This is one of a series of occasional articles highlighting conditions that may be more common than many doctors realise or may be missed at first presentation. The series advisers are Anthony Harnden, professor of primary care, Department of Primary Care Health Sciences, University of Oxford, and Richard Lehman, general practitioner, Banbury. To suggest a topic for this series, please email us at firstname.lastname@example.org.
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Competing interests I have read and understood the BMJ Group policy on declaration of interests and have no relevant interests to declare.
Provenance and peer review Not commissioned; externally peer reviewed.