Hives and angioedema

What hives are

Hives, or urticaria, are raised itchy weals in the skin — pink or pale, sometimes with a surrounding flare, anything from a few millimetres to large joined-up patches. They are caused by mast cells releasing histamine into the skin, which makes small blood vessels leak fluid into the surrounding tissue.

Two features identify them, and they are worth knowing because they distinguish hives from almost every other rash:

Individual weals come and go. Any one weal lasts from a few minutes up to 24 hours and then disappears completely, leaving no mark. New ones appear elsewhere meanwhile. A rash that stays in the same place for days is not urticaria.

They move. The overall rash shifts around the body over hours. That migration is characteristic.

So the useful thing to do is mark the edge of one weal with a pen and look again the next day. If that particular weal has gone, it is urticaria. If it is still there, it is something else.

Most hives are not an allergy

This is the part that conventional wisdom gets wrong, and it saves a lot of wasted effort.

Hives can certainly be allergic — to a food, a drug, an insect sting or latex — and in those cases the connection is usually obvious, because the hives appear within minutes to an hour of the exposure and the exposure was a discrete event.

But that is not the common case. In children, an episode of hives is usually caused by a viral infection, and often with no fever or other sign of illness to make that obvious. In adults, most hives turn out to have no identifiable trigger at all. Infections, some medicines, and physical triggers account for a good deal of the rest.

The practical consequence: hives appearing out of nowhere, with no single event preceding them by an hour or so, are unlikely to be a food allergy. Reconstructing the last three days of meals is the usual first instinct and rarely produces an answer. Where there is no clear immediate trigger, there is usually no allergen to find.

Acute and chronic

The single most useful question is how long it has been going on, because the answer puts it in one of two quite different categories.

Acute urticaria lasts less than six weeks, and most episodes settle within hours or days. This is the common kind, it is usually viral or unexplained, and it does not generally need investigating.

Chronic urticaria means weals continuing or recurring beyond six weeks. This is a different proposition, and it is where the assumption of allergy does the most harm.

Chronic urticaria, and why there is usually nothing to find

Chronic spontaneous urticaria is mostly idiopathic — meaning no cause is identified, not that the search was inadequate. The evidence points to an autoimmune mechanism in a substantial proportion of cases, with the immune system producing antibodies directed against its own IgE or the receptor it binds to. In other words the trigger is internal, which is precisely why no external one can be found.

People with chronic urticaria frequently spend months or years eliminating foods, replacing washing powder and buying commercial tests, on the reasonable-sounding assumption that something out there is causing it. That effort is almost always wasted, and the restricted diets it produces can do their own harm.

Chronic urticaria is unpleasant and can be genuinely wearing, particularly where it interferes with sleep. It is also treatable, and it tends to resolve eventually of its own accord, though that can take a long time.

When a physical trigger sets them off

A group of urticarias are brought on reliably by a physical stimulus rather than an allergen. They are worth knowing about because they explain hives that appear with no allergen in sight, and because the pattern is usually recognisable once named. In these, weals typically appear within about five minutes of the stimulus and fade within the hour.

The common ones: dermographism, where scratching or friction raises weals along the line of the contact; cold urticaria, from cold air, cold water or ice; cholinergic urticaria, from sweating — exercise, a hot shower, or strong emotion; delayed pressure urticaria, from sustained pressure such as a heavy bag strap or a seat belt; solar urticaria, from sun on skin unused to it; and less commonly reactions to heat, vibration and water itself.

Angioedema

Angioedema is swelling in the deeper layers of the skin or in mucous membranes, driven by a different mediator from the one behind ordinary weals. It often accompanies hives and sometimes occurs alone.

It typically affects the eyelids, lips, hands, feet and genitals, producing swelling that is firm and often more uncomfortable than itchy. It can also involve the tongue, the soft palate and the throat — and that is where it stops being a nuisance.

When it is an emergency

Hives alone, however widespread and uncomfortable, are not dangerous.

What is dangerous is hives occurring with signs that the reaction involves more than the skin: swelling of the tongue, throat or lips affecting breathing or swallowing; difficulty breathing, wheeze or a hoarse voice; feeling faint, light-headed or collapsing; or severe abdominal pain and vomiting in the context of a reaction. That combination is anaphylaxis.

It needs adrenaline by intramuscular injection into the outer thigh, given early, and an ambulance on 999 — made even if the adrenaline appears to have worked, because symptoms can return. An adrenaline auto-injector is not something prescribed for hives on their own; it is for people at risk of anaphylaxis, which is a different assessment.

Treatment

The mainstay is a non-sedating antihistamine — cetirizine, loratadine, fexofenadine. Unlike contact dermatitis, hives genuinely are histamine-driven, so antihistamines work here.

Two points about dosing that often go unmentioned. The older sedating antihistamines are no longer recommended for urticaria, despite being what people often reach for. And where a standard dose is not controlling it, the established approach is to increase the dose of the same antihistamine — up to around four times the standard amount is used in practice — rather than to add a second different one, which does not appear to help. That is a decision for a doctor or pharmacist rather than something to undertake alone, but it is worth knowing it exists, because a great many people conclude antihistamines do not work for them when they have only ever tried one tablet a day.

Beyond that: cooling the skin helps, as does avoiding scratching and anything tight against the area. Aspirin and anti-inflammatory painkillers such as ibuprofen can aggravate urticaria and are worth avoiding during a flare; paracetamol is generally fine.

A short course of oral steroid is sometimes used for a severe acute episode that antihistamines are not controlling. Long-term steroids are not appropriate for this, given what they do over time.

Where chronic urticaria is still not controlled after several weeks on a properly increased antihistamine dose, referral is the next step. Specialist treatments exist for antihistamine-resistant chronic urticaria and work well for a majority of the people who need them.

When to get help

A pharmacist can advise on antihistamines for an ordinary episode.

A GP is worth seeing where hives have continued beyond six weeks, where they are disturbing sleep or daily life, where they keep returning, where there is swelling of the lips or eyelids, or where a specific drug is suspected — that last one needs recording so the drug can be avoided in future.

Call 999 for hives with any difficulty breathing, swelling of the tongue or throat, or faintness and collapse.

Formal allergy testing has a limited role here, and is worth reserving for cases where the history actually points at something. Testing broadly in chronic urticaria tends to produce incidental positive results that lead to needless avoidance rather than to the cause. The underlying mechanisms are covered separately, as is the distinction between hives and the rashes caused by contact with something, which stay where they are put.