Pityriasis rosea is a common skin condition affecting 0.15% of the general population. This condition is more often than not diagnosed clinically by appearance and symptoms.

The lesions of the condition often start with a herald patch over the chest or abdomen followed by a more generalized rash over the chest, abdomen, legs and arms. The characteristic appearance of the rash is alike a Christmas tree with triangular lines extending from the center of the back to the sides. (1-5)

Atypical appearances of pityriasis rosea

Other atypical appearances of pityriasis rosea include; (1-5)

• Inverse pityriasis rosea - In this the arms and legs may be affected but the trunk may be spared. In children face may be involved and the armpits and groin are involved.

• Localized pityriasis rosea - In this condition the lesion is highly localized to a single place and this makes diagnosis difficult.

• Gigantean pityriasis rosea shows large lesions which are lesser in number.

• Pityriasis rosea Urticata shows itchy lesions and urticarial or itchy patches all over the skin.

Other types include:

• Pustular pityriasis rosea (with pus oozing from the lesions)

• Vesicular pityriasis rosea (with blisters)

• Purpuric pityriasis rosea (with bleeding spots over the lesions)

• Pityriasis rosea that appear like erythema multiforme

Diagnosis of pityriasis rosea

Routine blood counts are prescribed. In most cases they are normal. Some patients may show a rise in white blood cell counts. In addition, there may be selective rise of lymphocytes (B lymphocytes) indicating that pityriasis rosea may be caused by an infection. There is also a raised Erythrocyte sedimentation rate (ESR) that is another marker of disease.

Sometimes a skin biopsy may be undertaken to look at the cells of the lesion more closely. The skin area is cleaned and numbed with a local anaesthetic. Then a small tissue sample is cut off from the lesion. The area is dressed with bandages.

Under the microscope the skin tissues with pityriasis rosea reveal infiltration of the skin cells with lymphocytes, histiocytes and rarely eosinophils. There are changes in the superficial cells of the skin called the epidermis with increased keratosis or dyskeratosis. In addition, some red blood cells may also be seen in the superficial skin layers.

Ruling out alternative diagnoses

While diagnosing pityriasis rosea other conditions that may lead to similar lesions need to be ruled out. This includes:

• Drug rashes that appear as pityriasis rosea. Some drugs like Arsenic compounds, barbiturates (sedatives), bismuth, captopril (used in high blood pressure), gold (used in rheumatoid arthritis), metronidazole (antibiotic), D-penicillamine (chelating agent used in certain poisonings), sotretinoin (anti-aging skin preparation), Clonidine (used in high blood pressure), Interferon (used in viral infections), Ketotifen fumarate, Hepatitis B vaccine and Bacillus Calmette-Guérin vaccine (BCG vaccine used against tuberculosis) need to be ruled out.

• To rule out syphilis that may lead to similar skin lesions, tests like VDRL test and Fluorescent Trepenomal Antibody test are prescribed.

• To rule out fungal infections that may mimic pityriasis rosea. The skin lesion is scraped with a sterile blunt edge of the scalpel and the scrapings are placed on a microscope glass slide. These are then stained with special dyes and examined under the microscope.

Other conditions that need to be ruled out include:

• Pityriasis versicolor

• Erythema multiforme

• Guttate Psoriasis

• Para psoriasis

• Seborrhoeic dermatitis

• Nummular Eczema

• Lichen planus