• The cause of most cases of chronic urticaria (62%) cannot be identified [Grattan et al, 2001; Kozel et al, 2003].
• An autoimmune reaction is thought to be associated with a high number of chronic urticaria cases without an identifiable cause [Leznoff and Sussman, 1989].
• Allergies to food and drugs are uncommon causes of chronic urticaria [Wanderer et al, 2000]. However, aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) may exacerbate proven chronic urticaria.
• Chronic infections and infestations — intestinal parasites, chronic sinusitis, cholecystitis, dental abscesses, Helicobacter pylori infection and viral hepatitis have all been linked to chronic urticaria [Grattan et al, 2001; Zuberbier, 2003; Powell et al, 2007].
• Internal diseases (e.g. connective tissue disorders) are a rare (1.6%) cause of chronic urticaria. There appears to be a link with the presence of thyroid auto-antibodies and chronic urticaria [Leznoff and Sussman, 1989].
• Malignancy has been linked with urticaria and may suggest a relapse of the malignancy. Both solid tumours and lymphomas have been associated. There is no strong evidence to confirm an association between malignancy and uncomplicated chronic urticaria, except occasionally in urticarial vasculitis and, more frequently, in acquired C1 esterase inhibitor deficiency (see the CKS topic on Angio-oedema and anaphylaxis). A recent cohort study suggests a possible link with some haematological malignancies but further research is needed [Lindelof et al, 1990; Soderberg et al, 2004].
• Physical triggers are more often a cause of acute urticaria, but sometimes may result in chronic urticaria.
• The role of Helicobacter pylori infection in chronic urticaria is unclear and most prospective studies have found no relationship. A systematic review of 10 studies showed that in people with chronic urticaria thought to be caused by H. pylori infection, eradication therapy improved the urticarial rash in approximately a third of cases [Federman et al, 2003]. Nevertheless, the conflicting evidence of a causative role of H. pylori infection in chronic urticaria remains uncertain.

[Kozel and Sabroe, 2005]