What is heat urticaria?
Heat urticaria is a rare type of physical or chronic inducible urticaria. It is also called contact heat urticaria and localised heat urticaria.
Heat urticaria occurs following the direct contact of a warm stimulus to the skin. The onset of urticaria is typically rapid (within minutes) and the urticaria resolves within 1–2 hours . It can also cause angioedema and systemic symptoms.
Urticarial weals in acute spontaneous urticaria
Who gets heat urticaria?
Heat urticaria is very rare, with few reported cases, so the exact prevalence is unknown. In one family, delayed heat urticaria was inherited in an autosomal dominant pattern (one parent had the condition and passed on the condition) . Most reported patients have been women aged 20–45 years, with familial cases having onset during childhood .
Chronic inducible urticaria, in general, has an estimated prevalence of 0.5% and predominantly occurs in women, at a rate of 74% of those afflicted . People with heat urticaria may develop another type of inducible urticaria such as cold urticaria or dermographism [1,4].
What causes heat urticaria?
Examples of stimuli that may cause heat urticaria may include:
- Submerging a limb in warm water
- Cooking over a gas stove
- Hot air (eg, from a hairdryer) or sun exposure
- Close contact with a heater or hot water bottle
- Hot food or drink .
The exact development process of heat urticaria is unknown. Heat on the skin activates mast cells to release proinflammatory mediators, mainly histamine. This causes small vessels in the skin to dilate, leading to a localised area of swelling, erythema, and wealing .
Unlike cholinergic urticaria (a heat-related urticaria caused by sweating), heat urticaria does not involve the activation of cholinergic (sweat) glands and it is independent of the whole core body temperature .
What are the clinical features of heat urticaria?
The clinical features of heat urticaria are usually confined to the area affected by the heat stimulus. There are three clinical subtypes of heat urticaria .
Immediate localised heat urticaria
Immediate localised heat urticaria is the most common type and presents as large, well-circumscribed, itchy weals .
- Weals typically appear within 10 minutes of contact with the hot stimulus on the skin or a mucosal surface.
- They are confined to the contact site.
- The weals resolve within 1–3 hours.
- Angioedema can occur.
- Half of the patients report systemic symptoms such as weakness, faintness, headache, nausea, and diarrhoea.
Immediate generalised heat urticaria
Some patients with heat urticaria develop generalised wealing if exposed to a hot environment .
- Heat stimuli can include bathing, hot air, and sun exposure.
- Cutaneous lesions are limited to areas in contact with the heat stimulus.
Delayed heat urticaria
Delayed heat urticaria presents as weals that cause an itch or burning sensation .
- There may be a family history of delayed heat urticaria.
- The onset of wealing is delayed by 0.5–2 hours after contact with the hot stimulus.
- Weals may persist for up to 24 hours.
- Angioedema does not occur .
How is heat urticaria diagnosed?
Heat urticaria is typically diagnosed by the clinical features of a localised area of urticaria following the application and removal of a heat stimulus to the skin .
The diagnosis is confirmed by provocation tests. A warm stimulus (a test tube with water > 44° or a heated cylinder) is applied to the inner forearm . Urticaria develops typically within minutes after the warm stimulus has been removed. The test can be repeated using different temperatures to determine sensitivity to heat and to monitor treatment response.
What is the differential diagnosis for heat urticaria?
Heat urticaria must be differentiated from cholinergic urticaria (generalised heat urticaria) in that it is localised to the skin in contact with the heat stimulus and doesn’t involve the activation of sweat glands systemically .
Urticaria is a common symptom and can result from many precipitating or unknown causes, as described below.
- Chronic spontaneous urticaria appears without a known precipitating cause, has no association with a heat stimulus, and is typically widespread.
- Cholinergic urticaria results from an increase in core body temperature and leads to sweating, which causes urticaria. Cholinergic urticaria typically causes pinpoint weals, with pronounced flushing on trunk and limbs. In severe cases, it can result in wheezing and anaphylaxis.
- Aquagenic urticaria is very rare. Wealing follows direct skin contact with water, irrespective of its temperature.
- Other forms of chronic inducible urticaria are dermographism, cold, delayed pressure, solar, contact, exercise-induced, and vibration urticaria. These are classified according to the stimulus that provokes the urticaria to develop and are confirmed by provocation testing.
What is the treatment for heat urticaria?
It is important to identify and avoid the triggering factors for heat urticaria.
First-line treatment is a non-sedating second-generation H1 antihistamine, such as cetirizine or loratadine 10 mg daily . If symptoms persist, the dose is increased, up to four times the standard dose each day . This is at least somewhat effective for about 60% of patients with heat urticaria .
Possible additional options may include:
- H2 antihistamines, such as ranitidine 
- Non-steroidal anti-inflammatory drugs, such as indomethacin or ketotifen 
- Omalizumab (an anti-immunoglobulin E medication) 
- Desensitisation therapy using daily hot baths; unfortunately, this risks inducing a systemic anaphylactoid reaction .
What is the outcome for heat urticaria?
Chronic inducible urticaria overall has a long duration, with lower rates of remission at 1 year when compared to chronic spontaneous urticaria . The impact on the quality of life can be significant.