Spots: could a sweat allergy be the cause?

Contrary to certain common misconceptions, an allergy to sweat does not refer to an irritant reaction to moisture, but to a specific dermatological condition: cholinergic urticaria.

Identified as early as 1924, this condition manifests through the sudden appearance of small red patches as soon as the body’s internal temperature rises. Whether triggered by intense physical exertion, a hot bath, or even a sudden bout of stress , the underlying cause remains the same: activation of the thermoregulatory system.

Beyond the visual aspect, it is the perceived sensation that is characteristic: patients rarely report typical itching, but instead describe intense tingling and prickling sensations from the moment the spots first appear.

Furthermore, the pattern of spread of these spots follows a specific physiological logic. They appear predominantly on the trunk and upper limbs, areas that are rich in sweat glands. Another noteworthy point is that cholinergic urticaria almost systematically spares the palms of the hands, the soles of the feet and the armpits, even though these areas are also rich in sweat glands.

Although the symptoms usually resolve in less than an hour once the body has cooled down, the impact on quality of life is significant. In the most severe cases, this reaction may be accompanied by more concerning systemic symptoms, such as angioedema. Also called Quincke’s oedema, this is a swelling of the subcutaneous tissues that can lead to breathing difficulties. Angioedema is a medical emergency.

For a long time, the mechanism of cholinergic urticaria remained unknown. Today, researchers agree that this condition is not homogeneous, but results from several distinct biological dysfunctions involving the nervous system, the skin microbiota and the cells involved in allergic reactions.

First of all, one of the most significant discoveries of recent years concerns the protein MGL_1304. This protein is secreted by the yeast Malassezia globosa, a fungus that is part of our skin microbiome, before mixing with sweat. In certain individuals, the immune system mistakenly identifies this protein as a threat and produces specific antibodies (IgE). Indeed, when we sweat, the sweat carries this antigen through the different layers of the skin, triggering a release of histamine by mast cells.

Another line of research focuses on acetylcholine, the neurotransmitter used by the nervous system to signal sweat glands to produce sweat. Normally, acetylcholine binds to M3 muscarinic receptors on the glands to trigger sweating. However, in some patients, a reduction in the expression of these M3 receptors or a decrease in the enzyme responsible for degrading acetylcholine is observed. Overwhelmed, the neurotransmitter then stimulates neighbouring mast cells, which also possess receptors for acetylcholine. This direct stimulation causes mast cell degranulation and the appearance of lesions, even before sweat reaches the surface of the skin.

Finally, a particular form of cholinergic urticaria is linked to a defect in sweat drainage, known as hypohidrosis. In this case, the ducts through which sweat should be excreted are blocked. This blockage forces the sweat to spread laterally into the surrounding tissues instead of exiting through the pore. This leakage of sweat causes immediate local inflammation, producing intense tingling and burning sensations. This subtype is often associated with dry skin and frequently worsens during the winter.

The first step in establishing a treatment plan for sweat allergy is to diagnose it. Diagnosis is based primarily on the clinical observation of symptoms, but it must sometimes be confirmed by provocation tests. The most common involves inducing a controlled increase in body temperature, either through supervised physical exercise or by means of a hot bath. If papules measuring 1 to 3 mm appear within a few minutes, the diagnosis is confirmed.

Note : It is also important to carry out a differential diagnosis to avoid confusing allergy to sweating with food-dependent exercise-induced anaphylaxis, in which the rash appears only if a specific food has been consumed before physical activity, or with localised heat urticaria, which develops only at the site of direct contact with a heat source.

Once cholinergic urticaria has been confirmed, second-generation H1 antihistamines constitute the cornerstone of treatment.

Their role is to block histamine receptors in order to prevent the formation of wheals and to alleviate burning sensations. Although these drugs are very effective for typical urticaria, cholinergic urticaria often proves more resistant. It is common for clinicians to suggest increasing the usual dosage in order to achieve satisfactory results. In particularly persistent forms, especially those associated with an imbalance in acetylcholine, adding an H2-type antihistamine or other long-term treatments such as omalizumab may be considered to provide sustained stabilisation of mast cells and to reduce the frequency of attacks. For information, omalizumab is an antibody designed to bind to IgE, which are produced in excess in allergic patients.

Additionally, it is of course necessary to maintain good hygiene, with the aim of limiting the contact time between sweat and the skin.

If it is not always possible to shower immediately to remove sweat, carrying cleansing wipes can be a useful temporary solution. Wearing breathable clothing is also beneficial, as it helps moisture to evaporate more rapidly. Finally, maintaining a healthy skin barrier by applying emollients daily is essential: well‑hydrated skin is less permeable to external allergens, thereby reducing the overall reactivity of the epidermis.

• KABASHIMA K. & al. Cholinergic urticaria: Pathogenesis-based categorization and its treatment options. Journal of the European Academy of Dermatology and Venereology (2011).

• NISHIGORI C. & al. Cholinergic urticaria: Epidemiology, physiopathology, new categorization, and management. Clinical Autonomic Research (2017).

• HIDE M. & al. Sweat allergy. Allergology International (2018).

• SINGH I. How histamine can treat circadian rhythm. Research Archive of Rising Scholars (2023).