Would you recognise acute haemorrhagic oedema of childhood (AHO) if you saw it? The condition is said to have been described first by Snow in 1913 and, although there have been only around 90 cases reported since then, it may be more common than that might imply because it may often be mistaken for Henoch-Schönlein purpura (HSP) (L A Gonggryp and colleagues, Pediatric Dermatology1998;15:91–6).

The condition affects children between the ages of 4 months and 2 years and is characterised by ecchymotic purpura on the face (ears, eyelids, cheeks) and limbs, the lesions often having a target-like or cockade appearance and accompanied by local oedema. (Cockade; a ribbon, knot of ribbons, rosette, etc, worn in the hat as a badge of office or party, or as part of a livery dress—Shorter Oxford English Dictionary.) The lesions illustrated seem to be about 2 or 3 cm in diameter and are circular, with an outline consisting of two dark red parallel lines (tramline effect) surrounding fairly normal looking skin. Other lesions look like bruises or haemorrhagic papules. Some are tender. The child usually seems well although some have a mild fever. The condition almost always resolves within a week or two but may recur, although when it does recur it usually does so early. There has been debate about whether it is a variant form of HSP but it seems milder, without the gastrointestinal, joint, or kidney involvement of HSP, and with a totally benign course. Histologically the skin lesions of AHO and HSP are similar. Two of four children in this report, from South Africa, had tuberculosis. Previously reported associations have been with vaccination, respiratory infection, drugs, streptococci, staphylococci, and adenoviruses.

Is there any point in distinguishing between AHO and HSP? The skin lesions are different, the age range of AHO is more restricted, extracutaneous involvement is rare, and it is entirely benign. So probably there is.