Spots: could a sweat allergy be the cause?

Contrary to some common beliefs, an allergy to sweat does not refer to an irritative reaction to moisture, but to a specific dermatological condition: cholinergic urticaria.

First identified in 1924, this condition manifests as the sudden appearance of small red patches whenever the body’s internal temperature rises. Whether it is triggered by intense physical exertion, a hot bath, or even a sudden stress following emotional event, the underlying cause remains the same: activation of the thermoregulatory system.

Beyond the visual aspect, the defining feature is how it feels: patients rarely report typical itching, but instead describe intense prickling sensations and tingling as soon as the spots appear.

Moreover, the pattern of spread of these bumps follows a specific physiological logic. They appear mainly on the trunk and upper limbs, areas that are rich in sweat glands. Another notable 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 disappear in less than an hour once the body has cooled down, the impact on quality of life is real. In the most severe cases, this reaction may be accompanied by more concerning systemic symptoms, such as angioedema. Also called Quincke’s edema, this is a swelling of the subcutaneous tissues that can lead to difficulty breathing. 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 instead results from several distinct biological dysfunctions involving the nervous system, the skin microbiota, and the cells involved in allergic reactions.

First, 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 some 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 activate sweating. However, in some patients, a reduced expression of these M3 receptors is observed, or a decrease in the enzyme responsible for breaking down acetylcholine. Overwhelmed, the neurotransmitter then stimulates neighboring mast cells, which also have acetylcholine receptors. This direct stimulation triggers mast cell degranulation and the appearance of bumps, even before the sweat reaches the surface of the skin.

Finally, a particular form of cholinergic urticaria is linked to a defect in sweat elimination, called hypohidrosis. In this case, the ducts through which sweat is supposed to exit are blocked. This blockage forces the sweat to spread sideways into the surrounding tissues instead of exiting through the pore. This leakage of sweat causes immediate local inflammation, creating intense tingling and burning sensations. This subtype is often associated with dry skin and frequently worsens during the winter.

The first step in developing 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 test involves inducing a controlled increase in body temperature, either through supervised physical exercise or a hot bath. If 1–3 mm papules appear within minutes, the diagnosis is confirmed.

Note : It is also important to perform a differential diagnosis to avoid confusing sweat allergy with food-dependent exercise-induced anaphylaxis, in which hives appear only if a specific food was consumed before exercising, or with localized 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 are the cornerstone of treatment.

Their role is to block histamine receptors in order to prevent the formation of wheals and to relieve burning sensations. Although these medications are very effective for typical urticaria, cholinergic urticaria is often more treatment-resistant. Physicians commonly recommend increasing the usual dosage to achieve satisfactory results. In particularly persistent forms, especially those associated with an acetylcholine imbalance, adding an H2‑type antihistamine or other long‑term treatments such as omalizumab may be considered to provide lasting stabilization of mast cells and reduce the frequency of flare‑ups. For reference, 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 goal of limiting the contact time between sweat and the skin.

If it is not always possible to shower immediately to remove perspiration, carrying cleansing wipes can be a temporary solution. Wearing breathable clothing is also an option, as it allows moisture to be evacuated as quickly as possible. Finally, maintaining a healthy skin barrier is essential by applying emollients daily: 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).