Differential Diagnosis

Common Diagnoses

  • Drug Reaction/Allergy
  • Flat Mole (Junctional Naevus)
  • Non-Specific Viral Exanthem
  • Sun-induced Freckles (Including Solar Lentigines)
  • Chloasma

Occasional Diagnoses

  • Post-inflammatory Hypo- or Hyperpigmentation
  • Café Au Lait Spot (Creamy Brown) and Mongolian Spot (Brown or Slate-Grey)
  • Berloque Dermatitis (Brown: Chemical Photosensitisation, e.g. Bergamot Oil)
  • Measles
  • Pityriasis Alba
  • Pityriasis Versicolor
  • Rubella
  • Vitiligo

Rare Diagnoses

  • Infections: Macular Syphilide, Tuberculoid Leprosy, Typhoid (Rose Spots in 40%)
  • Albright’s Syndrome
  • Neurofibromatosis (Associated with more than Six Café Au Lait Spots)
  • Pathological Freckles: Hutchinson’s Freckle, Peutz–Jeghers Syndrome
  • Naevus Anaemicus (Permanent Vasoconstriction Due to Neurovascular Abnormality)

Ready Reckoner

Key distinguishing features of the most common diagnoses

Drug ReactionFlat MoleViralFrecklesChloasma
Itching and BurningYesNoNoNoNo
Variable PigmentationNoYesNoPossiblePossible
<0.5 cm DiameterPossiblePossiblePossibleYesNo
Symmetrical, Muzzle AreaNoNoNoNoYes

Possible Investigations

  • 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.

Top Tips

  • 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.

Red Flags

  • 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.
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