metronidazole, barbiturates, clonidine, captopril and ketotifen, have also been implicated as causal. However, it is now believed these reports are pityriasis rosea-like drug eruptions, rather than true pityriasis rosea.

The herald patch and secondary lesions in pityriasis rosea are histologically similar.

The epidermis shows spongiosis, vesicles and patchy para-keratosis. The upper dermis exhibits oedema and a mononuclear cell infiltrate (mainly helper T-lymphocytes and Langerhans cells), from which there is exocytosis into the epidermis, leading to formation of subcorneal pustules.

Classical pityriasis rosea (80 per cent of cases) virtually always runs the same course.

Usually, there are no prodromal symptoms, although some patients describe malaise and a mild headache.

The herald patch is the first manifestation of the disease and classically appears on the thigh, upper arm, trunk or neck. It rarely occurs on the face, scalp or genitalia.

The herald patch is initially a bright red, round or oval plaque with diffuse fine scale that later fades, becoming tawny in colour, and the scale becomes more peripheral (collarette). The free edges of the scale lie internally.

The herald patch is larger than the secondary lesions and usually measures 2-5cm, but can be larger. Rarely, pityriasis rosea can present with more than one herald patch, or patches can coalesce to form larger areas.

A more generalised secondary eruption usually occurs one to two weeks later, although it may occur within a few hours or up to two months later.

Secondary lesions are smaller (1-2cm), dull pink and oval, and again develop a fine collarette of scale. Secondary lesions tend to be central on the torso, upper arms and thighs. Lesions of the oral mucosa are common but are often overlooked.

The secondary eruption lasts six weeks fading, with no residual trace.

Differential diagnoses

Tinea: Skin scrapings for mycology will help to exclude or prove this diagnosis.

Drug eruptions: Histology of lesions may help.

Seborrhoeic dermatitis: Can look pityriasiform.

Secondary syphilis: Mainly affects genitalia, and lesions are maculopapular.

Guttate psoriasis: More persistent rash, and lesions tend to have a silvery scale.

Pityriasis lichenoides: Lesions tend to be polymorphic, haemorrhagic and have a crust.

Pityriasis alba: Affects the face of young children.

Diagnosis and treatment
Classical pityriasis rosea is usually not difficult to recognise, given its predictable presentation. As such, it is easily diagnosed on history and clinical examination, and investigations are not normally needed.

However, the condition presents atypically in one fifth of cases. Therefore, the differential diagnosis should include certain conditions (see box).

Where there is doubt about the diagnosis, biopsy and histological examination of lesions can help. Skin scrapings for mycology can help to rule out tinea.

Blood tests are largely unhelpful, but mild abnormalities may be seen, such as raised ESR and lymphocytosis.

Most cases of pityriasis rosea are asymptomatic or have only mild symptoms. Simple reassurance that the disease is self-limiting and will settle without treatment is often sufficient.

For patients who do need treatment, either because of pruritus or embarrassment about their appearance, topical steroids can suppress the inflammatory component of the disease.

Emollients and oral antihistamines may have some benefit.

Phototherapy has been used for more than 30 years to treat pityriasis rosea and there is some evidence that UVB can reduce pruritus and disease severity. The risk of post-inflammatory hyperpigmentation may increase with phototherapy.

For patients with severe disease, oral corticosteroids may be helpful. Studies have shown that a two- to three-week course of reducing prednisolone can help to control the disease. However, oral corticosteroids should be used with caution because they can exacerbate the condition in some cases.

Oral erythromycin, dapsone and aciclovir have also been used successfully.

Pityriasis rosea is a common, self-limiting disease that is easy to identify in its classic form. Most cases are asymptomatic or only have mild symptoms. Simple reassurance that the disease will settle without treatment is usually all that is needed.

Most symptomatic or distressed patients respond to treatment with a combination of a topical steroid, emollient and oral antihistamine. Difficult or resistant cases may need further investigation and referral to secondary care for phototherapy.

Explanation of the usual benign nature and self-limiting course of the disease will help patients to cope with any symptoms and embarrassment they might experience.

  • Dr Waseem Chaudhry is a GPSI in dermatology in Caerphilly

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