Key distinguishing features of the most common diagnoses
|Drug Reaction||Flat Mole||Viral||Freckles||Chloasma|
|Itching and Burning||Yes||No||No||No||No|
|<0.5 cm Diameter||Possible||Possible||Possible||Yes||No|
|Symmetrical, Muzzle Area||No||No||No||No||Yes|
- There are very few relevant investigations to consider and they would be required only exceptionally, as the diagnosis is usually clinical: Skin scrapings for mycology or fluorescence under Wood’s light may help in the diagnosis of pityriasis versicolor; acute and convalescent serum samples may confirm rubella; serology for syphilis may be appropriate with an unusual macular rash; and very occasionally, a skin biopsy may be required to clinch an obscure diagnosis.
- A drug eruption can take two weeks to appear from the time of the first dose – so don’t be misled by the fact that a course of antibiotics may have been completed some days before the related drug rash develops
- Pityriasis versicolor may be misdiagnosed as vitiligo. If in doubt, take scrapings for mycology or examine under Wood’s light.
- Odd lines of hyperpigmentation on the sides of the neck are likely to be Berloque dermatitis – a photosensitive rash caused by oil of bergamot, present in perfumes.
- Hutchinson’s freckle is a giant, variegated freckle, seen in elderly sun-exposed skin. There is a high risk of malignant change, so refer.
- Rubella is rare, but may become commoner as a result of media coverage of ‘immunisation scares’. Establish whether or not a young woman presenting with a rubella-type rash is pregnant – if she is, confirm her rubella status.
- A child with very many freckles on and around the lips may have Peutz–Jeghers syndrome. This is associated with small bowel polyposis.
- Vitiligo tends to have a poor prognosis in Caucasians, especially if it is widespread and affecting lips and extremities.