Accurate figures are difficult to obtain because different methods have been used in the reported studies. Reported prevalence rates vary from 12-5 per 100 000 in England to 50-8 per 100 000 in certain groups in the US.4 Many series are hospital based and probably do not reflect the true incidence. African-American, Chinese and Malaysian women appear to be the populations with the highest prevalence. Published figures suggest an increasing incidence, but this may be because of greater awareness of the condition.

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