Urticaria, also known as hives, are pale red bumps or plaques (wheals) on the skin that appear suddenly and typically come and go, with each lesion lasting less than 24 hours. Hives usually itch, but can also burn or sting. Pressing the centre of a red hive makes it turn white (blanching). Hives are caused by excessive release of histamine and other mediators from mast cells in the upper layer of the skin. Release of such mediators deeper down in the dermis or subcutaneous layers of the skin can cause more diffuse areas of swelling, such as swelling of the lips or eyes (angioedema), which can occur in association with hives.
About 20% of people are affected by hives at some stage in their lives. Hives can appear in “crops” which come and go in different regions of the skin. Some episodes of hives are short lived (“acute hives,” which by definition last less than 6 weeks) and some forms appear, disappear and reappear long term (“chronic hives,” which typically last more than 6 weeks, or recur in episodes). Hives themselves are not life threatening, but in some cases associated symptoms such as swelling of the throat can cause complications.
Causes of acute and chronic hives
Acute hives can be caused by an allergic reaction, in which case the hives typically last less than 6 hours and may be associated with other symptoms such as flushing, swelling, vomiting, abdominal cramps or more severe symptoms such as respiratory symptoms. Allergies can occur to certain foods, medications, contact with plants or certain animals, insect stings, latex or blood transfusions. However, not all hives are caused by allergies! Acute hives but can also be caused by viral or bacterial illnesses (this is more common in children, and hives usually come and go over several days). Acute hives can even be caused by hormonal changes or emotional stress. If a person is not known to have an allergy, and hives come and go over a few days, a general rule of thumb is that it is not an allergic reaction but an “immune response,” most commonly to a viral illness.
Chronic hives usually occur daily or almost daily for more than 6 weeks; each hive lasts less than 24 hours and typically does not leave bruises or scars. Most often there is not an identifiable trigger, and the hives come “out of the blue.” The easiest way to explain this to a patient is that the immune system is being mischievous “from the inside” rather than an external allergen causing trouble “from the outside.”
Sometimes chronic hives are worsened by physical triggers such as heat, sweating, cold, water, sun, physical pressure on an area (“inducible urticaria”) or can be worsened by factors stimulating mediator release, such as stress, hormonal changes, non-steroidal anti-inflammatory drugs, alcohol or infections.
If the cause of the chronic urticaria cannot be identified- which is commonly the case- the condition is termed “chronic spontaneous urticaria”. About half of such cases are associated with an immune cause, in other words the immune system is overreacting to one’s self. Most cases dissipate over time- this can take months to years.
Diagnosis of hives
In diagnosing the cause of hives, the history is of utmost importance. In the case of acute hives, if an allergy is suspected, further tests such as skin prick tests, blood tests or even food challenge tests may be needed. If hives come and go over several days, and an immune mediated cause such as a virus is suspected, no further tests are usually needed.
In the case of chronic hives, patients often seek an “exact” cause: careful explanation of the lack of an external allergen but rather an “internal” cause of histamine over-release goes a long way in avoiding unnecessary investigations and interventions. If physical triggers are suspected, then they can be simulated to test for an urticarial response, for example the ice cube on the arm test for cold-induced urticaria, and induced exercise for cholinergic urticaria (which is triggered by a rise in body temperature and sweating).
Management of hives
Management will depend to an extent on the cause. If exact triggers (eg allergic response or physical triggers) are elicited, avoidance of the trigger is the prudent first step in management. Emergency management of allergic reactions will include antihistamines and intramuscular adrenaline for severe reactions. Unlike in allergic reactions, the hives of viral urticarias and the chronic urticarias are, in the vast majority of cases, an isolated skin issue without systemic complications.
In both acute and chronic urticaria, antihistamines are the first-line treatment, often in doses up to 4 times the usual dose, with preference given to newer non-sedating antihistamines. The duration of the antihistamines depends on the duration of the hives. For viral urticarias, one to two weeks of treatment usually suffices. For the chronic urticarias, often months to years of high dose antihistamines are required, with slow tapering when the hives seem to be settling.
In some cases, short courses of oral steroid may be needed, and other anti-inflammatories and antihistaminergic drugs may need to be added on. Occasionally, immune suppressant drugs or antibody therapy (eg omalizumab, which is an anti-IgE injection) may be needed in stubborn cases of chronic urticaria.
And that’s just scratching the surface…
The causes of hives are a-plenty. Sometimes, true allergic reactions are the culprit and may require some detective work to decipher. In the case of hives coming and going over a few days, the fear of a “new” allergy causing the symptoms is usually unfounded.
A visit to an allergist will be of great use for the majority of people presenting with hives to fine-tune diagnosis and management, and to dispel many common misconceptions about hives.