Itchy rashes

(from BMJ Learning module)

Lichen planus

Cause unknown, but does have an association with Hep C virus, so consider checking.

The rash, like acute leg ischaemia, is described by 6 Ps:

  • Planar (flat-topped)
  • Polygonal
  • Purple
  • Pruritic
  • Papules
  • Plaques

I knew about Wickham’s striae in the mouth (though I’m not sure I would have remembered the name) but didn’t remember that it can also affect the lower back, genitals, scalp and nails.

Discoid eczema

This can be mistaken for psoriasis (red scaly plaques on extensor surfaces). However:

  • The plaques are not usually as well-defined or as thick.
  • Discoid eczema doesn’t have the silvery-white scale
  • It’s extremely itchy

Note that the steroid cream for discoid eczema has to be potent or very potent.

Guttate psoriasis

Is usually very itchy. The lesions are drop-shaped (hence ‘guttate’) and are initially flat before becoming raised. Scale may or may not be visible. There may be no history of psoriasis. Ask about history of sore throat about 7 – 10 days prior; this may be the most helpful diagnostic feature.

Pityriasis rosea can be differentiated from guttate psoriasis by the herald patch and collarette of scale.

Scabies

Very itchy rash which is worse at night and after a shower.

Papules and nodules on the penile shaft of an adult male or around the areola of an adult female are very likely to be scabies.

Babies and small children may have papules on their palms and soles.

The burrows may show up as scaling. They’re easier to see with dermoscopy, which can show up the mite (‘delta-wing’ head and translucent body) and eggs as well as typical S-shaped burrows. As well as finger and webs, they may show up on the sides of the feet, the waist, and the axillae. Use gloves for examination.

Scabies normally needs about 15 minutes of close bodily contact to be transmitted, though crusted scabies can be transmitted after brief contact. It is rarely transmitted via bedding/furniture. It takes about six weeks from transmission before the generalised itch develops.

Correct diagnosis is important; empirical treatment can worsen other types of itchy rash, due to the irritation.

Tinea incognito

This can be caused not just by inappropriate steroid treatment of a dermal rash, but also by other causes of immunosuppression such as HIV, diabetes, and immunosuppressant medication. When steroids are used, the rash initially seems to clear before coming back worse (which can lead to a vicious circle).

Asymmetrical rashes should be strongly suspected as being fungal. Tinea incognito may also have pustules around the edge.

Diagnose with skin scrapings.

Topical antifungals aren’t usually enough and oral antifungal drugs will be needed for tinea incognito.

Allergic rashes

A first-time allergy will normally show up 7 to 14 days after the rash; if a rash shows up within 48 hours of starting a drug that the patient has previously tolerated with no problems (or hasn’t previously had before) then this is not likely to be an allergy.

A good general tip…

When examining for a generalised itchy rash, check the centre of the back. This is probably not going to have been scratched, as it’s the hardest part to reach, so if there are no lesions there then this suggests that the lesions on the other parts of the skin may have been caused by the scratching and may, for example, have an underlying metabolic cause.

About Dr Sarah

I'm a GP with a husband and two young children.
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