Key distinguishing features of the most common diagnoses
Likely:Uric acid (if possible gout)
Possible:FBC, ESR/CRP, LFT, TFT
Small Print:Autoimmune studies, serology, CXR, ASO titre
- FBC/ESR/CRP: WCC and ESR/CRP raised in significant infection; Hb may be reduced (normochromic normocytic) in connective tissue disorder
- Autoimmune studies: If connective tissue disorder a possibility
- Serology: May help if suspect infective cause for erythema multiforme; also useful in assessing immune status in a pregnant woman exposed to slapped cheek syndrome, and in diagnosis of HIV infection and Lyme disease (though current guidance states that a diagnosis of Lyme disease should be made clinically in patients with erythema migrans).
- Uric acid: To confirm clinical suspicion of gout (when attack has subsided) especially if considering allopurinol.
- LFT, TFT: If palmar erythema present in non-pregnant patient – to detect alcohol excess or hyperthyroidism.
- Other investigations for erythema nodosum: If a non-drug cause is possible, investigations likely to include CXR (for TB, sarcoidosis) and ASO titre (for streptococcal infection).
- Toxic erythema caused by drugs tends to be itchy; if due to infection, it does not irritate but is accompanied by fever.
- Remember that there is often a delay before a drug causes toxic erythema – therefore, symptoms may only appear after a course of treatment (especially antibiotics) has been completed.
- ‘Deck-chair legs’ is erythema of the lower legs, sometimes with oedema and blistering, in the immobile. It tends to be mistakenly diagnosed as persistent or recurrent cellulitis.
- A violent local erythema, rapidly darkening and blistering and recurring at the same site, suggests a fixed drug eruption.
- Remember to take a drug history, including over-the-counter medications. This may reveal the underlying cause in toxic erythema, erythema nodosum and multiforme, and phototoxicity.
- Erythema nodosum and multiforme may be caused by significant disease, including, very occasionally, malignancy. If the patient is generally unwell or has other significant symptoms, investigate urgently or refer.
- Take a travel history – lyme disease is endemic in forested areas. If not diagnosed and treated early, it can have significant complications.
- Erythema multiforme with blistering and ulceration of the mucous membranes is Stevens– Johnson syndrome. Though rare, it is a very serious illness requiring urgent hospital treatment.
- Enquire about joint symptoms – many causes of erythema (e.g. erythema multiforme, butterfly rash, livedo reticularis) can be linked to a connective tissue isorder
- Remember that parvovirus can cause serious problems in pregnancy – check serology in women with suggestive symptoms, or exposure to a case.